crushtool man page

crushtool — CRUSH map manipulation tool


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


crushtool is a utility that lets you create, compile, decompile
and test 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):

The tool has four modes of operation.

--compile|-c map.txt
will compile a plaintext map.txt into a binary map file.
--decompile|-d map
will take the compiled map and decompile it into a plaintext source file, suitable for editing.
--build --num_osds {num-osds} layer1 ...
will create map with the given layer structure. See below for a detailed explanation.
will perform a dry run of a CRUSH mapping for a range of input object names. See below for a detailed explanation.

Unlike other Ceph tools, crushtool does not accept generic options such as --debug-crush from the command line. They can, however, be provided via the CEPH_ARGS environment variable. For instance, to silence all output from the CRUSH subsystem:

CEPH_ARGS="--debug-crush 0" crushtool ...

Running Tests with --test

The test mode will use the input crush map ( as specified with -i map ) and perform a dry run of CRUSH mapping or random placement ( if --simulate is set ). On completion, two kinds of reports can be created. 1) The --show-... option outputs human readable information on stderr. 2) The --output-csv option creates CSV files that are documented by the --help-output option.


For each rule, displays the mapping of each object. For instance:

CRUSH rule 1 x 24 [11,6]

shows that object 24 is mapped to devices [11,6] by rule 1. At the end of the mapping details, a summary of the distribution is displayed. For instance:

rule 1 (metadata) num_rep 5 result size == 5:    1024/1024

shows that rule 1 which is named metadata successfully mapped 1024 objects to result size == 5 devices when trying to map them to num_rep 5 replicas. When it fails to provide the required mapping, presumably because the number of tries must be increased, a breakdown of the failures is displayed. For instance:

rule 1 (metadata) num_rep 10 result size == 8:   4/1024
rule 1 (metadata) num_rep 10 result size == 9:   93/1024
rule 1 (metadata) num_rep 10 result size == 10:  927/1024

shows that although num_rep 10 replicas were required, 4 out of 1024 objects ( 4/1024 ) were mapped to result size == 8 devices only.


Displays which object failed to be mapped to the required number of devices. For instance:

bad mapping rule 1 x 781 num_rep 7 result [8,10,2,11,6,9]

shows that when rule 1 was required to map 7 devices, it could map only six : [8,10,2,11,6,9].


Displays the expected and actual utilisation for each device, for each number of replicas. For instance:

device 0: stored : 951      expected : 853.333
device 1: stored : 963      expected : 853.333

shows that device 0 stored 951 objects and was expected to store 853. Implies --show-statistics.

Displays the same as --show-utilization but does not suppress output when the weight of a device is zero. Implies --show-statistics.

Displays how many attempts were needed to find a device mapping. For instance:

0:     95224
1:      3745
2:      2225

shows that 95224 mappings succeeded without retries, 3745 mappings succeeded with one attempts, etc. There are as many rows as the value of the --set-choose-total-tries option.


Creates CSV files (in the current directory) containing information documented by --help-output. The files are named after the rule used when collecting the statistics. For instance, if the rule : 'metadata' is used, the CSV files will be:


The first line of the file shortly explains the column layout. For instance:

Device ID, Absolute Weight
--output-name NAME

Prepend NAME to the file names generated when --output-csv is specified. For instance --output-name FOO will create files:


The --set-... options can be used to modify the tunables of the input crush map. The input crush map is modified in memory. For example:

$ crushtool -i mymap --test --show-bad-mappings
bad mapping rule 1 x 781 num_rep 7 result [8,10,2,11,6,9]

could be fixed by increasing the choose-total-tries as follows:

$ crushtool -i mymap --test
--show-bad-mappings --set-choose-total-tries 500

Building a Map with --build

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

Each layer consists of:

bucket ( uniform | list | tree | straw ) size

The bucket is the type of the buckets in the layer (e.g. "rack"). Each bucket name will be built by appending a unique number to the bucket string (e.g. "rack0", "rack1"...).

The second component is the type of bucket: straw should be used most of the time.

The third component is the maximum size of the bucket. A size of zero means a bucket of infinite capacity.


Suppose we have two rows with two racks each and 20 nodes per rack. Suppose each node contains 4 storage devices for Ceph OSD Daemons. This configuration allows us to deploy 320 Ceph OSD Daemons. Lets assume a 42U rack with 2U nodes, leaving an extra 2U for a rack switch.

To reflect our hierarchy of devices, nodes, racks and rows, we would execute the following:

$ crushtool -o crushmap --build --num_osds 320 \
       node straw 4 \
       rack straw 20 \
       row straw 2 \
       root straw 0
# id        weight  type name       reweight
-87 320     root root
-85 160             row row0
-81 80                      rack rack0
-1  4                               node node0
0   1                                       osd.0   1
1   1                                       osd.1   1
2   1                                       osd.2   1
3   1                                       osd.3   1
-2  4                               node node1
4   1                                       osd.4   1
5   1                                       osd.5   1

CRUSH rulesets are created so the generated crushmap can be tested. They are the same rulesets as the one created by default when creating a new Ceph cluster. They can be further edited with:

# decompile
crushtool -d crushmap -o map.txt

# edit
emacs map.txt

# recompile
crushtool -c map.txt -o crushmap


crushtool is part of Ceph, a massively scalable, open-source, distributed storage system. Please refer to the Ceph documentation at http://ceph.com/docs for more information.

See Also

ceph(8), osdmaptool(8),


John Wilkins, Sage Weil, Loic Dachary

Referenced By

monmaptool(8), osdmaptool(8).

Explore man page connections for crushtool(8).

Ceph Sep 29, 2016