Synnefo » snf-ganeti

.TH HBAL 1 2009-03-23 htools "Ganeti H-tools"

.SH NAME

hbal \- Cluster balancer for Ganeti

.SH SYNOPSIS

.B hbal

.B "[backend options...]"

.B "[algorithm options...]"

.B "[reporting options...]"

.B hbal

.B --version

.TP

Backend options:

.BI "[ -m " cluster " ]"

|

.BI "[ -L[" path "] [-X]]"

|

.BI "[ -t " data-file " ]"

.TP

Algorithm options:

.BI "[ --max-cpu " cpu-ratio " ]"

.BI "[ --min-disk " disk-ratio " ]"

.BI "[ -l " limit " ]"

.BI "[ -e " score " ]"

.BI "[ -O " name... " ]"

.B "[ --no-disk-moves ]"

.BI "[ -U " util-file " ]"

.TP

Reporting options:

.BI "[ -C[" file "] ]"

.BI "[ -p[" fields "] ]"

.B "[ --print-instances ]"

.B "[ -o ]"

.B "[ -v... | -q ]"

.SH DESCRIPTION

hbal is a cluster balancer that looks at the current state of the

cluster (nodes with their total and free disk, memory, etc.) and

instance placement and computes a series of steps designed to bring

the cluster into a better state.

The algorithm used is designed to be stable (i.e. it will give you the

same results when restarting it from the middle of the solution) and

reasonably fast. It is not, however, designed to be a perfect

algorithm \(em it is possible to make it go into a corner from which

it can find no improvement, because it looks only one "step" ahead.

By default, the program will show the solution incrementally as it is

computed, in a somewhat cryptic format; for getting the actual Ganeti

command list, use the \fB-C\fR option.

.SS ALGORITHM

The program works in independent steps; at each step, we compute the

best instance move that lowers the cluster score.

The possible move type for an instance are combinations of

failover/migrate and replace-disks such that we change one of the

instance nodes, and the other one remains (but possibly with changed

role, e.g. from primary it becomes secondary). The list is:

.RS 4

.TP 3

\(em

failover (f)

.TP

\(em

replace secondary (r)

.TP

\(em

replace primary, a composite move (f, r, f)

.TP

\(em

failover and replace secondary, also composite (f, r)

.TP

\(em

replace secondary and failover, also composite (r, f)

.RE

We don't do the only remaining possibility of replacing both nodes

(r,f,r,f or the equivalent f,r,f,r) since these move needs an

exhaustive search over both candidate primary and secondary nodes, and

is O(n*n) in the number of nodes. Furthermore, it doesn't seems to

give better scores but will result in more disk replacements.

.SS PLACEMENT RESTRICTIONS

At each step, we prevent an instance move if it would cause:

.RS 4

.TP 3

\(em

a node to go into N+1 failure state

.TP

\(em

an instance to move onto an offline node (offline nodes are either

read from the cluster or declared with \fI-O\fR)

.TP

\(em

an exclusion-tag based conflict (exclusion tags are read from the

cluster and/or defined via the \fI--exclusion-tags\fR option)

.TP

\(em

a max vcpu/pcpu ratio to be exceeded (configured via \fI--max-cpu\fR)

.TP

\(em

min disk free percentage to go below the configured limit (configured

via \fI--min-disk\fR)

.SS CLUSTER SCORING

As said before, the algorithm tries to minimise the cluster score at

each step. Currently this score is computed as a sum of the following

components:

.RS 4

.TP 3

\(em

standard deviation of the percent of free memory

.TP

\(em

standard deviation of the percent of reserved memory

.TP

\(em

standard deviation of the percent of free disk

.TP

\(em

count of nodes failing N+1 check

.TP

\(em

count of instances living (either as primary or secondary) on

offline nodes

.TP

\(em

count of instances living (as primary) on offline nodes; this differs

from the above metric by helping failover of such instances in 2-node

clusters

.TP

\(em

standard deviation of the ratio of virtual-to-physical cpus (for

primary instances of the node)

.TP

\(em

standard deviation of the dynamic load on the nodes, for cpus,

memory, disk and network

.RE

The free memory and free disk values help ensure that all nodes are

somewhat balanced in their resource usage. The reserved memory helps

to ensure that nodes are somewhat balanced in holding secondary

instances, and that no node keeps too much memory reserved for

N+1. And finally, the N+1 percentage helps guide the algorithm towards

eliminating N+1 failures, if possible.

