Will changing the replication size from 2 to 3 cause huge I/O resources
to be used, or does this happen quietly in the background?
On 2016-04-06 00:40, Christian Balzer wrote:
Hello,
Brian already mentioned a number very pertinent things, I've got a few
more:
On Tue, 05 Apr 2016 10:48:49 -0400 dan@xxxxxxxxxxxxxxxxx wrote:
In a 12 OSD setup, the following config is there:
(OSDs * 100)
Total PGs = ----------
pool size
The PGcalc page at http://ceph.com/pgcalc/ is quite helpful and
contains a
lot of background info as well.
As Brian said, you can never decrease PG count, but growing it is also
a
very I/O intensive operation and you want to avoid that as much as
possible.
So with 12 OSD's and a pool size of 2 replicas, this would equal Total
PGs of 600 as per this url:
PGcalc with a target of 200 PGs per OSD (doubling of cluster size
expected) gives us 1024, which is also what I would go for myself.
However if this a production cluster and your OSDs are NOT RAID1 or
very
very reliable, fast and well monitored SSDs you're basically asking
Murphy
to come visit, destroying your data while eating babies and washing
them
down with bath water.
The default replication size was changed to 3 for a very good reason,
there are plenty of threads in this ML about failure scenarios and
probabilities.
Christian
http://docs.ceph.com/docs/master/rados/operations/placement-groups/#preselection
Yet in the same page, at the top it says:
Between 10 and 50 OSDs set pg_num to 4096
Our use is for shared hosting so there are lots of small writes and
reads. Which of these would be correct?
Also is it a simple process to update PGs on a live system without
affecting service?
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