[Gluster-devel] Change in rebalance graph

[Raghavendra Gowdappa]

dht: Add readdir-ahead in rebalance graph if parallel-readdir is on
    
    Issue:
    The value of linkto xattr is generally the name of the dht's
    next subvol, this requires that the next subvol of dht is not
    changed for the life time of the volume. But with parallel
    readdir enabled, the readdir-ahead loaded below dht, is optional.
    The linkto xattr for first subvol, when:
    - parallel readdir is enabled : "<volname>-readdir-head-0"
    - plain distribute volume : "<volname>-client-0"
    - distribute replicate volume : "<volname>-afr-0"
    
    The value of linkto xattr is "<volname>-readdir-head-0" when
    parallel readdir is enabled, and is "<volname>-client-0" if
    its disabled. But the dht_lookup takes care of healing if it
    cannot identify which linkto subvol, the xattr points to.
    
    In dht_lookup_cbk, if linkto xattr is found to be "<volname>-client-0"
    and parallel readdir is enabled, then it cannot understand the
    value "<volname>-client-0" as it expects "<volname>-readdir-head-0".
    In that case, dht_lookup_everywhere is issued and then the linkto file
    is unlinked and recreated with the right linkto xattr. The issue is
    when parallel readdir is enabled, mount point accesses the file
    that is currently being migrated. Since rebalance process doesn't
    have parallel-readdir feature, it expects "<volname>-client-0"
    where as mount expects "<volname>-readdir-head-0". Thus at some point
    either the mount or rebalance will fail.
    
    Solution:
    Enable parallel-readdir for rebalance as well and then do not
    allow enabling/disabling parallel-readdir if rebalance is in
    progress.
    
    Change-Id: I241ab966bdd850e667f7768840540546f5289483
    BUG: 1436090
    Signed-off-by: Poornima G <pgurusid at redhat.com>
    Reviewed-on: https://review.gluster.org/17056
    Smoke: Gluster Build System <jenkins at build.gluster.org>
    NetBSD-regression: NetBSD Build System <jenkins at build.gluster.org>
    CentOS-regression: Gluster Build System <jenkins at build.gluster.org>


Patch: https://review.gluster.org/17056
owners: Me and Poornima

PS: readdir-ahead is loaded in rebalance graph on top of each subvolume of DHT.

regards,
Raghavendra