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 "[-C]"

.B "[-p]"

.B "[-o]"

.BI "[-l" limit "]"

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

.BI "[-m " cluster "]"

.BI "[-n " nodes-file " ]"

.BI "[-i " instances-file "]"

.B hbal

.B --version

.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 to do so 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 - it is possible to make it go into a corner from which it

can find no improvement, because it only look 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 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

coefficient of variance of the percent of free memory

.TP

\(em

coefficient of variance of the percent of reserved memory

.TP

\(em

coefficient of variance of the percent of free disk

.TP

\(em

percentage of nodes failing N+1 check

.TP

\(em

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

offline nodes

.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 percentage, we use

the coefficient of variance since this brings the values into the same

unit so to speak, and with a restrict domain of values (between zero

and one). The percentage of N+1 failures, while also in this numeric

range, doesn't actually has the same meaning, but it has shown to work

well.

The other alternative, using for N+1 checks the coefficient of

variance of (N+1 fail=1, N+1 pass=0) across nodes could hint the

algorithm to make more N+1 failures if most nodes are N+1 fail

already. Since this (making N+1 failures) is not allowed by other

rules of the algorithm, so the N+1 checks would simply not work

anymore in this case.

The offline instances percentage (meaning the percentage 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.

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

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

values 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 OTHER POSSIBLE METRICS

It would be desirable to add more metrics to the algorithm, especially

dynamically-computed metrics, such as:

.RS 4

.TP 3

\(em

CPU usage of instances, combined with VCPU versus PCPU count

.TP

\(em

Disk IO usage

.TP

\(em

Network IO

.RE

.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.

.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.

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 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

.RE

.TP

.B -o, --oneline

Only shows a one-line 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

.RE

.TP

.BI "-n" nodefile ", --nodes=" nodefile

The name of the file holding node information (if not collecting via

RAPI), instead of the default

.I nodes

file.

.TP

.BI "-i" instancefile ", --instances=" instancefile

The name of the file holding instance information (if not collecting

via RAPI), instead of the default

.I instances

file.

.TP

.BI "-m" cluster

Collect data not from files but directly from the

.I cluster

given as an argument via RAPI. This work for both Ganeti 1.2 and

Ganeti 2.0.

.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

.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 zero.

.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 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 algorithm doesn't deal with non-\fBdrbd\fR instances, and chokes

on input data which has such instances.

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 hn1 "(1), " hscan "(1), " ganeti "(7), " gnt-instance "(8), "

.BR gnt-node "(8)"