[LU-9252] osd: Cannot really disable cache on OSS

• Type: Improvement
• Resolution: Duplicate
• Priority: Minor
• Labels: performance
• Severity: 3
• Assignee: Peter Jones
• Reporter: CEA

Description

When deactivating caching on an OSS through the osd proc files: read_cache_enable and writethrough_cache_enable, it only simulates Lustre's behaviour without cache.

It would be interesting though, for Lustre's OSSes to be able to allocate their memory outside of the pagecache for it seems to be a performance issues when using SSD disks: it creates a CPU overload and 65% of CPU ressources are spent to allocate/free memory from the pagecache.

Issue Links

• is related to: LU-11347 Do not use pagecache for SSD I/O when read/write cache are disabled

Comments

[Andreas Dilger]

This was fixed via LU-11347 to preallocate IO buffers in this case.

[Andreas Dilger]

To clarify your concern, the issue is that even with read/write cache disabled the OSS is still adding the pages to the mapping for the inode and then dropping it after the read/write is complete.

It should be possible to change this caching behaviour for osd-ldiskfs to preallocate IO buffers per thread, in a manner similar to what Lustre 1.8 did, but I don't think it would be possible to do this for osd-zfs.