Re: [PATCH md 0 of 4] Introduction

[Paul Clements <paul.clements@steeleye.com>]

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

-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html