==========================================
 crushtool -- CRUSH map manipulation tool
==========================================

.. program:: crushtool

Synopsis
========

| **crushtool** ( -d *map* | -c *map.txt* | --build *numosds*
  *layer1* *...* ) [ -o *outfile* [ --clobber ]]


Description
===========

**crushtool** is a utility that lets you create, compile, and
decompile CRUSH map files.

CRUSH is a pseudo-random data distribution algorithm that efficiently
maps input values (typically data objects) across a heterogeneous,
hierarchically structured device map. The algorithm was originally
described in detail in the following paper (although it has evolved
some since then):

       http://www.ssrc.ucsc.edu/Papers/weil-sc06.pdf

The tool has four modes of operation.

.. option:: -c map.txt

   will compile a plaintext map.txt into a binary map file.

.. option:: -d map

   will take the compiled map and decompile it into a plaintext source
   file, suitable for editing.

.. option:: --build --num_osds {num-osds} layer1 ...

   will create a relatively generic map with the given layer
   structure. See below for examples.

.. option:: --test ...
   will perform a dry run of a CRUSH mapping for a range of input object 
   names, see crushtool --help for more information. 

Options
=======

.. option:: -o outfile

   will specify the output file.

.. option:: --clobber

   will allow the tool to overwrite an existing outfile (it will normally refuse).


Building a map
==============

The build mode will generate relatively generic hierarchical maps. The
first argument simply specifies the number of devices (leaves) in the
CRUSH hierarchy. Each layer describes how the layer (or raw devices)
preceding it should be grouped.

Each layer consists of::

       name ( uniform | list | tree | straw ) size

The first element is the name for the elements in the layer
(e.g. "rack"). Each element's name will be append a number to the
provided name.

The second component is the type of CRUSH bucket.

The third component is the maximum size of the bucket. If the size is
0, a single bucket will be generated that includes everything in the
preceding layer.


Example
=======

Suppose we have 128 devices, each grouped into shelves with 4 devices
each, and 8 shelves per rack. We could create a three level hierarchy
with::

       crushtool --build 128 shelf uniform 4 rack straw 8 root straw 0 -o map

To adjust the default (generic) mapping rules, we can run::

       # decompile
       crushtool -d map -o map.txt

       # edit
       vi map.txt

       # recompile
       crushtool -c map.txt -o map


Availability
============

**crushtool** is part of the Ceph distributed file system. Please
refer to the Ceph documentation at http://ceph.com/docs for more
information.


See also
========

:doc:`ceph <ceph>`\(8),
:doc:`osdmaptool <osdmaptool>`\(8),
:doc:`mkcephfs <mkcephfs>`\(8)