[Qemu-devel] Re: [PATCH +stable] block: don't attempt to merge overlapping requests

[Michael Tokarev]

18.05.2010 23:37, Stefan Hajnoczi wrote:
> I just caught up on mails and saw you had already mentioned that
> overlapping writes from the guest look fishy in the "the>1Tb block
> issue".  Cache mode might still be interesting because it affects how
> guest virtio-blk chooses queue ordering mode.

What I can say for sure is that the issue mentioned (>1Tb block)
occurs regardless of the cache mode.  I tried all 3, with exactly
the same results (well, not entirely exactly, since the prob
depends on timing too, and timing is different depending on the
cache mode), and performed all further tests with cache=writeback
since it's the mode which lets the guest to finish mkfs'ing the
1.5Tb "disk" in a reasonable time (or else it takes hours).

But actually I don't quite see where that dependency is: guest
does not know how its data is cached on host...

/mjt