Add design spec for geo-rep support for sharded volumes

Change-Id: I100823a6d96f225c5b2c96defaa79866d619cee7
Signed-off-by: Kotresh HR <[email protected]>
Reviewed-on: http://review.gluster.org/13788
Reviewed-by: Aravinda VK <[email protected]>
Reviewed-by: Niels de Vos <[email protected]>
Tested-by: Niels de Vos <[email protected]>

Author: kotreshhr

under_review/geo-rep-sharding-support.md

@@ -0,0 +1,157 @@
+Feature
+-------
+Support geo-replication for sharded volumes.
+
+Summary
+-------
+This features helps geo-replicate the large files stored on sharded volume. The
+requirement is that the slave volume also should be sharded.
+
+Owners
+------
+[Kotresh HR]([email protected])
+[Aravinda VK]([email protected])
+
+Current status
+--------------
+Traditionally changelog xlator, sitting just above the posix records the changes
+at brick level and geo-replication picks up these files that are
+modified/created and syncs them over gluster mount to slave. This works well as
+long as a file in gluster volume is represented by a single file at the brick
+level. But with the introduction of sharding in gluster, a file in gluster
+volume could be represented by multiple files at the brick level spawning
+different bricks. Hence the traditional way syncing files using changelog
+results in related files being synced as different files all together. So there
+has to be some understanding between geo-replication and sharding to tell all
+those sharded files are related. Hence this feature.
+
+Related Feature Requests and Bugs
+---------------------------------
+   1. [Mask sharding translator for geo-replication client](https://bugzilla.redhat.com/show_bug.cgi?id=1275972)
+   2. [All other related changes for geo-replication](https://bugzilla.redhat.com/show_bug.cgi?id=1284453)
+
+Detailed Description
+--------------------
+Sharding breaks the file into multiple small files based on agreed upon
+shard-size(usually 4MB, 64MB...) and helps distribute the one big file well
+across sub-volumes. Let's say 4MB is the shard size, the first 4MB of the file
+is saved with the actual filename, say file1. The next 4MB will be it's first
+shard with the filename <GFID>.1 and it follows. So shards will be saved as
+<GFID>.1, <GFID>.2, <GFID>.3.......<GFID>.n where GFID is the gfid of file1.
+
+The shard xlator is placed just above DHT on client stack, shard determines to
+which shard the write/read belongs to based on offset and gives specific
+<GFID>.n file to DHT. Each of the sharded files are stored under a special
+directory called ".shard" in respective sub-volumes as hashed by DHT.
+
+For more information on Gluster sharding please go through following links.
+   1. <https://gluster.readthedocs.org/en/release-3.7.0/Features/shard>
+   2. <http://blog.gluster.org/2015/12/introducing-shard-translator>
+   3. <http://blog.gluster.org/2015/12/sharding-what-next-2>
+
+To make geo-rep work with sharded files, we got two options.
+
+   1. Somehow record only the main gfid and bname on changes to any shard:
+         This would simplify the design but lacks performance as geo-rep has to
+         sync all the shards from single brick and rsync might take more time
+         calculating checksums to find out delta if shards of the file are
+         placed in different nodes by DHT.
+
+   2. Let geo-rep sync each mainfile and each sharded files separately:
+         This approach overcomes the performance issue but the solution needs
+         to be implemented carefully considering all the cases. As for this,
+         geo-rep client is given the access by sharding xlator to sync each
+         shards as different files, hence the rsync need not calculate
+         check-sums over  wire and sync the shard as if it's a single file.
+         The xattrs maintained by the main file to track the shard-size and
+         file-size is also synced. Here multiple bricks participate in syncing
+         the shard with respect to where the shard is hashed.
+
+   Keeping performance in mind, the second approach is chosen!!!
+
+So the key here is that sharding xlator is masked for geo-replication
+(gsyncd client). It syncs all the sharded files as separate files as if no
+sharding xlator is loaded. Since xattrs of the main file is also synced from
+master, while reading from non geo-rep clients from slave, the data is intact.
+It could be possible that geo-rep wouldn't have synced all the shards of a file
+from master, during which, it is expected to get inconsistent data as any way
+geo-rep is eventually consistent model.
+
+So this brings in certain prerequisite configurations:
+
+   1. If master is a sharded volume, slave also needs to be sharded volume.
+   2. Geo-rep sync-engine must be 'rsync'. tarssh is not supported for sharding
+      configuration.
+
+Benefit to GlusterFS
+--------------------
+The sharded volumes can be geo-replicated. The main use case is in the
+hyperconvergence scenario where the large VM images are stored in sharded
+gluster volumes and needs to be geo-replicated for disaster recovery.
+
+Scope
+-----
+#### Nature of proposed change
+No new translators are written as part of this feature.
+The modification spawns sharding, gfid-access translators
+and geo-replication.
+
+   1. <http://review.gluster.org/#/c/12438>
+   2. <http://review.gluster.org/#/c/12732>
+   3. <http://review.gluster.org/#/c/12729>
+   4. <http://review.gluster.org/#/c/12721>
+   5. <http://review.gluster.org/#/c/12731>
+   6. <http://review.gluster.org/#/c/13643>
+
+#### Implications on manageability
+No implication to manageability. Ther is no change in the way geo-replication
+is setup.
+
+#### Implications on presentation layer
+No implication to NFS/SAMBA/UFO/FUSE/libglusterfsclient
+
+#### Implications on persistence layer
+No implications to LVM/XFS/RHEL.
+
+#### Implications on 'GlusterFS' backend
+No implication to brick's data format, layout changes
+
+#### Modification to GlusterFS metadata
+No modifications to metatdata. No new extended attributes used,
+internal hidden files to keep the metadata
+
+#### Implications on 'glusterd'
+None
+
+How To Test
+-----------
+   1. Setup master gluster volume and enable sharding
+   2. Setup slave gluster volume and enable sharding
+   3. Create geo-replication session between master and slave volume.
+   4. Make sure geo-rep config 'use_tarssh' is set to false
+   5. Make sure geo-rep config 'sync_xattrs' is set to true
+   6. Start geo-replication
+   7. Write a large file greater than shard size and check for the same
+      on slave volume.
+
+User Experience
+---------------
+   Following configuration should be done
+   1. If master is a sharded volume, slave also needs to be sharded volume.
+   2. Geo-rep sync-engine must be 'rsync'. tarssh is not supported for sharding
+      configuration.
+   3. Geo-replication config option 'sync_xattrs' should be set to true.
+
+Dependencies
+------------
+No dependencies apart from the sharding feature:)
+
+Documentation
+-------------
+
+Status
+------
+Completed
+
+Comments and Discussion
+-----------------------