[ceph-users] cephfs kernel client blocks when removing large files

Dylan McCulloch dmc at unimelb.edu.au
Sun Oct 7 23:57:18 PDT 2018


Hi all,


We have identified some unexpected blocking behaviour by the ceph-fs kernel client.


When performing 'rm' on large files (100+GB), there appears to be a significant delay of 10 seconds or more, before a 'stat' operation can be performed on the same directory on the filesystem.


Looking at the kernel client's mds inflight-ops, we observe that there are pending

UNLINK operations corresponding to the deleted files.


We have noted some correlation between files being in the client page cache and the blocking behaviour. For example, if the cache is dropped or the filesystem remounted the blocking will not occur.


Test scenario below:


/mnt/cephfs_mountpoint type ceph (rw,relatime,name=ceph_filesystem,secret=<hidden>,noshare,acl,wsize=16777216,rasize=268439552,caps_wanted_delay_min=1,caps_wanted_delay_max=1)


Test1:

1) unmount & remount:


2) Add 10 x 100GB files to a directory:


for i in {1..10}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$i.txt count=102400 bs=1048576; done


3) Delete all files in directory:


for i in {1..10};do rm -f /mnt/cephfs_mountpoint/file$i.txt; done


4) Immediately perform ls on directory:


time ls /mnt/cephfs_mountpoint/test1


Result: delay ~16 seconds

real    0m16.818s

user    0m0.000s

sys     0m0.002s



Test2:


1) unmount & remount


2) Add 10 x 100GB files to a directory

for i in {1..10}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$i.txt count=102400 bs=1048576; done


3) Either a) unmount & remount; or b) drop caches


echo 3 >/proc/sys/vm/drop_caches


4) Delete files in directory:


for i in {1..10};do rm -f /mnt/cephfs_mountpoint/file$i.txt; done


5) Immediately perform ls on directory:


time ls /mnt/cephfs_mountpoint/test1


Result: no delay

real    0m0.010s

user    0m0.000s

sys     0m0.001s


Our understanding of ceph-fs’ file deletion mechanism, is that there should be no blocking observed on the client. http://docs.ceph.com/docs/mimic/dev/delayed-delete/ .

It appears that if files are cached on the client, either by being created or accessed recently  it will cause the kernel client to block for reasons we have not identified.


Is this a known issue, are there any ways to mitigate this behaviour?

Our production system relies on our client’s processes having concurrent access to the file system, and access contention must be avoided.


An old mailing list post that discusses changes to client’s page cache behaviour may be relevant.

http://lists.ceph.com/pipermail/ceph-users-ceph.com/2015-October/005692.html


Client System:


OS: RHEL7

Kernel: 4.15.15-1


Cluster: Ceph: Luminous 12.2.8


Thanks,

Dylan

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