replication, snapshot, cluster questions

[Shrp]

I had a series of questions regarding NAS implementations. Any information you have or could point me to would be helpful. Thanks!

How does data replicate in various situations (clustering, disaster recovery, mirroring)? What network protocol, how often (and is this configurable), and is it block or file based?

What is done during a "snapshot"? How often is it reasonable to configure this being done?

This may differ among vendors, but typically with a clustered NAS, how is the data split? does each NAS contain identical copies of the data? or is it stored more like a striped volume?

 

[Shrp]

Also, does anyone have experiences with a NAS-like implementation using the hardware and OS they already had, without buying a separate product?

 

[RMGBELGIUM]

I can only speak about NetApp, but they have async replication ( over tcpip ), in which they transfer the modified blocks every certain interval (called SnapMirror ), and they have synchr replication ( which is called SyncMirror ).Replication is block based. During a snapshot, pointers are moved around, making sure that data is never overwritten, and changes to a block are always written to a new block. When restoring a previous snapshot, the pointers are simply repointed to the original blocks. The smalles granularity are hourly snapshots ( local snapshots ), and when you use the Async replication SnapMirror ( which uses snapshots to transfer the changed blocks ), the smallest interval is 5 minutes. The biggest problem with snapshots ( or any type of backup ) is garanteeing the Application consistency.You need to quiesce your application before a consistent snapshot or backup can be taken ( cfr Hot Backup for Oracle ).
Regarding the clustering part,NetApp has different solutions varying on the distance that is required between both storage controllers.Local cluster, stretched metro cluster (up to few hundred meters ), and fabric metro cluster ( multiple kilometers ).
Hope this helps a little ...

rgds,

R.

NetApp Certified NCDA/NCIE-SAN