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 ]
Algorithm options:
[ --max-cpu cpu-ratio ] [ --min-disk disk-ratio ] [ -l limit ] [
-e score ] [ -O name... ] [ --no-disk-moves ] [ -U util-file ] [
--evac-mode ] [ --exclude-instances inst... ]
Reporting options:
[ -C[file] ] [ -p[fields] ] [ --print-instances ] [ -o ] [ -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)
— 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 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
— 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
instance 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.
It is possible to customise the listed information by passing a
comma‐separated list of field names to this option (the field
list is currently undocumented). By default, the node list will
contain these informations:
F a character denoting the status of the node, with ’-’
meaning an offline node, ’*’ meaning N+1 failure and
blank meaning a good node
Name the node name
t_mem the total node memory
n_mem the memory used by the node itself
i_mem the memory used by instances
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
f_mem the free node memory
r_mem the reserved node memory, which is the amount of free
memory needed for N+1 compliance
t_dsk total disk
f_dsk free disk
pcpu the number of physical cpus on the node
vcpu the number of virtual cpus allocated to primary instances
pri number of primary instances
sec number of secondary instances
p_fmem percent of free memory
p_fdsk percent of free disk
r_cpu ratio of virtual to physical cpus
lCpu the dynamic CPU load (if the information is available)
lMem the dynamic memory load (if the information is available)
lDsk the dynamic disk load (if the information is available)
lNet the dynamic net load (if the information is available)
--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, --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
-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 hbal 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.
-escore, --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).
--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.
--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.
--exclude-instances instances
This parameter marks the given instances (as a comma-separated
list) from being moved during the rebalance. Note that the
instances must be given their full name (as reported by Ganeti).
-Uutil-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.
-tdatafile, --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.
-mcluster
Collect data directly from the cluster given as an argument via
RAPI. If the argument doesn’t contain a colon (:), then it is
converted into a fully‐built URL via prepending https:// and
appending the default RAPI port, otherwise it’s considered a
fully‐specified URL and is used as‐is.
-L[path]
Collect data directly from the master daemon, which is to be
contacted via the luxi (an internal Ganeti protocol). An
optional path 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
--localstatedir=/var is used.
-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.
-lN, --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 between zero and 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 one doesn’t make sense though, as that means no disk
space can be used on it.
--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.
-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.
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).
ENVIRONMENT
If the variables HTOOLS_NODES and HTOOLS_INSTANCES 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.
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).
EXAMPLE
Note that this example 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).
SEE ALSO
hspace(1), hscan(1), hail(1), ganeti(7), gnt-instance(8), gnt-node(8)
COPYRIGHT
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.
On Debian systems, the complete text of the GNU General Public License
can be found in /usr/share/common-licenses/GPL.