Re: too many PGs per OSD (326 > max 300) warning when ALL PGs are 256

David Turner <david.turner@xxxxxxxxxxxxxxxx> · Tue, 11 Oct 2016 13:10:22 +0000

On Oct 11, 2016, at 1:21 AM, Thomas HAMEL <hmlth@xxxxxxxxxx> wrote:

Hello,
 I have the same problem and I wanted to make a few remarks.

One of the main advice on pg count in the docs is: if you have less than 5 osds, use 128 pgs per pool. This kind of rule of thumb is really what you are looking for when you begin. But this one is very misleading, especially if you have 3 or 4 osds. Pgcalc
 may be the right tool, but it's hard to understand when you begin. I think it would be helpfull to lower the pg per pool advice to 64.

My other point is that the vast majority of advices and tutorials on this issue tells you to raise the warning threshold (sometimes to thousands of pgs per osds) or remove it entirely. That is very tempting , but the official docs never hints at when it's safe
 to do so. Do I need RAM? CPU? IOPS?

This is terrifying to think about increasing that threshold. You may have enough system resources while running healthy, but when recovering, especially during peering, your memory usage can more than double. If you have too many pgs per osd and/or too
 many osds per machine based on how much memory you have in each host you very well could end up in an eternal peering situation. Peering causes more memory usage, the Linux OOM killer is killing your osds causing more peering, and the only way to get out of
 the situation is running to the datacenter with buckets of RAM.

Thomas Hamel

David Turner |
Cloud Operations Engineer |
StorageCraft
 Technology Corporation

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Le 10 octobre 2016 23:00:33 GMT+02:00, David Turner <david.turner@xxxxxxxxxxxxxxxx> a écrit :

The default it uses can be controlled in your ceph.conf file.  The ceph-deploy tool is a generic ceph deployment tool which does not have presets for rados gateway deployments or
 other specific deployments.  When creating pools you can specify the amount of pgs in them with the tool so that it doesn't use your defaults.

You are correct that when creating a lot of pools that it is wiser to specify a smaller amount of pgs, but that is a manual step on your end and best to use the pgcalc tool to know what you should be aiming at.  I haven't used ceph with rgw so I can't speak
 to optimal settings here, but I can only imagine that there are several tutorials and guides out there.

David Turner |
Cloud Operations Engineer |
StorageCraft
 Technology Corporation

380 Data Drive Suite 300 |
Draper |
Utah |
84020

Office:
801.871.2760 |
Mobile:
385.224.2943

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From: Andrus, Brian Contractor [bdandrus@xxxxxxx]

Sent: Monday, October 10, 2016 12:18 PM

To: David Turner; ceph-users@xxxxxxxxxxxxxx

Subject: RE: [ceph-users] too many PGs per OSD (326 > max 300) warning when ALL PGs are 256

David,
Thanks for the info. I am getting an understanding of how this works.
Now I used the ceph-deploy tool to create the rgw pools. It seems then that the tool isn’t the best at creating the pools necessary for an rgw gateway as it made all of them the default sizes for pg_num/pgp_num
Perhaps, then, it is wiser to have a very low default for those so the ceph-deploy tool doesn assign a large value to something that will merely hold control or other metadata?

Brian Andrus
ITACS/Research Computing
Naval Postgraduate School
Monterey, California
voice: 831-656-6238

From: David Turner [mailto:david.turner@xxxxxxxxxxxxxxxx]

Sent: Monday, October 10, 2016 10:33 AM

To: Andrus, Brian Contractor <bdandrus@xxxxxxx>;
ceph-users@xxxxxxxxxxxxxx

Subject: RE: [ceph-users] too many PGs per OSD (326 > max 300) warning when ALL PGs are 256

You have 11 pools with 256 pgs, 1 pool with 128 and 1 pool with 64... that's 3,008 pgs in your entire cluster.  Multiply that number by your replica size and divide
 by how many OSDs you have in your cluster and you'll see what your average PGs per osd is.  Based on the replica size you shared, that's a total number of 6,528 copies of PGs to be divided amongst the OSDS in your cluster.  Your cluster will be in warning
 if that number is greater than 300 per OSD, like you're seeing.  When designing your cluster and how many pools, pgs, and replica size you will be setting, please consult the pgcalc tool found here
http://ceph.com/pgcalc/.  You cannot reduce the number of PGs in a pool, so the easiest way to resolve this issue is mostly likely going to be destroying pools and recreating them with the proper number
 of PGs.

The PG number should be based on what percentage of the data in your cluster will be in this pool.  If I'm planning to have about 1024 PGs total in my cluster and I give 256 PGs to 4 different pools, then what I'm saying is that each of those 4 pools will have
 the exact same amount of data as each other.  On the other hand, if I believe that 1 of those pools will have 90% of the data and the other 3 pools will have very little data, then I'll probably give the larger pool 1024 PGs and the rest of them 64 PGs (or
 less depending on what I'm aiming for).  It is beneficial to keep the pg_num and pgp_num counts to base 2 numbers.

David Turner | Cloud Operations Engineer |
StorageCraft Technology Corporation

380 Data Drive Suite 300 | Draper | Utah | 84020

Office: 801.871.2760 |
Mobile: 385.224.2943

If you are not the intended recipient of this message or received it erroneously, please notify the sender and delete it, together with any attachments, and be advised that any dissemination
 or copying of this message is prohibited.

From: ceph-users [ceph-users-bounces@xxxxxxxxxxxxxx]
 on behalf of Andrus, Brian Contractor [bdandrus@xxxxxxx]

Sent: Monday, October 10, 2016 11:14 AM

To: ceph-users@xxxxxxxxxxxxxx

Subject: [ceph-users] too many PGs per OSD (326 > max 300) warning when ALL PGs are 256

Ok, this is an odd one to me…
I have several pools, ALL of them are set with pg_num and pgp_num = 256. Yet, the warning about too many PGs per OSD is showing up.
Here are my pools:

pool 0 'rbd' replicated size 3 min_size 2 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 134 flags hashpspool stripe_width 0
pool 1 'cephfs_data' replicated size 3 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 203 flags hashpspool crash_replay_interval 45 stripe_width 0
pool 2 'cephfs_metadata' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 64 pgp_num 64 last_change 196 flags hashpspool stripe_width 0
pool 3 'vmimages' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 128 pgp_num 128 last_change 213 flags hashpspool stripe_width 0
        removed_snaps [1~3]
pool 25 '.rgw.root' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6199 flags hashpspool stripe_width 0
pool 26 'default.rgw.control' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6202 flags hashpspool stripe_width 0
pool 27 'default.rgw.data.root' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6204 flags hashpspool stripe_width 0
pool 28 'default.rgw.gc' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6205 flags hashpspool stripe_width 0
pool 29 'default.rgw.log' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6206 flags hashpspool stripe_width 0
pool 30 'default.rgw.users.uid' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6211 flags hashpspool stripe_width 0
pool 31 'default.rgw.meta' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6214 flags hashpspool stripe_width 0
pool 32 'default.rgw.buckets.index' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6216 flags hashpspool stripe_width 0
pool 33 'default.rgw.buckets.data' replicated size 2 min_size 1 crush_ruleset 0 object_hash rjenkins pg_num 256 pgp_num 256 last_change 6218 flags hashpspool stripe_width 0

so why would the warning show up, and how do I get it to go away and stay away?

Brian Andrus
ITACS/Research Computing
Naval Postgraduate School
Monterey, California
voice: 831-656-6238

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