The Write On An Origin Of A Snapshot

Sending I/O to a device for which we have a snapshot:

• If the start sector of the I/O belongs to a chunk that is found in the exceptions table, just perform a normal write to the origin volume.
• Otherwise, if there's a pending copy of this chunk, place the I/O on the chunk's queue.
• Otherwise (this is the first write for this chunk), look for a free entry in the exceptions table.
• If there were no free entries, mark this snapshot as invalid (since it is full), and return an I/O error.
• If there was a free entry, assign it to our chunk, and send a copy request to the kcopyd kernel-daemon.
• When the copy was complete, update the entry in the exceptions table, perform the original I/O and all other I/Os that might have gotten queued for this chunk.

Originally written by guy keren