Re: [Qemu-devel] Re: [PATCH v2] virtio-blk physical block size

[Jamie Lokier <jamie@shareable.org>]

Avi Kivity wrote:
> Physical block size is the what the logical block size would have been 
> is software didn't suck.  In theory they should be the same, but since 
> compatibility reaons clamp the logical block size to 512, they have to 
> differ.  A disk may have a physical block size of 4096 and emulate 
> logical block size of 512 on top of that using read-modify-write.
> 
> Or so I understand it.

I think that's right, but a side effect is that if you get a power
failure during the read-modify-write, bytes anywhere in 4096 sector
may be incorrect, so journalling (etc.) needs to use 4096 byte blocks
for data integrity, even though the drive emulates smaller writes.

-- Jamie