Provided by: ganeti-htools_2.9.3-1_amd64 
NAME
hbal - Cluster balancer for Ganeti
SYNOPSIS
hbal {backend options...} [algorithm options...] [reporting options...]
hbal --version
Backend options:
{ -m cluster | -L[ path ] [-X] | -t data-file | -I path }
Algorithm options:
[ --max-cpu cpu-ratio ] [ --min-disk disk-ratio ] [ -l limit ] [ -e score ] [ -g delta ] [ --min-gain-
limit threshold ] [ -O name... ] [ --no-disk-moves ] [ --no-instance-moves ] [ -U util-file ] [ --evac-
mode ] [ --select-instances inst... ] [ --exclude-instances inst... ]
Reporting options:
[ -C[ file ] ] [ -p[ fields ] ] [ --print-instances ] [ -S file ] [ -v... | -q ]
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: 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 -C option.
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:
• 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.
PLACEMENT RESTRICTIONS
At each step, we prevent an instance move if it would cause:
• a node to go into N+1 failure state
• an instance to move onto an offline node (offline nodes are either read from the cluster or declared
with -O; drained nodes are considered offline)
• an exclusion-tag based conflict (exclusion tags are read from the cluster and/or defined via the
--exclusion-tags option)
• a max vcpu/pcpu ratio to be exceeded (configured via --max-cpu)
• min disk free percentage to go below the configured limit (configured via --min-disk)
CLUSTER SCORING
As said before, the algorithm tries to minimise the cluster score at each step. Currently this score is
computed as a weighted sum of the following components:
• standard deviation of the percent of free memory
• standard deviation of the percent of reserved memory
• standard deviation of the percent of free disk
• count of nodes failing N+1 check
• count of instances living (either as primary or secondary) on offline nodes; in the sense of hbal (and
the other htools) drained nodes are considered offline
• 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
• standard deviation of the ratio of virtual-to-physical cpus (for primary instances of the node)
• standard deviation of the dynamic load on the nodes, for cpus, memory, disk and network
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 instances over a day and by
computing the delta of the cpu values, and feed that via the -U 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
:)
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.
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 --exclusion-tags) or via adding
them to the cluster tags:
--exclusion-tags=a,b
This will make all instance tags of the form a:*, b:* be considered for the exclusion map
cluster tags htools:iextags:a, htools:iextags:b
This will make instance tags a:*, b:* be considered for the exclusion map. More precisely, the
suffix of cluster tags starting with htools:iextags: will become the prefix of the exclusion tags.
Both the above forms mean that two instances both having (e.g.)
the tag a:foo or b:bar won't end on the same node.
OPTIONS
The options that can be passed to the program are as follows:
-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 -L 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.
-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. See the man page htools(1) for more details about this option.
--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.
-O name
This option (which can be given multiple times) will mark nodes as being offline. This means a
couple of things:
• instances won't be placed on these nodes, not even temporarily; e.g. the replace primary move
is not available if the secondary node is offline, since this move requires a failover.
• these nodes will not be included in the score calculation (except for the percentage of
instances on offline nodes)
Note that algorithm will also mark as offline any nodes which are reported by RAPI as such, or
that have "?" in file-based input in any numeric fields.
-e score, --min-score=score
This parameter denotes the minimum score we are happy with and alters the computation in two ways:
• if the cluster has the initial score lower than this value, then we don't enter the algorithm at
all, and exit with success
• during the iterative process, if we reach a score lower than this value, we exit the algorithm
The default value of the parameter is currently 1e-9 (chosen empirically).
-g delta, --min-gain=delta
Since the balancing algorithm can sometimes result in just very tiny improvements, that bring less
gain that they cost in relocation time, this parameter (defaulting to 0.01) represents the minimum
gain we require during a step, to continue balancing.
--min-gain-limit=threshold
The above min-gain option will only take effect if the cluster score is already below threshold
(defaults to 0.1). The rationale behind this setting is that at high cluster scores (badly
balanced clusters), we don't want to abort the rebalance too quickly, as later gains might still
be significant. However, under the threshold, the total gain is only the threshold value, so we
can exit early.
--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.
--no-instance-moves
This parameter prevents hbal from using instance moves (i.e. "gnt-instance migrate/failover")
operations. This will only use the slow disk-replacement operations, and will also provide a
worse balance, but can be useful if moving instances around is deemed unsafe or not preferred.
--evac-mode
This parameter restricts the list of instances considered for moving to the ones living on
offline/drained nodes. It can be used as a (bulk) replacement for Ganeti's own gnt-node evacuate,
with the note that it doesn't guarantee full evacuation.
--select-instances=instances
This parameter marks the given instances (as a comma-separated list) as the only ones being moved
during the rebalance.
--exclude-instances=instances
This parameter marks the given instances (as a comma-separated list) from being moved during the
rebalance.
-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.
