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

[Adi Kriegisch <adi@cg.tuwien.ac.at>]

On Mon, Feb 28, 2011 at 10:55:12AM +0000, Jan Beulich wrote:
> >>> On 28.02.11 at 11:06, Ian Campbell <Ian.Campbell@citrix.com> 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 about
that on the kernel mailing lists[1] and there is an interesting document about
that available from Mike Snitzer[2].
Those values are important for 4K block size drives, for SSDs and -- as in my
case -- for RAID levels with checksums.
A quick test with a samba server installed in Dom0 revealed that those
values do not need to be honoured by Samba to get full write speed. I/O
scheduler seems to be the one that needs those values.

-- Adi

[1] http://marc.info/?l=linux-ide&m=124058535512850&w=4
[2] http://people.redhat.com/msnitzer/docs/io-limits.txt