Peter T. Breuer wrote:
Neil - can you describe for me (us all?) what is meant by intentlogging here.
Since I wrote a lot of the code, I guess I'll try...
Well, I can guess - I suppose the driver marks the bitmap before a write (or group of writes) and unmarks it when they have completed successfully. Is that it?
Yes. It marks the bitmap before writing (actually queues up the bitmap and normal writes in bunches for the sake of performance). The code is actually (loosely) based on your original bitmap (fr1) code.
If so, how does it manage to mark what it is _going_ to do (without
psychic powers) on the disk bitmap?
That's actually fairly easy. The pages for the bitmap are locked in memory, so you just dirty the bits you want (which doesn't actually incur any I/O) and then when you're about to perform the normal writes, you flush the dirty bitmap pages to disk.
Once the writes are complete, a thread (we have the raid1d thread doing this) comes back along and flushes the (now clean) bitmap pages back to disk. If the pages get dirty again in the meantime (because of more I/O), we just leave them dirty and don't touch the disk.
Then resync would only deal with the marked blocks.
Right. It clears the bitmap once things are back in sync.
-- Paul
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