Except for the N+1 failures and offline instances counts, we use the

standard deviation since when used with values within a fixed range

(we use percents expressed as values between zero and one) it gives

consistent results across all metrics (there are some small issues

related to different means, but it works generally well). The 'count'

type values will have higher score and thus will matter more for

balancing; thus these are better for hard constraints (like evacuating

nodes and fixing N+1 failures). For example, the offline instances

count (i.e. the number of instances living on offline nodes) will

cause the algorithm to actively move instances away from offline

nodes. This, coupled with the restriction on placement given by

offline nodes, will cause evacuation of such nodes.

The dynamic load values need to be read from an external file (Ganeti

doesn't supply them), and are computed for each node as: sum of

primary instance cpu load, sum of primary instance memory load, sum of

primary and secondary instance disk load (as DRBD generates write load

on secondary nodes too in normal case and in degraded scenarios also

read load), and sum of primary instance network load. An example of

how to generate these values for input to hbal would be to track "xm

list" for instance over a day and by computing the delta of the cpu

values, and feed that via the \fI-U\fR option for all instances (and

keep the other metrics as one). For the algorithm to work, all that is

needed is that the values are consistent for a metric across all

instances (e.g. all instances use cpu% to report cpu usage, and not

something related to number of CPU seconds used if the CPUs are

different), and that they are normalised to between zero and one. Note

that it's recommended to not have zero as the load value for any

instance metric since then secondary instances are not well balanced.

On a perfectly balanced cluster (all nodes the same size, all

instances the same size and spread across the nodes equally), the

values for all metrics would be zero. This doesn't happen too often in

practice :)

.SS OFFLINE INSTANCES

Since current Ganeti versions do not report the memory used by offline

(down) instances, ignoring the run status of instances will cause

wrong calculations. For this reason, the algorithm subtracts the

memory size of down instances from the free node memory of their

primary node, in effect simulating the startup of such instances.

.SS EXCLUSION TAGS

The exclusion tags mechanism is designed to prevent instances which

run the same workload (e.g. two DNS servers) to land on the same node,

which would make the respective node a SPOF for the given service.

It works by tagging instances with certain tags and then building

exclusion maps based on these. Which tags are actually used is

configured either via the command line (option \fI--exclusion-tags\fR)

or via adding them to the cluster tags:

.TP

.B --exclusion-tags=a,b

This will make all instance tags of the form \fIa:*\fR, \fIb:*\fR be

considered for the exclusion map

.TP

cluster tags \fBhtools:iextags:a\fR, \fBhtools:iextags:b\fR

This will make instance tags \fIa:*\fR, \fIb:*\fR be considered for

the exclusion map. More precisely, the suffix of cluster tags starting

with \fBhtools:iextags:\fR will become the prefix of the exclusion

tags.

.P

Both the above forms mean that two instances both having (e.g.) the

tag \fIa:foo\fR or \fIb:bar\fR won't end on the same node.

.SH OPTIONS

The options that can be passed to the program are as follows:

.TP

.B -C, --print-commands

Print the command list at the end of the run. Without this, the

program will only show a shorter, but cryptic output.

Note that the moves list will be split into independent steps, called

"jobsets", but only for visual inspection, not for actually

parallelisation. It is not possible to parallelise these directly when

executed via "gnt-instance" commands, since a compound command

(e.g. failover and replace\-disks) must be executed serially. Parallel

execution is only possible when using the Luxi backend and the

\fI-L\fR option.

The algorithm for splitting the moves into jobsets is by accumulating

moves until the next move is touching nodes already touched by the

current moves; this means we can't execute in parallel (due to

resource allocation in Ganeti) and thus we start a new jobset.

.TP

.B -p, --print-nodes

Prints the before and after node status, in a format designed to allow

the user to understand the node's most important parameters.

