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Date: Tue, 5 Jan 2010 20:18:01 +0000
From: Jamie Lokier <jamie@shareable.org>
Subject: Re: [Qemu-devel] Re: [PATCH v2] virtio-blk physical block size
Message-ID: <20100105201801.GK14376@shareable.org>
References: <1262018363-15871-1-git-send-email-avi@redhat.com>
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To: Avi Kivity <avi@redhat.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.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