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

[Keir Fraser <keir.xen@xxxxxxxxx>]

On 16/09/2012 11:37, "James Harper" <james.harper@xxxxxxxxxxxxxxxx> wrote:

>>> 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. ;)
>> 
> 
> So a sector-size of 4096 would basically be a 512e device, allowing the
> underlying OS to communicate in 512 byte blocks but knowing that things will
> work best in 4096 byte sized transfers aligned to multiples of 4096 bytes,
> right?

My recollection is that blkfront is required to submit only appropriately
-sized and -aligned requests; i.e. it's not merely advisory. I remember this
got added for CD-ROM support and if they had worked without this, I'm sure
we wouldn't have bothered!

 -- Keir

> James
> 



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