On Wed, Feb 13, 2019 at 11:16 AM Manoj Pillai <mpillai@xxxxxxxxxx> wrote:
On Wed, Feb 13, 2019 at 10:51 AM Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx> wrote:On Tue, Feb 12, 2019 at 5:38 PM Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx> wrote:All,We've found perf xlators io-cache and read-ahead not adding any performance improvement. At best read-ahead is redundant due to kernel read-aheadOne thing we are still figuring out is whether kernel read-ahead is tunable. From what we've explored, it _looks_ like (may not be entirely correct), ra is capped at 128KB. If that's the case, I am interested in few things:* Are there any realworld applications/usecases, which would benefit from larger read-ahead (Manoj says block devices can do ra of 4MB)?kernel read-ahead is adaptive but influenced by the read-ahead setting on the block device (/sys/block/<dev>/queue/read_ahead_kb), which can be tuned. For RHEL specifically, the default is 128KB (last I checked) but the default RHEL tuned-profile, throughput-performance, bumps that up to 4MB. It should be fairly easy to rig up a test where 4MB read-ahead on the block device gives better performance than 128KB read-ahead.
Thanks Manoj. To add to what Manoj said and give more context here, Glusterfs being a fuse-based fs is not exposed as a block device. So, that's the first problem of where/how to tune and I've listed other problems earlier.
-- Manoj* Is the limit on kernel ra tunable a hard one? IOW, what does it take to make it to do higher ra? If its difficult, can glusterfs read-ahead provide the expected performance improvement for these applications that would benefit from aggressive ra (as glusterfs can support larger ra sizes)?I am still inclined to prefer kernel ra as I think its more intelligent and can identify more sequential patterns than Glusterfs read-ahead [1][2].and at worst io-cache is degrading the performance for workloads that doesn't involve re-read. Given that VFS already have both these functionalities, I am proposing to have these two translators turned off by default for native fuse mounts.For non-native fuse mounts like gfapi (NFS-ganesha/samba) we can have these xlators on by having custom profiles. Comments?regards,Raghavendra
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