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.TH HBAL 1 2009-03-23 htools "Ganeti H-tools" |
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.SH NAME |
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hbal \- Cluster balancer for Ganeti |
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|
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.SH SYNOPSIS |
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.B hbal |
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.B "[-C]" |
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.B "[-p]" |
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.B "[-o]" |
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.B "[-v... | -q]" |
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.BI "[-l" limit "]" |
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.BI "[-O" name... "]" |
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.BI "[-e" score "]" |
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.BI "[-m " cluster "]" |
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.BI "[-n " nodes-file " ]" |
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.BI "[-i " instances-file "]" |
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|
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.B hbal |
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.B --version |
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|
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.SH DESCRIPTION |
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hbal is a cluster balancer that looks at the current state of the |
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cluster (nodes with their total and free disk, memory, etc.) and |
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instance placement and computes a series of steps designed to bring |
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the cluster into a better state. |
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|
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The algorithm to do so is designed to be stable (i.e. it will give you |
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the same results when restarting it from the middle of the solution) |
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and reasonably fast. It is not, however, designed to be a perfect |
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algorithm - it is possible to make it go into a corner from which it |
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can find no improvement, because it only look one "step" ahead. |
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|
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By default, the program will show the solution incrementally as it is |
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computed, in a somewhat cryptic format; for getting the actual Ganeti |
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command list, use the \fB-C\fR option. |
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|
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.SS ALGORITHM |
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|
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The program works in independent steps; at each step, we compute the |
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best instance move that lowers the cluster score. |
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|
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The possible move type for an instance are combinations of |
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failover/migrate and replace-disks such that we change one of the |
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instance nodes, and the other one remains (but possibly with changed |
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role, e.g. from primary it becomes secondary). The list is: |
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.RS 4 |
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.TP 3 |
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\(em |
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failover (f) |
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.TP |
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\(em |
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replace secondary (r) |
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.TP |
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\(em |
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replace primary, a composite move (f, r, f) |
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.TP |
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\(em |
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failover and replace secondary, also composite (f, r) |
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.TP |
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\(em |
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replace secondary and failover, also composite (r, f) |
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.RE |
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|
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We don't do the only remaining possibility of replacing both nodes |
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(r,f,r,f or the equivalent f,r,f,r) since these move needs an |
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exhaustive search over both candidate primary and secondary nodes, and |
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is O(n*n) in the number of nodes. Furthermore, it doesn't seems to |
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give better scores but will result in more disk replacements. |
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|
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.SS CLUSTER SCORING |
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|
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As said before, the algorithm tries to minimise the cluster score at |
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each step. Currently this score is computed as a sum of the following |
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components: |
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.RS 4 |
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.TP 3 |
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\(em |
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coefficient of variance of the percent of free memory |
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.TP |
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\(em |
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coefficient of variance of the percent of reserved memory |
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.TP |
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\(em |
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coefficient of variance of the percent of free disk |
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.TP |
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\(em |
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percentage of nodes failing N+1 check |
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.TP |
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\(em |
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percentage of instances living (either as primary or secondary) on |
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offline nodes |
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.RE |
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|
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The free memory and free disk values help ensure that all nodes are |
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somewhat balanced in their resource usage. The reserved memory helps |
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to ensure that nodes are somewhat balanced in holding secondary |
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instances, and that no node keeps too much memory reserved for |
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N+1. And finally, the N+1 percentage helps guide the algorithm towards |
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eliminating N+1 failures, if possible. |
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|