-S filename, --save-cluster=filename
If given, the state of the cluster before the balancing is saved to the given file plus the
extension "original" (i.e. filename.original), and the state at the end of the balancing is saved
to the given file plus the extension "balanced" (i.e. filename.balanced). This allows re-feeding
the cluster state to either hbal itself or for example hspace via the -t option.
-t datafile, --text-data=datafile
Backend specification: 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. The option is
described in the man page htools(1).
-m cluster
Backend specification: collect data directly from the cluster given as an argument via RAPI. The
option is described in the man page htools(1).
-L [path]
Backend specification: collect data directly from the master daemon, which is to be contacted via
LUXI (an internal Ganeti protocol). The option is described in the man page htools(1).
-X When using the Luxi backend, hbal can also execute the given commands. The execution method is to
execute the individual jobsets (see the -C 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.
The execution of the job series can be interrupted, see below for signal handling.
-l N, --max-length=N
Restrict the solution to this length. This can be used for example to automate the execution of
the balancing.
--max-cpu=cpu-ratio
The maximum virtual to physical cpu ratio, as a floating point number greater than or equal to
one. For example, specifying cpu-ratio as 2.5 means that, for a 4-cpu machine, a maximum of 10
virtual cpus should be allowed to be in use for primary instances. A value of exactly one means
there will be no over-subscription of CPU (except for the CPU time used by the node itself), and
values below one do not make sense, as that means other resources (e.g. disk) won't be fully
utilised due to CPU restrictions.
--min-disk=disk-ratio
The minimum amount of free disk space remaining, as a floating point number. For example,
specifying disk-ratio as 0.25 means that at least one quarter of disk space should be left free on
nodes.
-G uuid, --group=uuid
On an multi-group cluster, select this group for processing. Otherwise hbal will abort, since it
cannot balance multiple groups at the same time.
-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.
-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.
-V, --version
Just show the program version and exit.
SIGNAL HANDLING
When executing jobs via LUXI (using the -X option), normally hbal will execute all jobs until either one
errors out or all the jobs finish successfully.
Since balancing can take a long time, it is possible to stop hbal early in two ways:
• by sending a SIGINT (^C), hbal will register the termination request, and will wait until the currently
submitted jobs finish, at which point it will exit (with exit code 0 if all jobs finished correctly,
otherwise with exit code 1 as usual)
• by sending a SIGTERM, hbal will immediately exit (with exit code 2); it is the responsibility of the
user to follow up with Ganeti and check the result of the currently-executing jobs
Note that in any situation, it's perfectly safe to kill hbal, either via the above signals or via any
other signal (e.g. SIGQUIT, SIGKILL), since the jobs themselves are processed by Ganeti whereas hbal
(after submission) only watches their progression. In this case, the user will have to query Ganeti for
job results.
EXIT STATUS
The exit status of the command will be zero, unless for some reason the algorithm failed (e.g. wrong
node or instance data), invalid command line options, or (in case of job execution) one of the jobs has
failed.
Once job execution via Luxi has started (-X), if the balancing was interrupted early (via SIGINT, or via
--max-length) but all jobs executed successfully, then the exit status is zero; a non-zero exit code
means that the cluster state should be investigated, since a job failed or we couldn't compute its status
and this can also point to a problem on the Ganeti side.
BUGS
The program does not check all its input data for consistency, and sometime aborts with cryptic errors
messages with invalid data.
The algorithm is not perfect.
EXAMPLE
Note that these examples are not for the latest version (they don't have full node data).
Default output
With the default options, the program shows each individual step and the improvements it brings in
cluster score:
$ 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
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 -C and -p options:
$ 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
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).
REPORTING BUGS
Report bugs to project website (http://code.google.com/p/ganeti/) or contact the developers using the
Ganeti mailing list (ganeti@googlegroups.com).
SEE ALSO
Ganeti overview and specifications: ganeti(7) (general overview), ganeti-os-interface(7) (guest OS
definitions), ganeti-extstorage-interface(7) (external storage providers).
Ganeti commands: gnt-cluster(8) (cluster-wide commands), gnt-job(8) (job-related commands), gnt-node(8)
(node-related commands), gnt-instance(8) (instance commands), gnt-os(8) (guest OS commands), gnt-
storage(8) (storage commands), gnt-group(8) (node group commands), gnt-backup(8) (instance import/export
commands), gnt-debug(8) (debug commands).
Ganeti daemons: ganeti-watcher(8) (automatic instance restarter), ganeti-cleaner(8) (job queue cleaner),
ganeti-noded(8) (node daemon), ganeti-masterd(8) (master daemon), ganeti-rapi(8) (remote API daemon).
Ganeti htools: htools(1) (generic binary), hbal(1) (cluster balancer), hspace(1) (capacity calculation),
hail(1) (IAllocator plugin), hscan(1) (data gatherer from remote clusters), hinfo(1) (cluster information
printer), mon-collector(7) (data collectors interface).
COPYRIGHT
Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 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.
On Debian systems, the complete text of the GNU General Public License can be found in /usr/share/common-
licenses/GPL.
Ganeti HBAL(1)