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PIT - point in time copy

Posted on 2012-03-12
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Last Modified: 2012-08-14
Hi,

How does PIT works? For an example if I have source volume and target voume, how does PIT work? First it copies the data from source to target, and then monitor the changes and once there is a change, it copies the data?
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Question by:mokkan
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by:woolmilkporc
woolmilkporc earned 500 total points
ID: 37713592
There are many implementations of PIT copy, such as "split mirror", "snapshot", "flash copy"  etc.

This is a Whitepaper from the 2002 Storage Conference outlining the common approaches and giving some insight into their functional principles:

http://www.storageconference.org/2002/papers/d05bp-aaz.pdf

"2002" seems a long time ago, but the basic concepts mentioned in the paper are still valid (and in use) nonetheless.

wmp
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by:mokkan
ID: 37720344
Thank you, I read it but not getting the full concept. It is basically keep track of the changes  perdiatically?
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by:woolmilkporc
woolmilkporc earned 500 total points
ID: 37720380
It always keeps track of all changes, because every I/O must be intercepted.

Depending on the implementation changes are recorded in a flash area while the original keeps unchanged, or the changes are applied to the original while a "before image" of the original is copied to the flash area.
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by:mokkan
ID: 37720435
For an example if the data  corrupted and if I want to decide the restore data, can I get pick a time?
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woolmilkporc earned 500 total points
ID: 37720475
Not with the mentioned flash/snapshot technologies.
You must decide whether to use the unchanged original originating from the time where the snapshot was initiated, or to use the updated copy.
Of course you can initiate several snapshots periodically, so you can decide which one is still good and which one is corrupted.

It seems what you're talking about is rather some kind of asynchronous (delayed) replication, where you can stop the replication in a certain time window once you noticed that corruption has occurred. You can then continue using the replica, which is still good given you actually hit the time window to stop the replication.
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