Wednesday, September 28, 2011

Hitachi Copy On Write (COW) Snapshot & Shadow Image

Logically, Copy On Write (COW) Snapshot & Shadow Image are the same thing to a host. It doesn't know if the volume being used is a V-VOL or an S-VOL.

The real difference is the impact it will have on the back end disks, space utilized and cache. With COW, the backup will hit the P-VOL as the V-VOL is only a pointer copy of the P-VOL. When an address is referenced on the V-VOL it is redirected back to the P-VOL, obviously since it's pointer based, the V-VOL is considerably smaller. With Shadow Image, the S-VOL is a full copy of the P-VOL, so as long as the pair is split, the P-VOL will remain relatively untouched. Being a full copy, the S-VOL and P-VOL are 1 to 1 from a capacity perspective.