Re: PG Calculations

[Karol Kozubal <Karol.Kozubal@xxxxxxxxx>]

Dan, I think your interpretation is indeed correct.

The documentation on this page looks to be saying this. http://ceph.com/docs/master/rados/operations/placement-groups/

Increasing the number of placement groups reduces the variance in per-OSD load across your cluster. We recommend approximately 50-100 placement groups per OSD to balance out memory and CPU requirements and per-OSD load. For a single pool of objects, you can use the following formula:

Then lower on the same page…

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 groups per OSD so that you arrive at a reasonable total number of placement groups that provides reasonably low variance per OSD without taxing system resources or making the peering process too slow.

However a confirmation from InkTank would be nice.

Karol

From: Dan Van Der Ster <daniel.vanderster@xxxxxxx>
Date: Friday, March 14, 2014 at 10:55 AM
To: "Bradley.McNamara@xxxxxxxxxxx" <Bradley.McNamara@xxxxxxxxxxx>
Cc: "ceph-users@xxxxxxxxxxxxxx" <ceph-users@xxxxxxxxxxxxxx>
Subject: Re: PG Calculations

Hi,
Since you didn't get an immediate reply from a developer, I'm going to be bold and repeat my interpretation that the documentation implies, perhaps not clearly enough, that the 50-100 PGs per OSD rule should be applied for the total of all pools, not per pool. I hope a dev will correct me if I'm wrong.

With your config you must have an avg 400 PGs per OSD. Do you find peering/backfilling/recovery to be responsive? How is the CPU and memory usage of your OSDs during backfilling?

Cheers, Dan

-- Dan van der Ster || Data & Storage Services || CERN IT Department --

-------- Original Message --------
From: "McNamara, Bradley" <Bradley.McNamara@xxxxxxxxxxx>
Sent: Thursday, March 13, 2014 08:03 PM
To: ceph-users@xxxxxxxxxxxxxx
Subject: PG Calculations

There was a very recent thread discussing PG calculations, and it made me doubt my cluster setup.  So, Inktank, please provide some clarification.

 

I followed the documentation, and interpreted that documentation to mean that PG and PGP calculation was based upon a per-pool calculation.  The recent discussion introduced a slightly different formula adding in the total number of pools: 

 

# OSD * 100 / 3

 

vs.

 

# OSD’s * 100 / (3 * # pools)

 

My current cluster has 24 OSD’s, replica size of 3, and the standard three pools, RBD, DATA, and METADATA.  My current total PG’s is 3072, which by the second formula is way too many.  So, do I have too many?  Does it need to be addressed, or can it wait until I add more OSD’s, which will bring the ratio closer to ideal?  I’m currently using only RBD and CephFS, no RadosGW.

 

Thank you!

 

Brad

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