It is possible to customise the listed information by passing a

comma\(hyseparated list of field names to this option (the field list is

currently undocumented). By default, the node list will contain these

informations:

.RS

.TP

.B F

a character denoting the status of the node, with '\-' meaning an

offline node, '*' meaning N+1 failure and blank meaning a good node

.TP

.B Name

the node name

.TP

.B t_mem

the total node memory

.TP

.B n_mem

the memory used by the node itself

.TP

.B i_mem

the memory used by instances

.TP

.B x_mem

amount memory which seems to be in use but cannot be determined why or

by which instance; usually this means that the hypervisor has some

overhead or that there are other reporting errors

.TP

.B f_mem

the free node memory

.TP

.B r_mem

the reserved node memory, which is the amount of free memory needed

for N+1 compliance

.TP

.B t_dsk

total disk

.TP

.B f_dsk

free disk

.TP

.B pcpu

the number of physical cpus on the node

.TP

.B vcpu

the number of virtual cpus allocated to primary instances

.TP

.B pri

number of primary instances

.TP

.B sec

number of secondary instances

.TP

.B p_fmem

percent of free memory

.TP

.B p_fdsk

percent of free disk

.TP

.B r_cpu

ratio of virtual to physical cpus

.TP

.B lCpu

the dynamic CPU load (if the information is available)

.TP

.B lMem

the dynamic memory load (if the information is available)

.TP

.B lDsk

the dynamic disk load (if the information is available)

.TP

.B lNet

the dynamic net load (if the information is available)

.RE

.TP

.B --print-instances

Prints the before and after instance map. This is less useful as the

node status, but it can help in understanding instance moves.

.TP

.B -o, --oneline

Only shows a one\(hyline output from the program, designed for the case

when one wants to look at multiple clusters at once and check their

status.

The line will contain four fields:

.RS

.RS 4

.TP 3

\(em

initial cluster score

.TP

\(em

number of steps in the solution

.TP

\(em

final cluster score

.TP

\(em

improvement in the cluster score

.RE

.RE

.TP

.BI "-O " name

This option (which can be given multiple times) will mark nodes as

being \fIoffline\fR. This means a couple of things:

.RS

.RS 4

.TP 3

\(em

instances won't be placed on these nodes, not even temporarily;

e.g. the \fIreplace primary\fR move is not available if the secondary

node is offline, since this move requires a failover.

.TP

\(em

these nodes will not be included in the score calculation (except for

the percentage of instances on offline nodes)

.RE

Note that hbal will also mark as offline any nodes which are reported

by RAPI as such, or that have "?" in file\(hybased input in any numeric

fields.

.RE

.TP

.BI "-e" score ", --min-score=" score

This parameter denotes the minimum score we are happy with and alters

the computation in two ways:

.RS

.RS 4

.TP 3

\(em

if the cluster has the initial score lower than this value, then we

don't enter the algorithm at all, and exit with success

.TP

\(em

during the iterative process, if we reach a score lower than this

value, we exit the algorithm

.RE

The default value of the parameter is currently \fI1e-9\fR (chosen

empirically).

.RE

.TP

.BI "--no-disk-moves"

This parameter prevents hbal from using disk move (i.e. "gnt\-instance

replace\-disks") operations. This will result in a much quicker

balancing, but of course the improvements are limited. It is up to the

user to decide when to use one or another.

.TP

.BI "-U" util-file

This parameter specifies a file holding instance dynamic utilisation

information that will be used to tweak the balancing algorithm to

equalise load on the nodes (as opposed to static resource usage). The

file is in the format "instance_name cpu_util mem_util disk_util

net_util" where the "_util" parameters are interpreted as numbers and

the instance name must match exactly the instance as read from

Ganeti. In case of unknown instance names, the program will abort.

If not given, the default values are one for all metrics and thus

dynamic utilisation has only one effect on the algorithm: the

equalisation of the secondary instances across nodes (this is the only

metric that is not tracked by another, dedicated value, and thus the

disk load of instances will cause secondary instance

equalisation). Note that value of one will also influence slightly the

primary instance count, but that is already tracked via other metrics

and thus the influence of the dynamic utilisation will be practically

insignificant.

