Am 14.12.2011 12:47, schrieb Paolo Bonzini: > On 12/14/2011 12:13 PM, Kevin Wolf wrote: >> As we discussed before, the really interesting point here is defaults, >> and whatever you choose to do is wrong in some respect. >> >> So it looks like you chose to make the virtual device default to the >> host block size. > > ... wait wait, I default to 512. :) > > Here is the rationale. 512-over-4k may be slow, but is safe (but it is > not slow if you align partitions properly). 4k-over-512 is unsafe. So, > defaulting to 512 seemed the right thing after all.
Which means bounce buffers by default on 4k hosts. Is this going to become our next cache=writethrough? At some point 4k disks will be in wide use, but we'll still be stuck with a slow default of 512. No matter what we decide here, I think it might really be a good idea to save the block size in the image and use that as the default if nothing else is specified on the command line. Kevin
