1 HBAL(1) Ganeti | Version @GANETI_VERSION@
2 =========================================
7 hbal \- Cluster balancer for Ganeti
12 **hbal** {backend options...} [algorithm options...] [reporting options...]
19 { **-m** *cluster* | **-L[** *path* **] [-X]** | **-t** *data-file* }
23 **[ --max-cpu *cpu-ratio* ]**
24 **[ --min-disk *disk-ratio* ]**
27 **[ -g *delta* ]** **[ --min-gain-limit *threshold* ]**
29 **[ --no-disk-moves ]**
30 **[ --no-instance-moves ]**
31 **[ -U *util-file* ]**
33 **[ --select-instances *inst...* ]**
34 **[ --exclude-instances *inst...* ]**
39 **[ -p[ *fields* ] ]**
40 **[ --print-instances ]**
48 hbal is a cluster balancer that looks at the current state of the
49 cluster (nodes with their total and free disk, memory, etc.) and
50 instance placement and computes a series of steps designed to bring
51 the cluster into a better state.
53 The algorithm used is designed to be stable (i.e. it will give you the
54 same results when restarting it from the middle of the solution) and
55 reasonably fast. It is not, however, designed to be a perfect
56 algorithm--it is possible to make it go into a corner from which
57 it can find no improvement, because it looks only one "step" ahead.
59 By default, the program will show the solution incrementally as it is
60 computed, in a somewhat cryptic format; for getting the actual Ganeti
61 command list, use the **-C** option.
66 The program works in independent steps; at each step, we compute the
67 best instance move that lowers the cluster score.
69 The possible move type for an instance are combinations of
70 failover/migrate and replace-disks such that we change one of the
71 instance nodes, and the other one remains (but possibly with changed
72 role, e.g. from primary it becomes secondary). The list is:
75 - replace secondary (r)
76 - replace primary, a composite move (f, r, f)
77 - failover and replace secondary, also composite (f, r)
78 - replace secondary and failover, also composite (r, f)
80 We don't do the only remaining possibility of replacing both nodes
81 (r,f,r,f or the equivalent f,r,f,r) since these move needs an
82 exhaustive search over both candidate primary and secondary nodes, and
83 is O(n*n) in the number of nodes. Furthermore, it doesn't seems to
84 give better scores but will result in more disk replacements.
86 PLACEMENT RESTRICTIONS
87 ~~~~~~~~~~~~~~~~~~~~~~
89 At each step, we prevent an instance move if it would cause:
91 - a node to go into N+1 failure state
92 - an instance to move onto an offline node (offline nodes are either
93 read from the cluster or declared with *-O*)
94 - an exclusion-tag based conflict (exclusion tags are read from the
95 cluster and/or defined via the *--exclusion-tags* option)
96 - a max vcpu/pcpu ratio to be exceeded (configured via *--max-cpu*)
97 - min disk free percentage to go below the configured limit
98 (configured via *--min-disk*)
103 As said before, the algorithm tries to minimise the cluster score at
104 each step. Currently this score is computed as a sum of the following
107 - standard deviation of the percent of free memory
108 - standard deviation of the percent of reserved memory
109 - standard deviation of the percent of free disk
110 - count of nodes failing N+1 check
111 - count of instances living (either as primary or secondary) on
113 - count of instances living (as primary) on offline nodes; this
114 differs from the above metric by helping failover of such instances
116 - standard deviation of the ratio of virtual-to-physical cpus (for
117 primary instances of the node)
118 - standard deviation of the dynamic load on the nodes, for cpus,
119 memory, disk and network
121 The free memory and free disk values help ensure that all nodes are
122 somewhat balanced in their resource usage. The reserved memory helps
123 to ensure that nodes are somewhat balanced in holding secondary
124 instances, and that no node keeps too much memory reserved for
125 N+1. And finally, the N+1 percentage helps guide the algorithm towards
126 eliminating N+1 failures, if possible.
128 Except for the N+1 failures and offline instances counts, we use the
129 standard deviation since when used with values within a fixed range
130 (we use percents expressed as values between zero and one) it gives
131 consistent results across all metrics (there are some small issues
132 related to different means, but it works generally well). The 'count'
133 type values will have higher score and thus will matter more for
134 balancing; thus these are better for hard constraints (like evacuating
135 nodes and fixing N+1 failures). For example, the offline instances
136 count (i.e. the number of instances living on offline nodes) will
137 cause the algorithm to actively move instances away from offline
138 nodes. This, coupled with the restriction on placement given by
139 offline nodes, will cause evacuation of such nodes.
