[Bugs] [Bug 1084508] read-ahead not working if open-behind is turned on

bugzilla at redhat.com bugzilla at redhat.com
Tue Apr 26 13:27:15 UTC 2016


--- Comment #4 from Raghavendra G <rgowdapp at redhat.com> ---
(In reply to Raghavendra G from comment #3)
> I think the issue is because of open-behind using anonymous-fd. See the
> following option in open-behind:
>         { .key  = {"read-after-open"},
>           .type = GF_OPTION_TYPE_BOOL,
>           .default_value = "no",
> 	  .description = "read is sent only after actual open happens and real "
>           "fd is obtained, instead of doing on anonymous fd (similar to
> write)",
>         },
> The read-ahead cache is per-fd and stored in the context of fd. If
> open-behind is using anonymous fds for doing reads, read is never sent on
> the fd which read-ahead has seen (during open from application). So, there
> is no read-ahead cache.

This RCA is not valid. The reason is during read-request, fd is stored in local
and in response cache is stored on the fd stored in local. So, even though
open-behind sends read on anonymous fd, read-ahead stores the cache in the fd
passed to application/kernel.

> Can you retry the tests by setting option "read-after-open" in open-behind
> to "yes"?
> [root at unused glusterfs]# gluster volume set dist-rep
> performance.read-after-open on
> volume set: success
> [root at unused glusterfs]# gluster volume info
> Volume Name: dist-rep
> Type: Distributed-Replicate
> Volume ID: 201492ff-9eb8-48f9-a647-59b89853e3d3
> Status: Created
> Number of Bricks: 2 x 2 = 4
> Transport-type: tcp
> Bricks:
> Brick1: booradley:/home/export-2/dist-rep1
> Brick2: booradley:/home/export-2/dist-rep2
> Brick3: booradley:/home/export-2/dist-rep3
> Brick4: booradley:/home/export-2/dist-rep4
> Options Reconfigured:
> performance.read-after-open: on
> performance.readdir-ahead: on

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