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Re: [Qemu-devel] Re: [PATCH v2] virtio-blk physical block size
From: |
Jamie Lokier |
Subject: |
Re: [Qemu-devel] Re: [PATCH v2] virtio-blk physical block size |
Date: |
Tue, 5 Jan 2010 20:18:01 +0000 |
User-agent: |
Mutt/1.5.13 (2006-08-11) |
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