141 The dynamic load values need to be read from an external file (Ganeti
142 doesn't supply them), and are computed for each node as: sum of
143 primary instance cpu load, sum of primary instance memory load, sum of
144 primary and secondary instance disk load (as DRBD generates write load
145 on secondary nodes too in normal case and in degraded scenarios also
146 read load), and sum of primary instance network load. An example of
147 how to generate these values for input to hbal would be to track ``xm
148 list`` for instances over a day and by computing the delta of the cpu
149 values, and feed that via the *-U* option for all instances (and keep
150 the other metrics as one). For the algorithm to work, all that is
151 needed is that the values are consistent for a metric across all
152 instances (e.g. all instances use cpu% to report cpu usage, and not
153 something related to number of CPU seconds used if the CPUs are
154 different), and that they are normalised to between zero and one. Note
155 that it's recommended to not have zero as the load value for any
156 instance metric since then secondary instances are not well balanced.
158 On a perfectly balanced cluster (all nodes the same size, all
159 instances the same size and spread across the nodes equally), the
160 values for all metrics would be zero. This doesn't happen too often in
166 Since current Ganeti versions do not report the memory used by offline
167 (down) instances, ignoring the run status of instances will cause
168 wrong calculations. For this reason, the algorithm subtracts the
169 memory size of down instances from the free node memory of their
170 primary node, in effect simulating the startup of such instances.
175 The exclusion tags mechanism is designed to prevent instances which
176 run the same workload (e.g. two DNS servers) to land on the same node,
177 which would make the respective node a SPOF for the given service.
179 It works by tagging instances with certain tags and then building
180 exclusion maps based on these. Which tags are actually used is
181 configured either via the command line (option *--exclusion-tags*)
182 or via adding them to the cluster tags:
185 This will make all instance tags of the form *a:\**, *b:\** be
186 considered for the exclusion map
188 cluster tags *htools:iextags:a*, *htools:iextags:b*
189 This will make instance tags *a:\**, *b:\** be considered for the
190 exclusion map. More precisely, the suffix of cluster tags starting
191 with *htools:iextags:* will become the prefix of the exclusion tags.
193 Both the above forms mean that two instances both having (e.g.) the
194 tag *a:foo* or *b:bar* won't end on the same node.
199 The options that can be passed to the program are as follows:
202 Print the command list at the end of the run. Without this, the
203 program will only show a shorter, but cryptic output.
205 Note that the moves list will be split into independent steps,
206 called "jobsets", but only for visual inspection, not for actually
207 parallelisation. It is not possible to parallelise these directly
208 when executed via "gnt-instance" commands, since a compound command
209 (e.g. failover and replace-disks) must be executed
210 serially. Parallel execution is only possible when using the Luxi
211 backend and the *-L* option.
213 The algorithm for splitting the moves into jobsets is by
214 accumulating moves until the next move is touching nodes already
215 touched by the current moves; this means we can't execute in
216 parallel (due to resource allocation in Ganeti) and thus we start a
220 Prints the before and after node status, in a format designed to allow
221 the user to understand the node's most important parameters. See the
222 man page **htools**(1) for more details about this option.
225 Prints the before and after instance map. This is less useful as the
226 node status, but it can help in understanding instance moves.
229 Only shows a one-line output from the program, designed for the case
230 when one wants to look at multiple clusters at once and check their
233 The line will contain four fields:
235 - initial cluster score
236 - number of steps in the solution
237 - final cluster score
238 - improvement in the cluster score
241 This option (which can be given multiple times) will mark nodes as
242 being *offline*. This means a couple of things:
244 - instances won't be placed on these nodes, not even temporarily;
245 e.g. the *replace primary* move is not available if the secondary
246 node is offline, since this move requires a failover.
247 - these nodes will not be included in the score calculation (except
248 for the percentage of instances on offline nodes)
250 Note that algorithm will also mark as offline any nodes which are
251 reported by RAPI as such, or that have "?" in file-based input in
254 -e *score*, --min-score=*score*
255 This parameter denotes the minimum score we are happy with and alters
256 the computation in two ways:
258 - if the cluster has the initial score lower than this value, then we
259 don't enter the algorithm at all, and exit with success
260 - during the iterative process, if we reach a score lower than this
261 value, we exit the algorithm
263 The default value of the parameter is currently ``1e-9`` (chosen
266 -g *delta*, --min-gain=*delta*
267 Since the balancing algorithm can sometimes result in just very tiny
268 improvements, that bring less gain that they cost in relocation
269 time, this parameter (defaulting to 0.01) represents the minimum
270 gain we require during a step, to continue balancing.
