Ceph too many pgs per osd: all you need to know

Question

I am getting both of these errors at the same time. I can't decrease the pg count and I can't add more storage.

This is a new cluster, and I got these warning when I uploaded about 40GB to it. I guess because radosgw created a bunch of pools.

How can ceph have too many pgs per osd, yet have more object per pg than average with a too few pgs suggestion?

HEALTH_WARN too many PGs per OSD (352 > max 300); 
pool default.rgw.buckets.data has many more objects per pg than average (too few pgs?)

osds: 4 (2 per site 500GB per osd)
size: 2 (cross site replication)
pg:  64
pgp: 64
pools: 11

Using rbd and radosgw, nothing fancy.

Answer 1

I'm going to answer my own question in hopes that it sheds some light on the issue or similar misconceptions of ceph internals.

Fixing HEALTH_WARN too many PGs per OSD (352 > max 300) once and for all

When balancing placement groups you must take into account:

Data we need

• pgs per osd
• pgs per pool
• pools per osd
• the crush map
• reasonable default pg and pgp num
• replica count

I will use my set up as an example and you should be able to use it as a template for your own.

Data we have

• num osds : 4
• num sites: 2
• pgs per osd: ???
• pgs per pool: ???
• pools per osd: 10
• reasonable default pg and pgp num: 64 (... or is it?)
• replica count: 2 (cross site replication)
• the crush map

ID WEIGHT TYPE NAME UP/DOWN REWEIGHT PRIMARY-AFFINITY root ourcompnay site a rack a-esx.0 host prdceph-strg01 osd.0 up 1.00000 1.00000 osd.1 up 1.00000 1.00000 site b rack a-esx.0 host prdceph-strg02 osd.2 up 1.00000 1.00000 osd.3 up 1.00000 1.00000

Our goal is to fill in the '???' above with what we need to serve a HEALTH OK cluster. Our pools are created by the rados gateway when it initialises. We have a single default.rgw.buckets.data where all data is being stored the rest of the pools are adminitrastive and internal to cephs metadata and book keeping.

PGs per osd (what is a reasonable default anyway???)

The documentation would have us use this calculation to determine our pg count per osd:

 (osd * 100)
----------- = pgs UP to nearest power of 2
 replica count

It is stated that to round up is optimal. So with our current setup it would be:

 (4 * 100)
----------- = (200 to the nearest power of 2) 256
    2

• osd.1 ~= 256
• osd.2 ~= 256
• osd.3 ~= 256
• osd.4 ~= 256

This is the recommended max number of pgs per osd. So... what do you actually have currently? And why isn't it working? And if you set a 'reasonable default' and understand the above WHY ISN'T IT WORKING!!! >=[

Likely, a few reasons. We have to understand what those 'reasonable defaults' above actually mean, how ceph applies them and to where. One might misunderstand from the above that I could create a new pool like so:

ceph osd pool create <pool> 256 256

or I might even think I could play it safe and follow the documentation which states that (128 pgs for < 5 osds) can use:

ceph osd pool create <pool> 128 128

This is wrong, flat out. Because it in no way explains the relationship or balance between what ceph is actaully doing with these numbers technically the correct answer is:

ceph osd pool create <pool> 32 32

And let me explain why:

If like me you provisioned your cluster with those 'reasonable defaults' (128 pgs for < 5 osds) as soon as you tried to do anything with rados it created a whole bunch of pools and your cluster spazzed out. The reason is because I misunderstood the relationship between everything mentioned above.

• pools: 10 (created by rados)
• pgs per pool: 128 (recommended in docs)
• osds: 4 (2 per site)

10 * 128 / 4 = 320 pgs per osd

This ~320 could be a number of pgs per osd on my cluster. But ceph might distribute these differently. Which is exactly what's happening and is way over the 256 max per osd stated above. My cluster's HEALTH WARN is HEALTH_WARN too many PGs per OSD (368 > max 300).

Using this command we're able to see better the relationship between the numbers:

pool :17 18  19  20  21  22  14  23  15  24  16 | SUM
------------------------------------------------< - *total pgs per osd*
osd.0 35 36  35  29  31  27  30  36  32  27  28 | 361
osd.1 29 28  29  35  33  37  34  28  32  37  36 | 375
osd.2 27 33  31  27  33  35  35  34  36  32  36 | 376
osd.3 37 31  33  37  31  29  29  30  28  32  28 | 360
-------------------------------------------------< - *total pgs per pool*
SUM :128 128 128 128 128 128 128 128 128 128 128

There's a direct correlation between the number of pools you have and the number of placement groups that are assigned to them. I have 11 pools in the snippet above and they each have 128 pgs and that's too many!! My reasonable defaults are 64! So what happened??

