Re: [Qemu-devel] [PATCH for-2.6 0/2] block/mirror: Fix mirroring with guest I/O load

[Kevin Wolf <kwolf@redhat.com>]

Am 20.04.2016 um 00:59 hat Max Reitz geschrieben:
> Bug report: https://bugs.launchpad.net/qemu/+bug/1570134
> 
> If you are doing a mirror operation (I just tested with block-commit on
> the active layer, but I guess any mirroring will do) while the guest has
> rather heavy I/O load (or light I/O also, you just need to be more
> unlucky) will lead to the cache of mirror's bitmap iterator becoming
> stale and not reflect all dirty bits which are set in the drive's dirty
> bitmap.
> 
> Generally, this isn't bad because we just restart over once we are
> through, and this will refresh the iterator's cache.
> 
> But it is bad for the code which tries to find a contiguous range of
> dirty chunks. This code needs to clear the bits in the iterator, so it
> invokes hbitmap_iter_next() for every contiguous dirty chunk found. But
> then it has to make sure that this actually cleared that chunk's dirty
> bit: And if the iterator's cache is stale, this may not be the case.
> Then, we run into a failed assertion.
> 
> But detecting this discrepancy is easy and refreshing the iterator's
> cache is too; and then, the assertion holds.
> 
> Besides this (patch 2), the code which is supposed to wait for
> overlapping in-flight requests on the first chunk of a dirty range is
> dead. I didn't produce any problems regarding that, but I'm sure it's
> not good. Patch 1 fixes that.

Thanks, applied to the block branch.

Kevin