272 --min-gain-limit=*threshold*
273 The above min-gain option will only take effect if the cluster score
274 is already below *threshold* (defaults to 0.1). The rationale behind
275 this setting is that at high cluster scores (badly balanced
276 clusters), we don't want to abort the rebalance too quickly, as
277 later gains might still be significant. However, under the
278 threshold, the total gain is only the threshold value, so we can
282 This parameter prevents hbal from using disk move
283 (i.e. "gnt-instance replace-disks") operations. This will result in
284 a much quicker balancing, but of course the improvements are
285 limited. It is up to the user to decide when to use one or another.
288 This parameter prevents hbal from using instance moves
289 (i.e. "gnt-instance migrate/failover") operations. This will only use
290 the slow disk-replacement operations, and will also provide a worse
291 balance, but can be useful if moving instances around is deemed unsafe
295 This parameter restricts the list of instances considered for moving
296 to the ones living on offline/drained nodes. It can be used as a
297 (bulk) replacement for Ganeti's own *gnt-node evacuate*, with the
298 note that it doesn't guarantee full evacuation.
300 --select-instances=*instances*
301 This parameter marks the given instances (as a comma-separated list)
302 as the only ones being moved during the rebalance.
304 --exclude-instances=*instances*
305 This parameter marks the given instances (as a comma-separated list)
306 from being moved during the rebalance.
309 This parameter specifies a file holding instance dynamic utilisation
310 information that will be used to tweak the balancing algorithm to
311 equalise load on the nodes (as opposed to static resource
312 usage). The file is in the format "instance_name cpu_util mem_util
313 disk_util net_util" where the "_util" parameters are interpreted as
314 numbers and the instance name must match exactly the instance as
315 read from Ganeti. In case of unknown instance names, the program
318 If not given, the default values are one for all metrics and thus
319 dynamic utilisation has only one effect on the algorithm: the
320 equalisation of the secondary instances across nodes (this is the
321 only metric that is not tracked by another, dedicated value, and
322 thus the disk load of instances will cause secondary instance
323 equalisation). Note that value of one will also influence slightly
324 the primary instance count, but that is already tracked via other
325 metrics and thus the influence of the dynamic utilisation will be
326 practically insignificant.
328 -t *datafile*, --text-data=*datafile*
329 The name of the file holding node and instance information (if not
330 collecting via RAPI or LUXI). This or one of the other backends must
333 -S *filename*, --save-cluster=*filename*
334 If given, the state of the cluster before the balancing is saved to
335 the given file plus the extension "original"
336 (i.e. *filename*.original), and the state at the end of the
337 balancing is saved to the given file plus the extension "balanced"
338 (i.e. *filename*.balanced). This allows re-feeding the cluster state
339 to either hbal itself or for example hspace.
342 Collect data directly from the *cluster* given as an argument via
343 RAPI. If the argument doesn't contain a colon (:), then it is
344 converted into a fully-built URL via prepending ``https://`` and
345 appending the default RAPI port, otherwise it's considered a
346 fully-specified URL and is used as-is.
349 Collect data directly from the master daemon, which is to be
350 contacted via the luxi (an internal Ganeti protocol). An optional
351 *path* argument is interpreted as the path to the unix socket on
352 which the master daemon listens; otherwise, the default path used by
353 ganeti when installed with *--localstatedir=/var* is used.
356 When using the Luxi backend, hbal can also execute the given
357 commands. The execution method is to execute the individual jobsets
358 (see the *-C* option for details) in separate stages, aborting if at
359 any time a jobset doesn't have all jobs successful. Each step in the
360 balancing solution will be translated into exactly one Ganeti job
361 (having between one and three OpCodes), and all the steps in a
362 jobset will be executed in parallel. The jobsets themselves are
365 -l *N*, --max-length=*N*
366 Restrict the solution to this length. This can be used for example
367 to automate the execution of the balancing.
369 --max-cpu=*cpu-ratio*
370 The maximum virtual to physical cpu ratio, as a floating point number
371 greater than or equal to one. For example, specifying *cpu-ratio* as
372 **2.5** means that, for a 4-cpu machine, a maximum of 10 virtual cpus
373 should be allowed to be in use for primary instances. A value of
374 exactly one means there will be no over-subscription of CPU (except
375 for the CPU time used by the node itself), and values below one do not
376 make sense, as that means other resources (e.g. disk) won't be fully
377 utilised due to CPU restrictions.
379 --min-disk=*disk-ratio*
380 The minimum amount of free disk space remaining, as a floating point
381 number. For example, specifying *disk-ratio* as **0.25** means that
382 at least one quarter of disk space should be left free on nodes.
