Hi,
I have also seen this on a recent large scale/parallel test I can on a
test setup. After creating about a hundred million files, then removing
all of them - there were thousands of objects left in the cephfs data
pool corresponding to inodes that no longer existed. I.e. it looks like
the MDS is "leaking" data - does not correctly delete all objects
corresponding to files in the purge queue. We have never used any
snapshots on this file system, so it definitely isn't about snapshots
holding data. Also, we increased the purge queue settings
(filer_max_purge_ops, mds_max_purge_* and friends) and definitely waited
long enough for all deletes to be processed (the objects were there
weeks after the file system was emptied). This was on a squid 19.2.3
cluster.
In the tests that lead to this, both the file creation and deletions
were done in parallel using a few dozen clients - stressing the MDS for
sure. We had no crashes/problems on the cluster, we never had to do any
data recovery steps, i.e. the MDS appeared to work fine through the
tests. Also, this was a single MDS cluster, i.e. the problem isn't
related to subtrees moving across MDS's for example. The problem became
obvious since after removing all data from cephfs, we were expecting the
data pools to be empty, but they weren't. There were two pools, a
primary triple replicated one and an erasure coded one. Both of them
had stray objects.
I didn't pursue this further since I wasn't sure what useful information
I could gather for a bug report - but it is certainly a curious
observation that perhaps large, long living cephfs clusters might have
significant space tied up in these objects that should have been removed
but weren't. Short of a full scan of all objects and matching them to
inodes - it is hard to tell how much even.
Andras
On 7/2/26 12:17 PM, Andrej Filipčič via ceph-users wrote:
So, to follow up on this, I did some further investigation.
Checking for write amplification, I have copied 250TB of a mix of
small and large files (20M of them), and the stored space on EC pool
matched what was expected from actual data size to a few %. So EC
overhead was not really a factor. Also, after the removal of this data
set, the stored space was recovered as expected.
The I checked the full dump of cephfs and compared it to list of all
objects in the EC pool as follows:
rados -p cephfs_data_echdd ls > echdd.objectlist
find /ceph/ -printf "%i %p\n" > cephfs.inodes
this took several hours, while in the meantime, writing and removing
of data to cephfs was relatively low (few MB/s), so the impact of new
objects and files should have been minimal.
Then I selected all the objects of the form
1007b28abae.00000000
100cce97f6d.00000000
10067733861.00000000
100cc4646aa.00000000
200044d5c07.00000000
...
and checked if they match the files in the cephfs.inodes list
~3M of *.00000000 objects do not have the corresponding inode in the
cephfs.inodes
I did "rados stat" of ~70k of these objects (still running for all of
them), and almost all have timestamps from March and April these year.
On 4th of May I remember I have increased the mds purge queue values
to speed up the removal of files since OSDs were filling up too
quickly, and since then it seems the objects are not left uncleaned
any more.
With "rados getxattr objectid parent" I have checked several of them,
and they all belong to two very active projects which typically write
files with few GB/s all the time (and remove as well so space usage is
not increasing). The objects I have checked belonged to files that
were removed in cephfs, the projects have a separate file catalog
which is consistent with cephfs contents.
So, I do not understand why so many objects were left unremoved in EC
pool in the period of 2 months, but at least, 3M uncleaned files
explains 1.5PB of dark data by quick estimate.
We also scrubbed cephfs root and ~mdsdir several times and no
leftovers to remove were found.
I hope the problem is gone now, but I would still like a good advice
on how to proceed with the cleanup. I see these options:
1) remove the unmatched objects directly from EC pool with "rados
rm". But this might have undesired side effects or corruption.
2) create a new EC pool and migrate all the data there by copying
files in filesystem, and then destroy the old pool.
3) create a new filesystem with new EC pools and migrate the files.
4) run some advanced MDS disaster recovery procedure
(cephfs-data-scan), but this requires offline FS and I do not want to
recover old files from existing unmatched objects. Anyway, cephfs
seems to be healthy now.
Any good ideas?
Best,
Andrej
On 10. 6. 2026 01:36, Anthony D'Atri via ceph-users wrote:
).
There is ~5.6PiB stored on /ceph, shown by ceph.dir.rbytes with
132M files and 139M rentries. The pool shows 7PiB stored and
9.7PiB used consistent with 8+3 EC.
The layout for most files:
ceph.dir.layout="stripe_unit=16777216 stripe_count=1
object_size=16777216 pool=cephfs_data_echdd"
But there is 1.4PiB discrepancy between the pool and the filesystem
Do you have scrubs enabled? Which if any non-default config options
do you have set? Any undersized or degraded or backfilling PGs?
Which Ceph release? Do you have a sizable fraction of small files?
If you’re running Squid or earlier or don’t have EC optimizations
enabled, even a tiny file will allocate a multiple of 11*16=176 KB.
An 129KB file will consume 352KB, etc. If I understand those layout
options correctly.
If that’s what’s going on, going to Tentacle with EC optimizations
would gain you some efficiency for files newly [re]written. You
could also migrate small files to a replicated pool.
which I cannot explain and I suspect there are a lot of orphan
objects there. I have run mds scrub on / and ~mdsdir as well.
There is some mds damage on some old small files (~400 files),
which I do not think it's relevant here.
Hello,
We had a similar issue last year with a group of users that created
and removed files at a very high rate.
Have you read
https://urldefense.com/v3/__https://docs.clyso.com/docs/kb/cephfs/*cephfs-pool-data-usage-growth-without-explanation__;Iw!!DSb-azq1wVFtOg!XcB7cJMEzAXsAddGg1LH5ff1B33dit2O1vAhxnVlv2MMUrC85oUKgukdaytYJQqAfmmjiTWJhr7GLPIKSTbwQpbx18c$
?
We increased the purge rate parameters (very) aggressively to get
back to a comfortable situation (i.e. not a pool w/ near full
warnings).
Loïc.
--
_____________________________________________________________
prof. dr. Andrej Filipcic, E-mail: [email protected]
Department of Experimental High Energy Physics - F9
Jozef Stefan Institute, Jamova 39, P.o.Box 3000
SI-1001 Ljubljana, Slovenia
Tel.: +386-1-477-3674 Fax: +386-1-477-3166
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