While testing the fscache performance fixes [1] that were merged into 6.4-rc1
it appears that the caching no longer works. The client will write to the cache
but never reads.
I suspect this is related to known issue #1. However, I tested the client
with rsize less than, equal to, and greater than readahead, and in all cases
I see the issue.
If I apply both the patches [2], [3] from the known issues to 6.4-rc1 then the
cache works as expected. I suspect only patch [2] is required but have not
tested patch [2] without [3].
Testing
=======
For the test I was just using dd to read 300 x 1gb files from an NFS
share to fill the cache, then repeating the read.
In the first test run, /var/cache/fscache steadily filled until reaching
300 GB. The read I/O was less than 1 MB/s, and the write speed was fairly
constant 270 MB/s.
In the second run, /var/cache/fscache remained at 300 GB, so no new data was
being written. However, the read I/O remained at less than 1 MB/s and the write
rate at 270 MB/s.
/var/cache/fscache
| 1st run | 2nd run
disk usage | 0 -> 300 GB | 300 GB
read speed | < 1 MB/s | < 1 MB/s
write speed | 270 MB/s | 270 MB/s
This seems to imply that the already cached data was being read from the source
server and re-written to the cache.
Known Issues
============
1. Unit test setting rsize < readahead does not properly read from
fscache but re-reads data from the NFS server
* This will be fixed with another dhowells patch [2]:
"[PATCH v6 2/2] mm, netfs, fscache: Stop read optimisation when
folio removed from pagecache"
2. "Cache volume key already in use" after xfstest runs involving
multiple mounts
* Simple reproducer requires just two mounts as follows:
mount -overs=4.1,fsc,nosharecache -o
context=system_u:object_r:root_t:s0 nfs-server:/exp1 /mnt1
mount -overs=4.1,fsc,nosharecache -o
context=system_u:object_r:root_t:s0 nfs-server:/exp2 /mnt2
* This should be fixed with dhowells patch [3]:
"[PATCH v5] vfs, security: Fix automount superblock LSM init
problem, preventing NFS sb sharing"
References
==========
[1] https://lore.kernel.org/linux-nfs/20230220134308.1193219-1-dwysocha@xxxxxxxxxx/
[2] https://lore.kernel.org/linux-nfs/20230216150701.3654894-1-dhowells@xxxxxxxxxx/T/#mf3807fa68fb6d495b87dde0d76b5237833a0cc81
[3] https://lore.kernel.org/linux-kernel/217595.1662033775@xxxxxxxxxxxxxxxxxxxxxx/
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