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

[Christoph Hellwig <hch@lst.de>]

On Mon, Jan 04, 2010 at 01:38:51PM +1030, Rusty Russell wrote:
> 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?

Those should be the same for any sane interface.  They are for classical
disk devices with larger block sizes (MO, s390 dasd) and also for the
now appearing 4k sector scsi disks.  But in the ide world people are
concerned about dos/window legacy compatiblity so they came up with a
nasty hack:

 - there is a physical block size as used by the disk internally
   (4k initially)
 - all the interfaces to the operating system still happen in the
   traditional 512 byte blocks to not break any existing assumptions
 - to make sure modern operating systems can optimize for the larger
   physical sectors the disks expose this size, too.
 - even worse disks can also have alignment hacks for the traditional
   DOS partitions tables, so that the 512 byte block zero might even
   have an offset into the first larger physical block.  This is also
   exposed in the ATA identify information.

All in all I don't think this mess is a good idea to replicate in
virtio.  Virtio by defintion requires virtualization aware guests, so we
should just follow the SCSI way of larger real block sizes here.

> 
> Rusty.
> 
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