[Gluster-devel] Disabling read-ahead and io-cache for native fuse mounts

[Raghavendra Gowdappa]

On Tue, Feb 12, 2019 at 5:38 PM Raghavendra Gowdappa <rgowdapp at redhat.com>
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-ahead
>

One 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)?
* 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].
[1] https://www.kernel.org/doc/ols/2007/ols2007v2-pages-273-284.pdf
[2] https://lwn.net/Articles/155510/

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?
>
> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1665029
>
> regards,
> Raghavendra
>
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