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From: Rusty Russell <rusty@rustcorp.com.au>
Date: Mon, 4 Jan 2010 13:38:51 +1030
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Subject: [Qemu-devel] Re: [PATCH v2] virtio-blk physical block size
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To: Avi Kivity <avi@redhat.com>
Cc: qemu-devel@nongnu.org, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org

On Tue, 29 Dec 2009 03:09:23 am Avi Kivity wrote:
> This patch adds a physical block size attribute to virtio disks,
> corresponding to /sys/devices/.../physical_block_size.  It is defined as
> the request alignment which will not trigger RMW cycles.  This can be
> important for modern disks which use 4K physical sectors (though they
> still support 512 logical sectors), and for file-backed disk images (which
> have both the underlying filesystem block size and their own allocation
> granularity to consider).
> 
> Installers use this to align partitions to physical block boundaries.
> 
> Note the spec already defined blk_size as the performance rather than
> minimum alignment.  However the driver interpreted this as the logical
> block size, so I updated the spec to match the driver assuming the driver
> predates the spec and that this is an error.

I thought this was what I was doing, but I have shown over and over that
I have no idea about block devices.

Our current driver treats BLK_SIZE as the logical and physical size (see
blk_queue_logical_block_size).

I have no idea what "logical" vs. "physical" actually means.  Anyone?  Most
importantly, is it some Linux-internal difference or a real I/O-visible
distinction?

Rusty.