[Bugs] [Bug 1278565] samba-vfs-glusterfs reads all from a single brick in gluster replica

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1278565

Niels de Vos <ndevos at redhat.com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |atalur at redhat.com,
                   |                            |kdhananj at redhat.com,
                   |                            |pkarampu at redhat.com,
                   |                            |ravishankar at redhat.com
          Component|gluster-smb                 |replicate
           Assignee|ndevos at redhat.com           |bugs at gluster.org



--- Comment #2 from Niels de Vos <ndevos at redhat.com> ---
On a replicated volume READs are served by the brick that returned from a
LOOKUP first (decided by the AFR-xlator). To explain the reported behaviour, it
is important to know where the volume/share is mounted, and what server was
used for mounting.

For example, I guess you have a setup like this:
- 4 gluster storage servers
    - one of these storage servers exports the volume over Samba
- 1 client system (not on a storage server)

AFR takes care of the replication, and talks to the brick processes running on
all the storage servers. This is all done in the Gluster client side (fuse
mount, or vfs_glusterfs/libgfapi). When a file is opened for reading, a LOOKUP
is done as the first step, this is sent to all the bricks in the volume.

When mounting the Gluster volume over FUSE on a client system, AFR runs on the
client-side too. All storage servers are connected equally over a network.
Whichever storage server replies to the LOOKUP first, will be used to READ
from. The 1st replies will more or less come randomly from the different
storage servers, and the load that READ procedures cause are distributed
relatively evenly.

If the Gluster client (vfs_glusterfs/libgfapi) is running local on a Gluster
server, the brick on 'localhost' will most of the times be quickest in replying
to the LOOKUP. The other bricks are located over a network connection, and will
normally need more time to reply. You will see that in the "gluster volume
profile" output the server running Samba handles most of the READ requests.


I hope that this explains it well. Please let us know if my assumptions are
incorrect. The developers working on AFR have been added as CC on this bug and
they will be able to answer more details.

We might want to place a description like this in our documentation on
https://gluster.readthedocs.org/ too, but I'm leaving that for others to do
(feel free to copy/paste).

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