hbal.1

.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]"
.B "[-v... | -q]"
.BI "[-l" limit "]"
.BI "[-O" name... "]"
.BI "[-e" score "]"
.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
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.
.RE

.TP
.BI "-e" score ", --min-score=" score
This parameter denotes the minimum score we are happy with and alters
the computation in two ways:
.RS
.RS 4
.TP 3
\(em
if the cluster has the initial score lower than this value, then we
don't enter the algorithm at all, and exit with success
.TP
\(em
during the iterative process, if we reach a score lower than this
value, we exit the algorithm
.RE
The default value of the parameter is currently \fI1e-9\fR (chosen
empirically).
.RE

.TP
.BI "-n" nodefile ", --nodes=" nodefile
The name of the file holding node information (if not collecting via
RAPI), instead of the default \fInodes\fR file (but see below how to
customize the default value via the environment).

.TP
.BI "-i" instancefile ", --instances=" instancefile
The name of the file holding instance information (if not collecting
via RAPI), instead of the default \fIinstances\fR file (but see below
how to customize the default value via the environment).

.TP
.BI "-m" cluster
Collect data not from files but directly from the
.I 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 unchanged.

.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 one.

.TP
.B -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.

.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 ENVIRONMENT

If the variables \fBHTOOLS_NODES\fR and \fBHTOOLS_INSTANCES\fR 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 RAPI is used.

.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)"