[Gluster-users] read-subvolume

Allan Latham alatham at flexsys-group.de
Thu Jul 4 11:10:34 UTC 2013

Hi all

Does anyone use read-subvolume?

Has anyone tested read-subvolume?

Does read-subvolume work in such a way that if the file is present on
the local node the local copy is used rather than a remote one?

Alternatively is there any way to configure (or patch) gluster to always
prefer the local file?

I have read everything available and have found no answer.

Unison works very well in our environment but is not real time and needs
to be run every few minutes and/or be kicked off with inotify.

If I could get gluster to always read the local copy it would be a much
better drop in replacement.

This is a small scale deployment not a massive cluster but I can imagine
there are many potential users of gluster in this mode. It should beat
unison and similar solutions in every way - but it doesn't because it is
reading from the network even when it has a local up-to-date copy. This
can't be intended behaviour.

So what have I configured wrong?

Thanks in advance


On 02/07/13 13:38, Allan Latham wrote:
> Hi everyone
> I have installed 3.3.1-1 from the Debian repository you provide.
> I am using a simple 2 node cluster and running in replication mode. The
> connection between the nodes is limited to 100MB/sec (that's bits not
> bytes!). Usage will be mainly for read access and since there is always
> a local copy available [ exactly 2 replicas on exactly 2 machines ] I
> expect very fast read performance. Writes are low volume and very
> infrequent - performance is not an issue.
> Almost everything works as I would expect.
> Write speed is limited to 10Mb (bytes) per second which is what I would
> expect and is adequate for the application.
> But read speed is either super fast or 10Mb/sec. i.e. read operations
> take place on the local copy or the remote seemingly at random.
> This not the 'small files problem'. I am aware that Gluster must use
> network access for stat() etc. This is all about where the data comes
> from on a read(). If I do an m5dum on a 200Mb file it takes either half
> a second or 18 seconds.
> There is an option read-subvolume.
> I have tried to understand how this works from the documentation
> available and from the few examples on the web.
> I have added the option using:
> gluster volume set X read-subvolume Y
> It has no effect even after stopping and starting the volume,
> remounting, restarting gluster servers etc.
> What's more I fail to see how this option could ever work at all. The
> configuration changes caused by the above command are rolled out to both
> nodes - but what is right for one node is exactly the wrong
> configuration for the other node.
> Configs attached are in /var/lib/glusterd/vols/shared except
> glusterd.vol which is in /etc/glusterfs.
> Here is the output of the mount command filtered to just the glusterfs
> mount:
> on /gluster/rw/shared type fuse.glusterfs
> (rw,relatime,user_id=0,group_id=0,default_permissions,allow_other,max_read=131072)
> is local to this host.
> I would be very thankful if someone can enlighten me. I am obviously
> configuring this wrong. I may have missed something important.
> Best regards to all
> Allan
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