Re: Questions about pg num setting

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Is there a disadvantage to just always start pg_num and pgp_num with 
something low like 8, and then later increase it when necessary?

Question is then how to identify when necessary





-----Original Message-----
From: Christian Wuerdig [mailto:christian.wuerdig@xxxxxxxxx] 
Sent: dinsdag 2 januari 2018 19:40
To: 于相洋
Cc: Ceph-User
Subject: Re:  Questions about pg num setting

Have you had a look at http://ceph.com/pgcalc/?

Generally if you have too many PGs per OSD you can get yourself into 
trouble during recovery and backfilling operations consuming a lot more 
RAM than you have and eventually making your cluster unusable (some more 
info can be found here for example:
http://lists.ceph.com/pipermail/ceph-users-ceph.com/2016-October/013614.html
but there are other threads on the ML).
Also currently you cannot reduce the number of PGs for a pool so you are 
much better of starting with a lower value and then gradually increasing 
it.

The fact that the ceph developers introduced a config option which 
prevents users from increasing the number of PGs if it exceeds the 
configured limit should be a tell-tale sign that having too many PGs per 
OSD is considered a problem (see also
https://bugzilla.redhat.com/show_bug.cgi?id=1489064 and linked PRs)

On Wed, Dec 27, 2017 at 3:15 PM, 于相洋 <penglaiyxy@xxxxxxxxx> wrote:
> Hi cephers,
>
> I have two questions about pg number setting.
>
> First :
> My storage informaiton is show as belows:
> HDD: 10 * 8TB
> CPU: Intel(R) Xeon(R) CPU E5645 @ 2.40GHz (24 cores)
> Memery: 64GB
>
> As my HDD capacity and my Mem is too large, so I want to set as many 
> as 300 pgs to each OSD. Although 100 pgs per OSD is perferred. I want 
> to know what is the disadvantage of setting too many pgs?
>
>
> Second:
> At begin ,I can not judge the capacity proportion of my workloads, so 
> I can not set accurate pg numbers of different pools. How many pgs 
> should I set for each pools first?
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