Re: [Xen-devel] bug when using 4K sectors?

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 16/09/2012 08:00, "Joseph Glanville" <joseph.glanville@xxxxxxxxxxxxxx>
wrote:

> After reading through blkback it appears that it can only support 512
> byte sector sizes and removing this limitation would take quite abit
> of work.
> It uses hard coded bitshifts pervasively to convert between number of
> requests/pages and size of sectors etc. (that is all the >> 9
> everywhere)
> 
> I am going to see what I can about working on getting it to support 4k
> sectors too and eventually uncoupled logical/physical sizes but that
> would take even more work as far as I can tell.
> 
> Being able to use 4k sectors seems like it would provide pretty
> massive gains in performance just by being more efficient let alone
> increasing byte aligned writes to the underlying block storage system.

The PV blk transport may be based on 512-byte sectors, but the real sector
size is communicated between blkfront and blkback via xenbus (field
'sector-size') and blkfront is expected to only make requests that are
multiple of, and aligned according to, that real 'sector-size'.

I would kind of expect it to work, as CD-ROMs have a larger sector size (2kB
IIRC) and we support those...

Bashing your head against the PV blk transport code may be premature. ;)

 -- Keir



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