.TP

.BI "-t" datafile ", --text-data=" datafile

The name of the file holding node and instance information (if not

collecting via RAPI or LUXI). This or one of the other backends must

be selected.

.TP

.BI "-m" cluster

Collect data directly from the

.I cluster

given as an argument via RAPI. If the argument doesn't contain a colon

(:), then it is converted into a fully\(hybuilt URL via prepending

https:// and appending the default RAPI port, otherwise it's

considered a fully\(hyspecified URL and is used as\(hyis.

.TP

.BI "-L[" path "]"

Collect data directly from the master daemon, which is to be contacted

via the luxi (an internal Ganeti protocol). An optional \fIpath\fR

argument is interpreted as the path to the unix socket on which the

master daemon listens; otherwise, the default path used by ganeti when

installed with \fI--localstatedir=/var\fR is used.

.TP

.B "-X"

When using the Luxi backend, hbal can also execute the given

commands. The execution method is to execute the individual jobsets

(see the \fI-C\fR option for details) in separate stages, aborting if

at any time a jobset doesn't have all jobs successful. Each step in

the balancing solution will be translated into exactly one Ganeti job

(having between one and three OpCodes), and all the steps in a jobset

will be executed in parallel. The jobsets themselves are executed

serially.

.TP

.BI "-l" N ", --max-length=" N

Restrict the solution to this length. This can be used for example to

automate the execution of the balancing.

.TP

.BI "--max-cpu " cpu-ratio

The maximum virtual\(hyto\(hyphysical cpu ratio, as a floating point

number between zero and one. For example, specifying \fIcpu-ratio\fR

as \fB2.5\fR means that, for a 4\(hycpu machine, a maximum of 10

virtual cpus should be allowed to be in use for primary instances. A

value of one doesn't make sense though, as that means no disk space

can be used on it.

.TP

.BI "--min-disk " disk-ratio

The minimum amount of free disk space remaining, as a floating point

number. For example, specifying \fIdisk-ratio\fR as \fB0.25\fR means

that at least one quarter of disk space should be left free on nodes.

.TP

.B -v, --verbose

Increase the output verbosity. Each usage of this option will increase

the verbosity (currently more than 2 doesn't make sense) from the

default of one.

.TP

.B -q, --quiet

Decrease the output verbosity. Each usage of this option will decrease

the verbosity (less than zero doesn't make sense) from the default of

one.

.TP

.B -V, --version

Just show the program version and exit.

.SH EXIT STATUS

The exist status of the command will be zero, unless for some reason

the algorithm fatally failed (e.g. wrong node or instance data).

.SH ENVIRONMENT

If the variables \fBHTOOLS_NODES\fR and \fBHTOOLS_INSTANCES\fR are

present in the environment, they will override the default names for

the nodes and instances files. These will have of course no effect

when the RAPI or Luxi backends are used.

.SH BUGS

The program does not check its input data for consistency, and aborts

with cryptic errors messages in this case.

The algorithm is not perfect.

The output format is not easily scriptable, and the program should

feed moves directly into Ganeti (either via RAPI or via a gnt\-debug

input file).

.SH EXAMPLE

Note that this example are not for the latest version (they don't have

full node data).

.SS Default output

With the default options, the program shows each individual step and

the improvements it brings in cluster score:

.in +4n

.nf

.RB "$" " hbal"

Loaded 20 nodes, 80 instances

Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy.

Initial score: 0.52329131

Trying to minimize the CV...

