[Bugs] [Bug 1375959] Files not being opened with o_direct flag during random read operation ( Glusterfs 3.8.2)

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1375959



--- Comment #5 from Krutika Dhananjay <kdhananj at redhat.com> ---
(In reply to Pranith Kumar K from comment #3)
> Shekhar,
>    You need the following two options for O_DIRECT to be handled properly:
> 
> 1) performance.strict-o-direct on
> 2) network.remote-dio off
> 
> Pranith

@Pranith - Part of the problem seems to be that the anonymous fds created by
open-behind don't inherit the original open() flags. And when ob does wind
reads and writes on these anon fds, posix winds up using the usual
GF_ANON_FD_FLAGS (O_RDWR|O_LARGEFILE) value at the time of open() preceding the
invoked fd operation (mostly read/write).

FWIW, I did try the same test Shekhar ran on my test cluster and disabling
open-behind in addition to the two already known o-direct options that you
mentioned above, seemed to fix the issue of growing cache size as the test
progresses (although it didn't have any effect as per Shekhar when he tried the
same).

I am yet to confirm the theory (and the presence/absence of O_DIRECT flag at
the level of posix) through strace output of the bricks with and without
open-behind. I will do the same tomorrow and update the bug.

-Krutika

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