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Discussing snapshot features

In an earlier posting you explained why metadata snapshots have an impact on performance on the original volume, since write requests to the original volume must be "copied out" to the snapshot before the write is allowed. Isn't there any storage vendor that offers a snapshot feature based on a different principle such as one that acknowledges the write once it has reached the cache and only later proceeds to perform the additional operations while relying on dual-write or other, write-data-protection features?

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The answer to your question is yes, but I have not found this to be the case in software-based solutions. The HDS 9000 storage arrays use a cascaded snapshot approach where writes to the original volume is given an I/O complete back to the application immediately. The cascaded writes to the snapshot volume happen in ASYNC mode using dual writes from the cache in background mode. This alleviates many of the performance issues associated with writes to the primary volume when linked to an image copy. This is similar to metadata snapshooting, although there is an actual complete physical copy of the data. It's just as fast as creating a metadata copy of the disk from a storage pool but you also get the added benefit of RAID protected snapshots.

In the original question, I was speaking of metadata copy using software, not hardware.

Chris

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This was first published in November 2002

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