Hi Greg,
Nowhere in your test procedure do you mention syncing or flushing the files to disk. That is almost certainly the cause of the slowness
We have tested performing sync after file creation and the delay still occurs. (See Test3 results below)
To clarify, it appears the delay is observed only when ls is performed on the same directory in which the files were removed, provided the files have been recently cached.
e.g. rm -f /mnt/cephfs_mountpoint/file*; ls /mnt/cephfs_mountpoint
the client which wrote the data is required to flush it out before dropping enough file "capabilities" for the other client to do the rm.
Our tests are performed on the same host.
In Test1 the rm and ls are performed by the same client id. And for other tests in which an unmount & remount were performed, I would assume the unmount would cause that particular client id to terminate and drop any caps.
Do you still believe held caps are contributing to slowness in these test scenarios?
We’ve added 3 additional test cases below.
Test 3) Sync write (delay observed when writing files and syncing)
Test 4) Bypass cache (no delay observed when files are not written to cache)
Test 5) Read test (delay observed when removing files that have been read recently in to cache)
Test3: Sync Write - File creation, with sync after write.
1) unmount & remount:
2) Add 5 x 100GB files to a directory:
for i in {1..5}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$i.txt count=102400 bs=1048576;done
3) sync
4) Delete all files in directory:
for i in {1..5};do rm -f /mnt/cephfs_mountpoint/file$i.txt; done
5) Immediately perform ls on directory:
time ls /mnt/cephfs_mountpoint
real 0m8.765s
user 0m0.001s
sys 0m0.000s
Test4: Bypass cache - File creation, with nocache options for dd.
1) unmount & remount:
2) Add 5 x 100GB files to a directory:
for i in {1..5}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$i.txt count=102400 bs=1048576 oflag=nocache,sync iflag=nocache;done
3) sync
4) Delete all files in directory:
for i in {1..5};do rm -f /mnt/cephfs_mountpoint/file$i.txt; done
5) Immediately perform ls on directory:
time ls /mnt/cephfs_mountpoint
real 0m0.003s
user 0m0.000s
sys 0m0.001s
Test5: Read test - Read files into empty page cache, before deletion.
1) unmount & remount
2) Add 5 x 100GB files to a directory:
for i in {1..5}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$i.txt count=102400 bs=1048576;done
3) sync
4) unmount & remount #empty cache
5) read files (to add back to cache)
for i in {1..5};do cat /mnt/cephfs_mountpoint/file$i.txt > /dev/null; done
6) Delete all files in directory:
for i in {1..5};do rm -f /mnt/cephfs_mountpoint/file$i.txt; done
5) Immediately perform ls on directory:
time ls /mnt/cephfs_mountpoint
real 0m8.723s
user 0m0.000s
sys 0m0.001s
Regards,Dylan
From: Gregory Farnum <gfarnum@xxxxxxxxxx>
Sent: Wednesday, October 10, 2018 4:37:49 AM
To: Dylan McCulloch
Cc: ceph-users@xxxxxxxxxxxxxx
Subject: Re: cephfs kernel client blocks when removing large filesNowhere in your test procedure do you mention syncing or flushing the files to disk. That is almost certainly the cause of the slowness — the client which wrote the data is required to flush it out before dropping enough file "capabilities" for the other client to do the rm.-Greg
On Sun, Oct 7, 2018 at 11:57 PM Dylan McCulloch <dmc@xxxxxxxxxxxxxx> wrote:
_______________________________________________Hi all,
We have identified some unexpected blocking behaviour by the ceph-fs kernel client.
When performing 'rm' on large files (100+GB), there appears to be a significant delay of 10 seconds or more, before a 'stat' operation can be performed on the same directory on the filesystem.
Looking at the kernel client's mds inflight-ops, we observe that there are pending
UNLINK operations corresponding to the deleted files.
We have noted some correlation between files being in the client page cache and the blocking behaviour. For example, if the cache is dropped or the filesystem remounted the blocking will not occur.
Test scenario below:
/mnt/cephfs_mountpoint type ceph (rw,relatime,name=ceph_filesystem,secret=<hidden>,noshare,acl,wsize=16777216,rasize=268439552,caps_wanted_delay_min=1,caps_wanted_delay_max=1)
Test1:
1) unmount & remount:
2) Add 10 x 100GB files to a directory:
for i in {1..10}; do dd if=/dev/zero of=/mnt/cephfs_mountpoint/file$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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