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Add a manpage for hbal

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.TH HBAL 2 2009-03-13 htools "Ganeti H-tools"
.SH NAME
hbal \- Cluster balancer for Ganeti
.SH SYNOPSIS
.B hbal
.B "[-C]"
.B "[-p]"
.B "[-o]"
.B "-l"
.BI "[ -m " cluster "]"
.BI "[-n " nodes-file " ]"
.BI "[ -i " instances-file "]"
.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 indepentent 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:
- failover (f)
- replace secondary (r)
- replace primary, a composite move (f, r, f)
- failover and replace secondary, also composite (f, r)
- replace secondary and failover, also composite (r, f)
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 minimize the cluster score at
each step. Currently this score is computed as a sum of the following
components:
- coefficient of variance of the percent of free memory
- coefficient of variance of the percent of reserved memory
- coefficient of variance of the percent of free disk
- percentage of nodes failing N+1 check
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, 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 percentange 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.
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 OTHER POSSIBLE METRICS
It would be desirable to add more metrics to the algorithm, especially
dynamically-computed metrics, such as:
- CPU usage of instances, combined with VCPU versus PCPU count
- Disk IO usage
- Network IO
.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:
- a character denoting the N+1 status of the node, with blank
meaning pass and an asterisk ('*') meaning fail
- the node name
- the total node memory
- the free node memory
- the reserved node memory, which is the amount of free memory
needed for N+1 compliancy
- total disk
- free disk
- number of primary instances
- number of secondary instances
- percent of free memory
- percent of free disk
.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:
- initial cluster score
- number of steps in the solution
- final cluster score
- improvement in the cluster score
.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 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
.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
ganeti(7), gnt-instance(8), gnt-node(8)
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