[Bugs] [Bug 1428061] New: Halo Replication feature for AFR translator

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1428061

            Bug ID: 1428061
           Summary: Halo Replication feature for AFR translator
           Product: GlusterFS
           Version: mainline
         Component: core
          Assignee: bugs at gluster.org
          Reporter: vbellur at redhat.com
                CC: bugs at gluster.org



Halo Replication feature for AFR translator

Summary:
Halo Geo-replication is a feature which allows Gluster or NFS clients to write
locally to their region (as defined by a latency "halo" or threshold if you
like), and have their writes asynchronously propagate from their origin to the
rest of the cluster.  Clients can also write synchronously to the cluster
simply by specifying a halo-latency which is very large (e.g. 10seconds) which
will include all bricks.

In other words, it allows clients to decide at mount time if they desire
synchronous or asynchronous IO into a cluster and the cluster can support both
of these modes to any number of clients simultaneously.

There are a few new volume options due to this feature:
  halo-shd-latency:  The threshold below which self-heal daemons will
  consider children (bricks) connected.

  halo-nfsd-latency: The threshold below which NFS daemons will consider
  children (bricks) connected.

  halo-latency: The threshold below which all other clients will
  consider children (bricks) connected.

  halo-min-replicas: The minimum number of replicas which are to
  be enforced regardless of latency specified in the above 3 options.
  If the number of children falls below this threshold the next
  best (chosen by latency) shall be swapped in.

New FUSE mount options:
  halo-latency & halo-min-replicas: As descripted above.

This feature combined with multi-threaded SHD support (D1271745) results in
some pretty cool geo-replication possibilities.

Operational Notes:
- Global consistency is gaurenteed for synchronous clients, this is provided by
the existing entry-locking mechanism.
- Asynchronous clients on the other hand and merely consistent to their region.
 Writes & deletes will be protected via entry-locks as usual preventing
concurrent writes into files which are undergoing replication.  Read operations
on the other hand should never block.
- Writes are allowed from _any_ region and propagated from the origin to all
other regions.  The take away from this is care should be taken to ensure
multiple writers do not write the same files resulting in a gfid split-brain
which will require resolution via split-brain policies (majority, mtime &
size).  Recommended method for preventing this is using the nfs-auth feature to
define which region for each share has RW permissions, tiers not in the origin
region should have RO perms.

TODO:
- Synchronous clients (including the SHD) should choose clients from their own
region as preferred sources for reads.  Most of the plumbing is in place for
this via the child_latency array.
- Better GFID split brain handling & better dent type split brain handling
(i.e. create a trash can and move the offending files into it).
- Tagging in addition to latency as a means of defining which children you wish
to synchronously write to

Test Plan:
- The usual suspects, clang, gcc w/ address sanitizer & valgrind
- Prove tests

Reviewers: jackl, dph, cjh, meyering

Reviewed By: meyering

Subscribers: ethanr

Differential Revision: https://phabricator.fb.com/D1272053

Tasks: 4117827

Change-Id: I694a9ab429722da538da171ec528406e77b5e6d1
Signed-off-by: Kevin Vigor <kvigor at fb.com>
Reviewed-on: http://review.gluster.org/16099
NetBSD-regression: NetBSD Build System <jenkins at build.gluster.org>
Smoke: Gluster Build System <jenkins at build.gluster.org>
CentOS-regression: Gluster Build System <jenkins at build.gluster.org>
Reviewed-by: Shreyas Siravara <sshreyas at fb.com>

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