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Except for the N+1 failures and offline instances percentage, we use |
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the coefficient of variance since this brings the values into the same |
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unit so to speak, and with a restrict domain of values (between zero |
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and one). The percentage of N+1 failures, while also in this numeric |
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range, doesn't actually has the same meaning, but it has shown to work |
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well. |
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|
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The other alternative, using for N+1 checks the coefficient of |
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variance of (N+1 fail=1, N+1 pass=0) across nodes could hint the |
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algorithm to make more N+1 failures if most nodes are N+1 fail |
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already. Since this (making N+1 failures) is not allowed by other |
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rules of the algorithm, so the N+1 checks would simply not work |
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anymore in this case. |
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|
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The offline instances percentage (meaning the percentage of instances |
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living on offline nodes) will cause the algorithm to actively move |
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instances away from offline nodes. This, coupled with the restriction |
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on placement given by offline nodes, will cause evacuation of such |
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nodes. |
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|
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On a perfectly balanced cluster (all nodes the same size, all |
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instances the same size and spread across the nodes equally), all |
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values would be zero. This doesn't happen too often in practice :) |
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|
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.SS OFFLINE INSTANCES |
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|
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Since current Ganeti versions do not report the memory used by offline |
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(down) instances, ignoring the run status of instances will cause |
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wrong calculations. For this reason, the algorithm subtracts the |
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memory size of down instances from the free node memory of their |
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primary node, in effect simulating the startup of such instances. |
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|
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.SS OTHER POSSIBLE METRICS |
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|
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It would be desirable to add more metrics to the algorithm, especially |
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dynamically-computed metrics, such as: |
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.RS 4 |
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.TP 3 |
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\(em |
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CPU usage of instances, combined with VCPU versus PCPU count |
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.TP |
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\(em |
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Disk IO usage |
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.TP |
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\(em |
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Network IO |
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.RE |
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|
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.SH OPTIONS |
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The options that can be passed to the program are as follows: |
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.TP |
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.B -C, --print-commands |
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Print the command list at the end of the run. Without this, the |
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program will only show a shorter, but cryptic output. |
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.TP |
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.B -p, --print-nodes |
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Prints the before and after node status, in a format designed to allow |
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the user to understand the node's most important parameters. |
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|
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The node list will contain these informations: |
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.RS |
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.TP |
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.B F |
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a character denoting the status of the node, with '-' meaning an |
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offline node, '*' meaning N+1 failure and blank meaning a good node |
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.TP |
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.B Name |
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the node name |
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.TP |
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.B t_mem |
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the total node memory |
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.TP |
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.B n_mem |
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the memory used by the node itself |
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.TP |
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.B i_mem |
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the memory used by instances |
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.TP |
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.B x_mem |
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amount memory which seems to be in use but cannot be determined why or |
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by which instance; usually this means that the hypervisor has some |
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overhead or that there are other reporting errors |
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.TP |
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.B f_mem |
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the free node memory |
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.TP |
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.B r_mem |
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the reserved node memory, which is the amount of free memory needed |
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for N+1 compliance |
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.TP |
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.B t_dsk |
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total disk |
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.TP |
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.B f_dsk |
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free disk |
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.TP |
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.B pri |
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number of primary instances |
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.TP |
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.B sec |
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number of secondary instances |
