Synnefo » snf-ganeti

.TH HN1 1 2009-03-22 htools "Ganeti H-tools"

.SH NAME

hn1 \- N+1 fixer for Ganeti

.SH SYNOPSIS

.B hn1

.B "[-C]"

.B "[-p]"

.B "[-o]"

.BI "[ -m " cluster "]"

.BI "[-n " nodes-file " ]"

.BI "[ -i " instances-file "]"

.BI "[-d " depth "]"

.BI "[-r " max-removals "]"

.BI "[-L " max-delta "]"

.BI "[-l " min-delta "]"

.B hn1

.B --version

.SH DESCRIPTION

hn1 is a cluster N+1 fixer that tries to compute the minimum number of

moves needed for getting all nodes to be N+1 compliant.

The algorithm is designed to be a 'perfect' algorithm, so that we

always examine the entire solution space until we find the minimum

solution. The algorithm can be tweaked via the \fB-d\fR, \fB-r\fR,

\fB-L\fR and \fB-l\fR options.

By default, the program will show the solution in a somewhat cryptic

format; for getting the actual Ganeti command list, use the \fB-C\fR

option.

\fBNote:\fR this program is somewhat deprecated; \fBhbal(1)\fR gives

usually much faster results, and a better cluster. It is recommended

to use this program only when \fBhbal\fR doesn't give a N+1 compliant

cluster.

.SS ALGORITHM

The algorithm works in multiple rounds, of increasing \fIdepth\fR,

until we have a solution.

First, before starting the solution computation, we compute all the

N+1-fail nodes and the instances they hold. These instances are

candidate for replacement (and only these!).

The program start then with \fIdepth\fR one (unless overridden via the

\fB-d\fR option), and at each round:

  - it tries to remove from the cluster as many instances as the

    current depth in order to make the cluster N+1 compliant

  - then, for each of the possible instance combinations that allow

    this (unless the total size is reduced via the \fB-r\fR option),

    it tries to put them back on the cluster while maintaining N+1

    compliance

It might be that at a given round, the results are:

  - no instance combination that can be put back; this means it is not

    possible to make the cluster N+1 compliant with this number of

    instances being moved, so we increase the depth and go on to the

    next round

  - one or more successful result, in which case we take the one that

    has as few changes as possible (by change meaning a replace-disks

    needed)

The main problem with the algorithm is that, being an exhaustive

search, the CPU time required grows very very quickly based on

depth. On a 20-node, 80-instances cluster, depths up to 5-6 are

quickly computed, and depth 10 could already take days.

Since the algorithm is designed to prune the search space as quickly

as possible, is by luck we find a good solution early at a given

depth, then the other solutions which would result in a bigger delta

(the number of changes) will not be investigated, and the program will

finish fast. Since this is random and depends on where in the full

solution space the good solution will be, there are two options for

cutting down the time needed:

  - \fB-l\fR makes any solution that has delta lower than its

    parameter succeed instantly

  - \fB-L\fR makes any solution with delta higher than its parameter

    being rejected instantly (and not descend on the search tree)

.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

.BI "-n" nodefile ", --nodes=" nodefile

The name of the file holding node information (if not collecting via

RAPI), instead of the default

.I nodes

file.

.TP

.BI "-i" instancefile ", --instances=" instancefile

The name of the file holding instance information (if not collecting

via RAPI), instead of the default

.I instances

file.

.TP

.BI "-m" cluster

Collect data not from files but directly from the

.I cluster

given as an argument via RAPI. This work for both Ganeti 1.2 and

Ganeti 2.0.

.TP

.BI "-d" DEPTH ", --depth=" DEPTH

Start the algorithm directly at depth \fID\fR, so that we don't

examine lower depth. This will be faster if we know a solution is not

found a lower depths, and thus it's unneeded to search them.

.TP

.BI "-l" MIN-DELTA ", --min-delta=" MIN-DELTA

If we find a solution with delta lower or equal to \fIMIN-DELTA\fR,

consider this a success and don't examine further.

.TP

.BI "-L" MAX-DELTA ", --max-delta=" MAX-DELTA

If while computing a solution, it's intermediate delta is already

higher or equal to \fIMAX-DELTA\fR, consider this a failure and abort

(as if N+1 checks have failed).

.TP

.B -V, --version

Just show the program version and exit.

.SH EXIT STATUS

The exist status of the command will be zero, unless for some reason

the algorithm fatally failed (e.g. wrong node or instance data).

.SH BUGS

The program does not check its input data for consistency, and aborts

with cryptic errors messages in this case.

The algorithm doesn't know when it won't be possible to reach N+1

compliance at all, and will happily churn CPU for ages without

realising it won't reach a solution.

The algorithm is too slow.

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 SEE ALSO

.BR hbal "(1), " hscan "(1), " ganeti "(7), " gnt-instance "(8), "

.BR gnt-node "(8)"