Messages in this thread | | | Subject | Re: ext3 allocate-with-reservation latencies | From | "Stephen C. Tweedie" <> | Date | Wed, 06 Apr 2005 19:22:12 +0100 |
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Hi,
On Wed, 2005-04-06 at 17:53, Mingming Cao wrote:
> > Possible, but not necessarily nice. If you've got a nearly-full disk, > > most bits will be already allocated. As you scan the bitmaps, it may > > take quite a while to find a free bit; do you really want to (a) lock > > the whole block group with a temporary window just to do the scan, or > > (b) keep allocating multiple smaller windows until you finally find a > > free bit? The former is bad for concurrency if you have multiple tasks > > trying to allocate nearby on disk --- you'll force them into different > > block groups. The latter is high overhead.
> I am not quite understand what you mean about (a). In this proposal, we > will drop the lock before the scan.
s/lock/reserve/.
> And for (b), maybe I did not make myself clear: I am not proposing to > keeping allocating multiple smaller windows until finally find a free > bit. I mean, we book the window(just link the node into the tree) before > we drop the lock, if there is no free bit inside that window, we will go > back search for another window(call find_next_reserveable_window()), > inside it, we will remove the temporary window we just created and find > next window. SO we only have one temporary window at a time.
And that's the problem. Either we create small temporary windows, in which case we may end up thrashing through vast numbers of them before we find a bit that's available --- very expensive as the disk gets full --- or we use large windows but get worse layout when there are parallel allocators going on.
--Stephen
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