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 

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

From: ceph-users [ceph-users-boun...@lists.ceph.com] on behalf of Matteo 
Dacrema [mdacr...@enter.eu]
Sent: 18 September 2016 03:42
To: 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 ?

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