    1. instance14  node1:node10  => node16:node10 0.42109120 a=f r:node16 f

    2. instance54  node4:node15  => node16:node15 0.31904594 a=f r:node16 f

    3. instance4   node5:node2   => node2:node16  0.26611015 a=f r:node16

    4. instance48  node18:node20 => node2:node18  0.21361717 a=r:node2 f

    5. instance93  node19:node18 => node16:node19 0.16166425 a=r:node16 f

    6. instance89  node3:node20  => node2:node3   0.11005629 a=r:node2 f

    7. instance5   node6:node2   => node16:node6  0.05841589 a=r:node16 f

    8. instance94  node7:node20  => node20:node16 0.00658759 a=f r:node16

    9. instance44  node20:node2  => node2:node15  0.00438740 a=f r:node15

   10. instance62  node14:node18 => node14:node16 0.00390087 a=r:node16

   11. instance13  node11:node14 => node11:node16 0.00361787 a=r:node16

   12. instance19  node10:node11 => node10:node7  0.00336636 a=r:node7

   13. instance43  node12:node13 => node12:node1  0.00305681 a=r:node1

   14. instance1   node1:node2   => node1:node4   0.00263124 a=r:node4

   15. instance58  node19:node20 => node19:node17 0.00252594 a=r:node17

Cluster score improved from 0.52329131 to 0.00252594

.fi

.in

In the above output, we can see:

  - the input data (here from files) shows a cluster with 20 nodes and

    80 instances

  - the cluster is not initially N+1 compliant

  - the initial score is 0.52329131

The step list follows, showing the instance, its initial

primary/secondary nodes, the new primary secondary, the cluster list,

and the actions taken in this step (with 'f' denoting failover/migrate

and 'r' denoting replace secondary).

Finally, the program shows the improvement in cluster score.

A more detailed output is obtained via the \fB-C\fR and \fB-p\fR options:

.in +4n

.nf

.RB "$" " hbal"

Loaded 20 nodes, 80 instances

Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy.

Initial cluster status:

N1 Name   t_mem f_mem r_mem t_dsk f_dsk pri sec  p_fmem  p_fdsk

 * node1  32762  1280  6000  1861  1026   5   3 0.03907 0.55179

   node2  32762 31280 12000  1861  1026   0   8 0.95476 0.55179

 * node3  32762  1280  6000  1861  1026   5   3 0.03907 0.55179

 * node4  32762  1280  6000  1861  1026   5   3 0.03907 0.55179

 * node5  32762  1280  6000  1861   978   5   5 0.03907 0.52573

 * node6  32762  1280  6000  1861  1026   5   3 0.03907 0.55179

 * node7  32762  1280  6000  1861  1026   5   3 0.03907 0.55179

   node8  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node9  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

 * node10 32762  7280 12000  1861  1026   4   4 0.22221 0.55179

   node11 32762  7280  6000  1861   922   4   5 0.22221 0.49577

   node12 32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node13 32762  7280  6000  1861   922   4   5 0.22221 0.49577

   node14 32762  7280  6000  1861   922   4   5 0.22221 0.49577

 * node15 32762  7280 12000  1861  1131   4   3 0.22221 0.60782

   node16 32762 31280     0  1861  1860   0   0 0.95476 1.00000

   node17 32762  7280  6000  1861  1106   5   3 0.22221 0.59479

 * node18 32762  1280  6000  1396   561   5   3 0.03907 0.40239

 * node19 32762  1280  6000  1861  1026   5   3 0.03907 0.55179

   node20 32762 13280 12000  1861   689   3   9 0.40535 0.37068

Initial score: 0.52329131

Trying to minimize the CV...

    1. instance14  node1:node10  => node16:node10 0.42109120 a=f r:node16 f

    2. instance54  node4:node15  => node16:node15 0.31904594 a=f r:node16 f

    3. instance4   node5:node2   => node2:node16  0.26611015 a=f r:node16

    4. instance48  node18:node20 => node2:node18  0.21361717 a=r:node2 f

    5. instance93  node19:node18 => node16:node19 0.16166425 a=r:node16 f

    6. instance89  node3:node20  => node2:node3   0.11005629 a=r:node2 f

    7. instance5   node6:node2   => node16:node6  0.05841589 a=r:node16 f

    8. instance94  node7:node20  => node20:node16 0.00658759 a=f r:node16

    9. instance44  node20:node2  => node2:node15  0.00438740 a=f r:node15

   10. instance62  node14:node18 => node14:node16 0.00390087 a=r:node16

   11. instance13  node11:node14 => node11:node16 0.00361787 a=r:node16

   12. instance19  node10:node11 => node10:node7  0.00336636 a=r:node7

   13. instance43  node12:node13 => node12:node1  0.00305681 a=r:node1

   14. instance1   node1:node2   => node1:node4   0.00263124 a=r:node4

   15. instance58  node19:node20 => node19:node17 0.00252594 a=r:node17

Cluster score improved from 0.52329131 to 0.00252594

Commands to run to reach the above solution:

