How many PGs do you have - and how many are you increasing it to?

Increasing PG counts can be disruptive if you are increasing by a large 
proportion of the initial count because all the PG peering involved.  If you 
are doubling the amount of PGs it might be good to do it in stages to minimize 
peering.  For example if you are going from 1024 to 2048 - consider 4 increases 
of 256, allowing the cluster to stabilize in-between, rather that one event 
that doubles the number of PGs.

If you expect this cluster to grow, overshoot the recommended PG count by 50% 
or so.  This will allow you to minimize the PG increase events, and thusly 
impact to your users.

From: ceph-users 
<ceph-users-boun...@lists.ceph.com<mailto:ceph-users-boun...@lists.ceph.com>> 
on behalf of Matteo Dacrema <mdacr...@enter.eu<mailto:mdacr...@enter.eu>>
Date: Sunday, September 18, 2016 at 3:29 PM
To: Goncalo Borges 
<goncalo.bor...@sydney.edu.au<mailto:goncalo.bor...@sydney.edu.au>>, 
"ceph-users@lists.ceph.com<mailto:ceph-users@lists.ceph.com>" 
<ceph-users@lists.ceph.com<mailto:ceph-users@lists.ceph.com>>
Subject: [EXTERNAL] Re: [ceph-users] Increase PG number

Hi , thanks for your reply.

Yes, I’don’t any near full osd.

The problem is not the rebalancing process but the process of creation of new 
pgs.

I’ve only 2 host running Ceph Firefly version with 3 SSDs for journaling each.
During the creation of new pgs all the volumes attached stop to read or write 
showing high iowait.
Ceph -s tell me that there are thousand of slow requests.

When all the pgs are created slow request begin to decrease and the cluster 
start rebalancing process.

Matteo

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Il giorno 18 set 2016, alle ore 13:08, Goncalo Borges 
<goncalo.bor...@sydney.edu.au<mailto:goncalo.bor...@sydney.edu.au>> ha scritto:

Hi
I am assuming that you do not have any near full osd  (either before or along 
the pg splitting process) and that your cluster is healthy.

To minimize the impact on the clients during recover or operations like pg 
splitting, it is good to set the following configs. Obviously the whole 
operation will take longer to recover but the impact on clients will be 
minimized.

#  ceph daemon mon.rccephmon1 config show | egrep 
"(osd_max_backfills|osd_recovery_threads|osd_recovery_op_priority|osd_client_op_priority|osd_recovery_max_active)"
   "osd_max_backfills": "1",
   "osd_recovery_threads": "1",
   "osd_recovery_max_active": "1"
   "osd_client_op_priority": "63",
   "osd_recovery_op_priority": "1"

Cheers
G.
________________________________________
From: ceph-users 
[ceph-users-boun...@lists.ceph.com<mailto:ceph-users-boun...@lists.ceph.com>] 
on behalf of Matteo Dacrema [mdacr...@enter.eu<mailto:mdacr...@enter.eu>]
Sent: 18 September 2016 03:42
To: ceph-users@lists.ceph.com<mailto:ceph-users@lists.ceph.com>
Subject: [ceph-users] Increase PG number

Hi All,

I need to expand my ceph cluster and I also need to increase pg number.
In a test environment I see that during pg creation all read and write 
operations are stopped.

Is that a normal behavior ?

Thanks
Matteo
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