PG = Placement Group PGP = Placement Group for Placement purpose
pg_num = number of placement groups mapped to an OSD
When pg_num is increased for any pool, every PG of this pool splits into half, but they all remain mapped to their parent OSD.
Until this time, Ceph does not start rebalancing. Now, when you increase the pgp_num value for the same pool, PGs start to migrate from the parent to some other OSD, and cluster rebalancing starts. This is how PGP plays an important role.
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Karan Singh Systems Specialist , Storage Platforms CSC - IT Center for Science, Keilaranta 14, P. O. Box 405, FIN-02101 Espoo, Finland mobile: +358 503 812758 tel. +358 9 4572001 fax +358 9 4572302 http://www.csc.fi/****************************************************************
To be sure to understand, if I create 2 times replicated pool toto with 1024 pgs and 1 pgp, pg and data of pool toto will be mapped on only 2 OSDs and on 2 servers right ? Sent from my iPhone On 21 mai 2015, at 18:58, Florent MONTHEL <fmonthel@xxxxxxxxxxxxx> wrote:
Thanks Ilya for this clear explanation!
I'm searching that for a long time
Best practices is to have pg = pgp in order to "avoid" using of the same set of osd right ? (On a small cluster you will have)
Sent from my iPhone
On 21 mai 2015, at 07:49, Ilya Dryomov <idryomov@xxxxxxxxx> wrote:
On Thu, May 21, 2015 at 12:12 PM, baijiaruo@xxxxxxx <baijiaruo@xxxxxxx> wrote:
Re: what's the difference between pg and pgp?
pg-num is the number of PGs, pgp-num is the number of PGs that will be
considered for placement, i.e. it's the pgp-num value that is used by
CRUSH, not pg-num. For example, consider pg-num = 1024 and pgp-num
= 1. In that case you will see 1024 PGs but all of those PGs will map
to the same set of OSDs.
When you increase pg-num you are splitting PGs, when you increase
pgp-num you are moving them, i.e. changing sets of OSDs they map to.
Thanks,
Ilya
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