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

[Manoj Pillai]

On Wed, Feb 13, 2019 at 10:51 AM Raghavendra Gowdappa <rgowdapp at redhat.com>
wrote:

>
>
> 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)?
>

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.

-- 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].
> [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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