Re: [Xen-devel] blk[front|back] does not hand over disk parameters

["Jan Beulich" <JBeulich@xxxxxxxxxx>]

>>> On 28.02.11 at 12:54, Adi Kriegisch <adi@xxxxxxxxxxxxxxx> wrote:
> On Mon, Feb 28, 2011 at 10:55:12AM +0000, Jan Beulich wrote:
>> >>> On 28.02.11 at 11:06, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> [SNIP]
>> > It should be trivial to add this in a compatible manner since the
>> > frontend can just do what it does today if the nodes are missing and the
>> > backend wouldn't rely on the frontend doing anything useful with the
>> > information anyway.
>> 
>> Am I right in understanding that these numbers aren't used by
>> the block layer itself at all, but just get provided to userspace for
>> whatever optimization it can do? In that case, I can't really see
>> how passing through these values can really help general
>> performance (i.e. for apps not paying attention to these values).
> AFAIK these values are used by mkfs.* in userspace and by the I/O Schedulers
> in kernel space to optimize performance. There has been some discussions 

I grepped for io_min and a couple of derived variables (like
alignment_offset) but couldn't spot any I/O-relevant readers
under block/.

> about
> that on the kernel mailing lists[1] and there is an interesting document 
> about
> that available from Mike Snitzer[2].

I'll take a look at those.

Jan


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