I was misunderstandning how the 'reasonable defaults' were being used. When I set my default to 64, you can see ceph has taking my crush map into account where I have a failure domain between site a and site b. Ceph has to ensure that everything that's on site a is at least accessible on site b.

WRONG

site a
osd.0
osd.1 TOTAL of ~ 64pgs

site b
osd.2 
osd.3 TOTAL of ~ 64pgs

We needed a grand total of 64 pgs per pool so our reasonable defaults should've actually been set to 32 from the start!

If we use ceph osd pool create <pool> 32 32 what this amounts to is that the relationship between our pgs per pool and pgs per osd with those 'reasonable defaults' and our recommened max pgs per osd start to make sense:

---

So you broke your cluster ^_^

Don't worry we're going to fix it. The procedure here I'm afraid might vary in risk and time depending on how big your cluster. But the only way to get around altering this is to add more storage, so that the placement groups can redistribute over a larger surface area. OR we have to move everything over to newly created pools.

I'll show an example of moving the default.rgw.buckets.data pool:

old_pool=default.rgw.buckets.data
new_pool=new.default.rgw.buckets.data

create a new pool, with the correct pg count:

ceph osd pool create $new_pool 32

copy the contents of the old pool the new pool:

rados cppool $old_pool $new_pool

remove the old pool:

ceph osd pool delete $old_pool $old_pool --yes-i-really-really-mean-it

rename the new pool to 'default.rgw.buckets.data'

ceph osd pool rename $new_pool $old_pool

Now it might be a safe bet to restart your radosgws.

FINALLY CORRECT

site a
osd.0
osd.1 TOTAL of ~ 32pgs

site b
osd.2 
osd.3 TOTAL of ~ 32pgs

As you can see my pool numbers have incremented since they are added by pool id and are new copies. And our total pgs per osd is way under the ~256 which gives us room to add custom pools if required.

pool :  26 35 27 36 28 29 30 31 32 33 34 | SUM
-----------------------------------------------
osd.0   15 18 16 17 17 15 15 15 16 13 16 | 173
osd.1   17 14 16 15 15 17 17 17 16 19 16 | 179
osd.2   17 14 16 18 12 17 18 14 16 14 13 | 169
osd.3   15 18 16 14 20 15 14 18 16 18 19 | 183
-----------------------------------------------
SUM :   64 64 64 64 64 64 64 64 64 64 64 

Now you should test your ceph cluster with whatever is at your disposal. Personally I've written a bunch of python over boto that tests the infrastructure and return buckets stats and metadata rather quickly. They have ensured to me that the cluster is back to working order without any of the issues it suffered from previously. Good luck!

---

Fixing pool default.rgw.buckets.data has many more objects per pg than average (too few pgs?) once and for all

This quite literally means, you need to increase the pg and pgp num of your pool. So... do it. With everything mentioned above in mind. When you do this however note that the cluster will start backfilling and you can watch this process %: watch ceph -s in another terminal window or screen.

ceph osd pool set default.rgw.buckets.data pg_num 128
ceph osd pool set default.rgw.buckets.data pgp_num 128

Armed with the knowledge and confidence in the system provided in the above segment we can clearly understand the relationship and the influence of such a change on the cluster.

pool :  35 26 27 36 28 29 30 31 32 33 34 | SUM
----------------------------------------------
osd.0   18 64 16 17 17 15 15 15 16 13 16 | 222
osd.1   14 64 16 15 15 17 17 17 16 19 16 | 226
osd.2   14 66 16 18 12 17 18 14 16 14 13 | 218
osd.3   18 62 16 14 20 15 14 18 16 18 19 | 230
-----------------------------------------------
SUM :   64 256 64 64 64 64 64 64 64 64 64 

Can you guess which pool id is default.rgw.buckets.data? haha ^_^

Answer 2

In Ceph Nautilus (v14 or later), you can turn on "PG Autotuning". See this documentation and this blog entry for more information.

I accidentally created pools with live data that I could not migrate to repair the PGs. It took some days to recover, but the PGs were optimally adjusted with zero problems.