[Gluster-devel] Gluster Sharding and Geo-replication

[Aravinda]

Geo-replication and Sharding Team today discussed about the approach
to make Sharding aware Geo-replication. Details are as below

Participants: Aravinda, Kotresh, Krutika, Rahul Hinduja, Vijay Bellur

- Both Master and Slave Volumes should be Sharded Volumes with same
   configurations.
- In Changelog record changes related to Sharded files also. Just like
   any regular files.
- Sharding should allow Geo-rep to list/read/write Sharding internal
   Xattrs if Client PID is gsyncd(-1)
- Sharding should allow read/write of Sharded files(that is in .shards
   directory) if Client PID is GSYNCD
- Sharding should return actual file instead of returning the
   aggregated content when the Main file is requested(Client PID
   GSYNCD)

For example, a file f1 is created with GFID G1.

When the file grows it gets sharded into chunks(say 5 chunks).

     f1   G1
     .shards/G1.1   G2
     .shards/G1.2   G3
     .shards/G1.3   G4
     .shards/G1.4   G5

In Changelog, this is recorded as 5 different files as below

     CREATE G1 f1
     DATA G1
     META G1
     CREATE G2 PGS/G1.1
     DATA G2
     META G1
     CREATE G3 PGS/G1.2
     DATA G3
     META G1
     CREATE G4 PGS/G1.3
     DATA G4
     META G1
     CREATE G5 PGS/G1.4
     DATA G5
     META G1

Where PGS is GFID of .shards directory.

Geo-rep will create these files independently in Slave Volume and
syncs Xattrs of G1. Data can be read only when all the chunks are
synced to Slave Volume. Data can be read partially if main/first file
and some of the chunks synced to Slave.

Please add if I missed anything. C & S Welcome.

regards
Aravinda

On 08/11/2015 04:36 PM, Aravinda wrote:
> Hi,
>
> We are thinking different approaches to add support in Geo-replication 
> for Sharded Gluster Volumes[1]
>
> *Approach 1: Geo-rep: Sync Full file*
>    - In Changelog only record main file details in the same brick 
> where it is created
>    - Record as DATA in Changelog whenever any addition/changes to the 
> sharded file
>    - Geo-rep rsync will do checksum as a full file from mount and 
> syncs as new file
>    - Slave side sharding is managed by Slave Volume
> *Approach 2: Geo-rep: Sync sharded file separately*
>    - Geo-rep rsync will do checksum for sharded files only
>    - Geo-rep syncs each sharded files independently as new files
>    - [UNKNOWN] Sync internal xattrs(file size and block count) in the 
> main sharded file to Slave Volume to maintain the same state as in Master.
>    - Sharding translator to allow file creation under .shards dir for 
> gsyncd. that is Parent GFID is .shards directory
>    - If sharded files are modified during Geo-rep run may end up stale 
> data in Slave.
>    - Files on Slave Volume may not be readable unless all sharded 
> files sync to Slave(Each bricks in Master independently sync files to 
> slave)
>
> First approach looks more clean, but we have to analize the Rsync 
> checksum performance on big files(Sharded in backend, accessed as one 
> big file from rsync)
>
> Let us know your thoughts. Thanks
>
> Ref:
> [1] 
> http://www.gluster.org/community/documentation/index.php/Features/sharding-xlator
> -- 
> regards
> Aravinda
>
>
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