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doc: Add "nearest power of two" to PG rule-of-thumb

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Following an IRC discussion, it emerged that it would be helpful
to explain the merit of choosing a number of PGs per pool that is
a power of two, to keep PGs at roughly equal sizes in case of
PG splits.

See http://irclogs.ceph.widodh.nl/index.php?date=2014-03-12 for the
original discussion.

Signed-off-by: Florian Haas <florian@hastexo.com>
This commit is contained in:
Florian Haas 2014-03-12 19:31:54 +01:00
parent 9987e486ad
commit 047287afbe
1 changed files with 12 additions and 2 deletions
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14
doc/rados/operations/placement-groups.rst
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@ -44,9 +44,19 @@ to balance out memory and CPU requirements and per-OSD load. For a single pool
of objects, you can use the following formula:: of objects, you can use the following formula::
(OSDs * 100) (OSDs * 100)
Total PGs = ------------ Total PGs = ------------ *(rounded up to the nearest power of 2)*
Replicas Replicas
The rounding to the nearest power of two is optional, but recommended
if you want to ensure that all placement groups are roughly the same size.
As an example, for a cluster with 200 OSDs and a pool size of 3
replicas, you would estimate your number of PGs as follows:
(200 * 100)
----------- = 6667. Nearest power of 2: 8192
3
When using multiple data pools for storing objects, you need to ensure that you When using multiple data pools for storing objects, you need to ensure that you
balance the number of placement groups per pool with the number of placement balance the number of placement groups per pool with the number of placement
groups per OSD so that you arrive at a reasonable total number of placement groups per OSD so that you arrive at a reasonable total number of placement
@ -184,4 +194,4 @@ entirely. To mark the "unfound" objects as "lost", execute the following::
pg-concepts pg-concepts
.. _Create a Pool: ../pools#createpool .. _Create a Pool: ../pools#createpool