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.TP |
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.B p_fmem |
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percent of free memory |
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.TP |
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.B p_fdsk |
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percent of free disk |
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.RE |
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|
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.TP |
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.B -o, --oneline |
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Only shows a one-line output from the program, designed for the case |
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when one wants to look at multiple clusters at once and check their |
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status. |
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|
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The line will contain four fields: |
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.RS |
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.RS 4 |
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.TP 3 |
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\(em |
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initial cluster score |
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.TP |
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\(em |
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number of steps in the solution |
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.TP |
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\(em |
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final cluster score |
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.TP |
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\(em |
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improvement in the cluster score |
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.RE |
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.RE |
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|
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.TP |
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.BI "-O " name |
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This option (which can be given multiple times) will mark nodes as |
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being \fIoffline\fR. This means a couple of things: |
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.RS |
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.RS 4 |
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.TP 3 |
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\(em |
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instances won't be placed on these nodes, not even temporarily; |
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e.g. the \fIreplace primary\fR move is not available if the secondary |
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node is offline, since this move requires a failover. |
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.TP |
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\(em |
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these nodes will not be included in the score calculation (except for |
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the percentage of instances on offline nodes) |
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.RE |
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Note that hbal will also mark as offline any nodes which are reported |
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by RAPI as such, or that have "?" in file-based input in any numeric |
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fields. |
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.RE |
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|
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.TP |
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.BI "-e" score ", --min-score=" score |
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This parameter denotes the minimum score we are happy with and alters |
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the computation in two ways: |
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.RS |
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.RS 4 |
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.TP 3 |
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\(em |
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if the cluster has the initial score lower than this value, then we |
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don't enter the algorithm at all, and exit with success |
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.TP |
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\(em |
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during the iterative process, if we reach a score lower than this |
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value, we exit the algorithm |
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.RE |
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The default value of the parameter is currently \fI1e-9\fR (chosen |
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empirically). |
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.RE |
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|
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.TP |
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.BI "-n" nodefile ", --nodes=" nodefile |
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The name of the file holding node information (if not collecting via |
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RAPI), instead of the default \fInodes\fR file (but see below how to |
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customize the default value via the environment). |
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|
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.TP |
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.BI "-i" instancefile ", --instances=" instancefile |
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The name of the file holding instance information (if not collecting |
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via RAPI), instead of the default \fIinstances\fR file (but see below |
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how to customize the default value via the environment). |
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|
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.TP |
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.BI "-m" cluster |
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Collect data not from files but directly from the |
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.I cluster |
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given as an argument via RAPI. If the argument doesn't contain a colon |
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(:), then it is converted into a fully-built URL via prepending |
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https:// and appending the default RAPI port, otherwise it's |
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considered a fully-specified URL and is used as-is. |
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|
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.TP |
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.BI "-l" N ", --max-length=" N |
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Restrict the solution to this length. This can be used for example to |
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automate the execution of the balancing. |
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|
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.TP |
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.B -v, --verbose |
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Increase the output verbosity. Each usage of this option will increase |
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the verbosity (currently more than 2 doesn't make sense) from the |
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default of one. |
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|
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.TP |
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.B -q, --quiet |