384 -G *uuid*, --group=*uuid*
385 On an multi-group cluster, select this group for
386 processing. Otherwise hbal will abort, since it cannot balance
387 multiple groups at the same time.
390 Increase the output verbosity. Each usage of this option will
391 increase the verbosity (currently more than 2 doesn't make sense)
392 from the default of one.
395 Decrease the output verbosity. Each usage of this option will
396 decrease the verbosity (less than zero doesn't make sense) from the
400 Just show the program version and exit.
405 The exit status of the command will be zero, unless for some reason
406 the algorithm fatally failed (e.g. wrong node or instance data), or
407 (in case of job execution) any job has failed.
412 The program does not check its input data for consistency, and aborts
413 with cryptic errors messages in this case.
415 The algorithm is not perfect.
417 The output format is not easily scriptable, and the program should
418 feed moves directly into Ganeti (either via RAPI or via a gnt-debug
424 Note that these examples are not for the latest version (they don't
425 have full node data).
430 With the default options, the program shows each individual step and
431 the improvements it brings in cluster score::
434 Loaded 20 nodes, 80 instances
435 Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy.
436 Initial score: 0.52329131
437 Trying to minimize the CV...
438 1. instance14 node1:node10 => node16:node10 0.42109120 a=f r:node16 f
439 2. instance54 node4:node15 => node16:node15 0.31904594 a=f r:node16 f
440 3. instance4 node5:node2 => node2:node16 0.26611015 a=f r:node16
441 4. instance48 node18:node20 => node2:node18 0.21361717 a=r:node2 f
442 5. instance93 node19:node18 => node16:node19 0.16166425 a=r:node16 f
443 6. instance89 node3:node20 => node2:node3 0.11005629 a=r:node2 f
444 7. instance5 node6:node2 => node16:node6 0.05841589 a=r:node16 f
445 8. instance94 node7:node20 => node20:node16 0.00658759 a=f r:node16
446 9. instance44 node20:node2 => node2:node15 0.00438740 a=f r:node15
447 10. instance62 node14:node18 => node14:node16 0.00390087 a=r:node16
448 11. instance13 node11:node14 => node11:node16 0.00361787 a=r:node16
449 12. instance19 node10:node11 => node10:node7 0.00336636 a=r:node7
450 13. instance43 node12:node13 => node12:node1 0.00305681 a=r:node1
451 14. instance1 node1:node2 => node1:node4 0.00263124 a=r:node4
452 15. instance58 node19:node20 => node19:node17 0.00252594 a=r:node17
453 Cluster score improved from 0.52329131 to 0.00252594
455 In the above output, we can see:
457 - the input data (here from files) shows a cluster with 20 nodes and
459 - the cluster is not initially N+1 compliant
460 - the initial score is 0.52329131
462 The step list follows, showing the instance, its initial
463 primary/secondary nodes, the new primary secondary, the cluster list,
464 and the actions taken in this step (with 'f' denoting failover/migrate
465 and 'r' denoting replace secondary).