  echo step 1

  echo gnt\-instance migrate instance14

  echo gnt\-instance replace\-disks \-n node16 instance14

  echo gnt\-instance migrate instance14

  echo step 2

  echo gnt\-instance migrate instance54

  echo gnt\-instance replace\-disks \-n node16 instance54

  echo gnt\-instance migrate instance54

  echo step 3

  echo gnt\-instance migrate instance4

  echo gnt\-instance replace\-disks \-n node16 instance4

  echo step 4

  echo gnt\-instance replace\-disks \-n node2 instance48

  echo gnt\-instance migrate instance48

  echo step 5

  echo gnt\-instance replace\-disks \-n node16 instance93

  echo gnt\-instance migrate instance93

  echo step 6

  echo gnt\-instance replace\-disks \-n node2 instance89

  echo gnt\-instance migrate instance89

  echo step 7

  echo gnt\-instance replace\-disks \-n node16 instance5

  echo gnt\-instance migrate instance5

  echo step 8

  echo gnt\-instance migrate instance94

  echo gnt\-instance replace\-disks \-n node16 instance94

  echo step 9

  echo gnt\-instance migrate instance44

  echo gnt\-instance replace\-disks \-n node15 instance44

  echo step 10

  echo gnt\-instance replace\-disks \-n node16 instance62

  echo step 11

  echo gnt\-instance replace\-disks \-n node16 instance13

  echo step 12

  echo gnt\-instance replace\-disks \-n node7 instance19

  echo step 13

  echo gnt\-instance replace\-disks \-n node1 instance43

  echo step 14

  echo gnt\-instance replace\-disks \-n node4 instance1

  echo step 15

  echo gnt\-instance replace\-disks \-n node17 instance58

Final cluster status:

N1 Name   t_mem f_mem r_mem t_dsk f_dsk pri sec  p_fmem  p_fdsk

   node1  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node2  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node3  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node4  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node5  32762  7280  6000  1861  1078   4   5 0.22221 0.57947

   node6  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node7  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node8  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node9  32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node10 32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node11 32762  7280  6000  1861  1022   4   4 0.22221 0.54951

   node12 32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node13 32762  7280  6000  1861  1022   4   4 0.22221 0.54951

   node14 32762  7280  6000  1861  1022   4   4 0.22221 0.54951

   node15 32762  7280  6000  1861  1031   4   4 0.22221 0.55408

   node16 32762  7280  6000  1861  1060   4   4 0.22221 0.57007

   node17 32762  7280  6000  1861  1006   5   4 0.22221 0.54105

   node18 32762  7280  6000  1396   761   4   2 0.22221 0.54570

   node19 32762  7280  6000  1861  1026   4   4 0.22221 0.55179

   node20 32762 13280  6000  1861  1089   3   5 0.40535 0.58565

.fi

.in

Here we see, beside the step list, the initial and final cluster

status, with the final one showing all nodes being N+1 compliant, and

the command list to reach the final solution. In the initial listing,

we see which nodes are not N+1 compliant.

The algorithm is stable as long as each step above is fully completed,

e.g. in step 8, both the migrate and the replace\-disks are

done. Otherwise, if only the migrate is done, the input data is

changed in a way that the program will output a different solution

list (but hopefully will end in the same state).

.SH SEE ALSO

.BR hspace "(1), " hscan "(1), " hail "(1), "

.BR ganeti "(7), " gnt-instance "(8), " gnt-node "(8)"

.SH "COPYRIGHT"

.PP

Copyright (C) 2009 Google Inc. Permission is granted to copy,

distribute and/or modify under the terms of the GNU General Public

License as published by the Free Software Foundation; either version 2

of the License, or (at your option) any later version.

.PP

On Debian systems, the complete text of the GNU General Public License

can be found in /usr/share/common-licenses/GPL.