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Decrease the output verbosity. Each usage of this option will decrease |
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the verbosity (less than zero doesn't make sense) from the default of |
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one. |
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|
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.TP |
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.B -V, --version |
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Just show the program version and exit. |
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|
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.SH EXIT STATUS |
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|
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The exist status of the command will be zero, unless for some reason |
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the algorithm fatally failed (e.g. wrong node or instance data). |
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|
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.SH ENVIRONMENT |
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|
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If the variables \fBHTOOLS_NODES\fR and \fBHTOOLS_INSTANCES\fR are |
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present in the environment, they will override the default names for |
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the nodes and instances files. These will have of course no effect |
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when RAPI is used. |
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|
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.SH BUGS |
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|
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The program does not check its input data for consistency, and aborts |
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with cryptic errors messages in this case. |
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|
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The algorithm is not perfect. |
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|
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The output format is not easily scriptable, and the program should |
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feed moves directly into Ganeti (either via RAPI or via a gnt-debug |
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input file). |
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|
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.SH EXAMPLE |
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|
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Note that this example are not for the latest version (they don't have |
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full node data). |
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|
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.SS Default output |
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|
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With the default options, the program shows each individual step and |
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the improvements it brings in cluster score: |
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|
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.in +4n |
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.nf |
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.RB "$" " hbal" |
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Loaded 20 nodes, 80 instances |
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Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy. |
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Initial score: 0.52329131 |
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Trying to minimize the CV... |
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1. instance14 node1:node10 => node16:node10 0.42109120 a=f r:node16 f |
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2. instance54 node4:node15 => node16:node15 0.31904594 a=f r:node16 f |
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3. instance4 node5:node2 => node2:node16 0.26611015 a=f r:node16 |
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4. instance48 node18:node20 => node2:node18 0.21361717 a=r:node2 f |
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5. instance93 node19:node18 => node16:node19 0.16166425 a=r:node16 f |
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6. instance89 node3:node20 => node2:node3 0.11005629 a=r:node2 f |
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7. instance5 node6:node2 => node16:node6 0.05841589 a=r:node16 f |
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8. instance94 node7:node20 => node20:node16 0.00658759 a=f r:node16 |
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9. instance44 node20:node2 => node2:node15 0.00438740 a=f r:node15 |
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10. instance62 node14:node18 => node14:node16 0.00390087 a=r:node16 |
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11. instance13 node11:node14 => node11:node16 0.00361787 a=r:node16 |
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12. instance19 node10:node11 => node10:node7 0.00336636 a=r:node7 |
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13. instance43 node12:node13 => node12:node1 0.00305681 a=r:node1 |
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14. instance1 node1:node2 => node1:node4 0.00263124 a=r:node4 |
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15. instance58 node19:node20 => node19:node17 0.00252594 a=r:node17 |
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Cluster score improved from 0.52329131 to 0.00252594 |
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.fi |
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.in |
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|
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In the above output, we can see: |
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- the input data (here from files) shows a cluster with 20 nodes and |
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80 instances |
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- the cluster is not initially N+1 compliant |
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- the initial score is 0.52329131 |
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|
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The step list follows, showing the instance, its initial |
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primary/secondary nodes, the new primary secondary, the cluster list, |
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and the actions taken in this step (with 'f' denoting failover/migrate |
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and 'r' denoting replace secondary). |
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|
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Finally, the program shows the improvement in cluster score. |
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|
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A more detailed output is obtained via the \fB-C\fR and \fB-p\fR options: |
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|
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.in +4n |
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.nf |
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.RB "$" " hbal" |
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Loaded 20 nodes, 80 instances |
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Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy. |
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Initial cluster status: |
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N1 Name t_mem f_mem r_mem t_dsk f_dsk pri sec p_fmem p_fdsk |
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* node1 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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node2 32762 31280 12000 1861 1026 0 8 0.95476 0.55179 |
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* node3 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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* node4 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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* node5 32762 1280 6000 1861 978 5 5 0.03907 0.52573 |