467 Finally, the program shows the improvement in cluster score.
469 A more detailed output is obtained via the *-C* and *-p* options::
472 Loaded 20 nodes, 80 instances
473 Cluster is not N+1 happy, continuing but no guarantee that the cluster will end N+1 happy.
474 Initial cluster status:
475 N1 Name t_mem f_mem r_mem t_dsk f_dsk pri sec p_fmem p_fdsk
476 * node1 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
477 node2 32762 31280 12000 1861 1026 0 8 0.95476 0.55179
478 * node3 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
479 * node4 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
480 * node5 32762 1280 6000 1861 978 5 5 0.03907 0.52573
481 * node6 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
482 * node7 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
483 node8 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
484 node9 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
485 * node10 32762 7280 12000 1861 1026 4 4 0.22221 0.55179
486 node11 32762 7280 6000 1861 922 4 5 0.22221 0.49577
487 node12 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
488 node13 32762 7280 6000 1861 922 4 5 0.22221 0.49577
489 node14 32762 7280 6000 1861 922 4 5 0.22221 0.49577
490 * node15 32762 7280 12000 1861 1131 4 3 0.22221 0.60782
491 node16 32762 31280 0 1861 1860 0 0 0.95476 1.00000
492 node17 32762 7280 6000 1861 1106 5 3 0.22221 0.59479
493 * node18 32762 1280 6000 1396 561 5 3 0.03907 0.40239
494 * node19 32762 1280 6000 1861 1026 5 3 0.03907 0.55179
495 node20 32762 13280 12000 1861 689 3 9 0.40535 0.37068
497 Initial score: 0.52329131
498 Trying to minimize the CV...
499 1. instance14 node1:node10 => node16:node10 0.42109120 a=f r:node16 f
500 2. instance54 node4:node15 => node16:node15 0.31904594 a=f r:node16 f
501 3. instance4 node5:node2 => node2:node16 0.26611015 a=f r:node16
502 4. instance48 node18:node20 => node2:node18 0.21361717 a=r:node2 f
503 5. instance93 node19:node18 => node16:node19 0.16166425 a=r:node16 f
504 6. instance89 node3:node20 => node2:node3 0.11005629 a=r:node2 f
505 7. instance5 node6:node2 => node16:node6 0.05841589 a=r:node16 f
506 8. instance94 node7:node20 => node20:node16 0.00658759 a=f r:node16
507 9. instance44 node20:node2 => node2:node15 0.00438740 a=f r:node15
508 10. instance62 node14:node18 => node14:node16 0.00390087 a=r:node16
509 11. instance13 node11:node14 => node11:node16 0.00361787 a=r:node16
510 12. instance19 node10:node11 => node10:node7 0.00336636 a=r:node7
511 13. instance43 node12:node13 => node12:node1 0.00305681 a=r:node1
512 14. instance1 node1:node2 => node1:node4 0.00263124 a=r:node4
513 15. instance58 node19:node20 => node19:node17 0.00252594 a=r:node17
514 Cluster score improved from 0.52329131 to 0.00252594
516 Commands to run to reach the above solution:
518 echo gnt-instance migrate instance14
519 echo gnt-instance replace-disks -n node16 instance14
520 echo gnt-instance migrate instance14
522 echo gnt-instance migrate instance54
523 echo gnt-instance replace-disks -n node16 instance54
524 echo gnt-instance migrate instance54
526 echo gnt-instance migrate instance4
527 echo gnt-instance replace-disks -n node16 instance4
529 echo gnt-instance replace-disks -n node2 instance48
530 echo gnt-instance migrate instance48
532 echo gnt-instance replace-disks -n node16 instance93
533 echo gnt-instance migrate instance93
535 echo gnt-instance replace-disks -n node2 instance89
536 echo gnt-instance migrate instance89
538 echo gnt-instance replace-disks -n node16 instance5
539 echo gnt-instance migrate instance5
541 echo gnt-instance migrate instance94
542 echo gnt-instance replace-disks -n node16 instance94
544 echo gnt-instance migrate instance44
545 echo gnt-instance replace-disks -n node15 instance44
547 echo gnt-instance replace-disks -n node16 instance62
549 echo gnt-instance replace-disks -n node16 instance13
551 echo gnt-instance replace-disks -n node7 instance19
553 echo gnt-instance replace-disks -n node1 instance43
555 echo gnt-instance replace-disks -n node4 instance1
557 echo gnt-instance replace-disks -n node17 instance58
559 Final cluster status:
560 N1 Name t_mem f_mem r_mem t_dsk f_dsk pri sec p_fmem p_fdsk
561 node1 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
562 node2 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
563 node3 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
564 node4 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
565 node5 32762 7280 6000 1861 1078 4 5 0.22221 0.57947
566 node6 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
567 node7 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
568 node8 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
569 node9 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
570 node10 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
571 node11 32762 7280 6000 1861 1022 4 4 0.22221 0.54951
572 node12 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
573 node13 32762 7280 6000 1861 1022 4 4 0.22221 0.54951
574 node14 32762 7280 6000 1861 1022 4 4 0.22221 0.54951
575 node15 32762 7280 6000 1861 1031 4 4 0.22221 0.55408
576 node16 32762 7280 6000 1861 1060 4 4 0.22221 0.57007
577 node17 32762 7280 6000 1861 1006 5 4 0.22221 0.54105
578 node18 32762 7280 6000 1396 761 4 2 0.22221 0.54570
579 node19 32762 7280 6000 1861 1026 4 4 0.22221 0.55179
580 node20 32762 13280 6000 1861 1089 3 5 0.40535 0.58565
582 Here we see, beside the step list, the initial and final cluster
583 status, with the final one showing all nodes being N+1 compliant, and
584 the command list to reach the final solution. In the initial listing,
585 we see which nodes are not N+1 compliant.
587 The algorithm is stable as long as each step above is fully completed,
588 e.g. in step 8, both the migrate and the replace-disks are
589 done. Otherwise, if only the migrate is done, the input data is
590 changed in a way that the program will output a different solution
591 list (but hopefully will end in the same state).
593 .. vim: set textwidth=72 :