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* node6 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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* node7 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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node8 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
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node9 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
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* node10 32762 7280 12000 1861 1026 4 4 0.22221 0.55179 |
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node11 32762 7280 6000 1861 922 4 5 0.22221 0.49577 |
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node12 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
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node13 32762 7280 6000 1861 922 4 5 0.22221 0.49577 |
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node14 32762 7280 6000 1861 922 4 5 0.22221 0.49577 |
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* node15 32762 7280 12000 1861 1131 4 3 0.22221 0.60782 |
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node16 32762 31280 0 1861 1860 0 0 0.95476 1.00000 |
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node17 32762 7280 6000 1861 1106 5 3 0.22221 0.59479 |
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* node18 32762 1280 6000 1396 561 5 3 0.03907 0.40239 |
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* node19 32762 1280 6000 1861 1026 5 3 0.03907 0.55179 |
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node20 32762 13280 12000 1861 689 3 9 0.40535 0.37068 |
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|
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Initial score: 0.52329131 |
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Trying to minimize the CV... |
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1. instance14 node1:node10 => node16:node10 0.42109120 a=f r:node16 f |
421 |
2. instance54 node4:node15 => node16:node15 0.31904594 a=f r:node16 f |
422 |
3. instance4 node5:node2 => node2:node16 0.26611015 a=f r:node16 |
423 |
4. instance48 node18:node20 => node2:node18 0.21361717 a=r:node2 f |
424 |
5. instance93 node19:node18 => node16:node19 0.16166425 a=r:node16 f |
425 |
6. instance89 node3:node20 => node2:node3 0.11005629 a=r:node2 f |
426 |
7. instance5 node6:node2 => node16:node6 0.05841589 a=r:node16 f |
427 |
8. instance94 node7:node20 => node20:node16 0.00658759 a=f r:node16 |
428 |
9. instance44 node20:node2 => node2:node15 0.00438740 a=f r:node15 |
429 |
10. instance62 node14:node18 => node14:node16 0.00390087 a=r:node16 |
430 |
11. instance13 node11:node14 => node11:node16 0.00361787 a=r:node16 |
431 |
12. instance19 node10:node11 => node10:node7 0.00336636 a=r:node7 |
432 |
13. instance43 node12:node13 => node12:node1 0.00305681 a=r:node1 |
433 |
14. instance1 node1:node2 => node1:node4 0.00263124 a=r:node4 |
434 |
15. instance58 node19:node20 => node19:node17 0.00252594 a=r:node17 |
435 |
Cluster score improved from 0.52329131 to 0.00252594 |
436 |
|
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Commands to run to reach the above solution: |
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echo step 1 |
439 |
echo gnt-instance migrate instance14 |
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echo gnt-instance replace-disks -n node16 instance14 |
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echo gnt-instance migrate instance14 |
442 |
echo step 2 |
443 |
echo gnt-instance migrate instance54 |
444 |
echo gnt-instance replace-disks -n node16 instance54 |
445 |
echo gnt-instance migrate instance54 |
446 |
echo step 3 |
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echo gnt-instance migrate instance4 |
448 |
echo gnt-instance replace-disks -n node16 instance4 |
449 |
echo step 4 |
450 |
echo gnt-instance replace-disks -n node2 instance48 |
451 |
echo gnt-instance migrate instance48 |
452 |
echo step 5 |
453 |
echo gnt-instance replace-disks -n node16 instance93 |
454 |
echo gnt-instance migrate instance93 |
455 |
echo step 6 |
456 |
echo gnt-instance replace-disks -n node2 instance89 |
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echo gnt-instance migrate instance89 |
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echo step 7 |
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echo gnt-instance replace-disks -n node16 instance5 |
460 |
echo gnt-instance migrate instance5 |
461 |
echo step 8 |
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echo gnt-instance migrate instance94 |
463 |
echo gnt-instance replace-disks -n node16 instance94 |
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echo step 9 |
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echo gnt-instance migrate instance44 |
466 |
echo gnt-instance replace-disks -n node15 instance44 |
467 |
echo step 10 |
468 |
echo gnt-instance replace-disks -n node16 instance62 |
469 |
echo step 11 |
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echo gnt-instance replace-disks -n node16 instance13 |
471 |
echo step 12 |
472 |
echo gnt-instance replace-disks -n node7 instance19 |
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echo step 13 |
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echo gnt-instance replace-disks -n node1 instance43 |
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echo step 14 |
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echo gnt-instance replace-disks -n node4 instance1 |
477 |
echo step 15 |
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echo gnt-instance replace-disks -n node17 instance58 |
479 |
|
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Final cluster status: |
481 |
N1 Name t_mem f_mem r_mem t_dsk f_dsk pri sec p_fmem p_fdsk |
482 |
node1 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
483 |
node2 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
484 |
node3 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
485 |
node4 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
486 |
node5 32762 7280 6000 1861 1078 4 5 0.22221 0.57947 |
487 |
node6 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
488 |
node7 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
489 |
node8 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
490 |
node9 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
491 |
node10 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
492 |
node11 32762 7280 6000 1861 1022 4 4 0.22221 0.54951 |
493 |
node12 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
494 |
node13 32762 7280 6000 1861 1022 4 4 0.22221 0.54951 |
495 |
node14 32762 7280 6000 1861 1022 4 4 0.22221 0.54951 |
496 |
node15 32762 7280 6000 1861 1031 4 4 0.22221 0.55408 |
497 |
node16 32762 7280 6000 1861 1060 4 4 0.22221 0.57007 |
498 |
node17 32762 7280 6000 1861 1006 5 4 0.22221 0.54105 |
499 |
node18 32762 7280 6000 1396 761 4 2 0.22221 0.54570 |
500 |
node19 32762 7280 6000 1861 1026 4 4 0.22221 0.55179 |
501 |
node20 32762 13280 6000 1861 1089 3 5 0.40535 0.58565 |
502 |
|
503 |
.fi |
504 |
.in |
505 |
|
506 |
Here we see, beside the step list, the initial and final cluster |
507 |
status, with the final one showing all nodes being N+1 compliant, and |
508 |
the command list to reach the final solution. In the initial listing, |
509 |
we see which nodes are not N+1 compliant. |
510 |
|
511 |
The algorithm is stable as long as each step above is fully completed, |
512 |
e.g. in step 8, both the migrate and the replace-disks are |
513 |
done. Otherwise, if only the migrate is done, the input data is |
514 |
changed in a way that the program will output a different solution |
515 |
list (but hopefully will end in the same state). |
516 |
|
517 |
.SH SEE ALSO |
518 |
.BR hn1 "(1), " hscan "(1), " ganeti "(7), " gnt-instance "(8), " |
519 |
.BR gnt-node "(8)" |
520 |
|
521 |
.SH "COPYRIGHT" |
522 |
.PP |
523 |
Copyright (C) 2009 Google Inc. Permission is granted to copy, |
524 |
distribute and/or modify under the terms of the GNU General Public |
525 |
License as published by the Free Software Foundation; either version 2 |
526 |
of the License, or (at your option) any later version. |
527 |
.PP |
528 |
On Debian systems, the complete text of the GNU General Public License |
529 |
can be found in /usr/share/common-licenses/GPL. |