[dm-devel] [Lsf-pc] [LSF/MM TOPIC] a few storage topics

[Jan Kara]

On Wed 25-01-12 14:35:52, Wu Fengguang wrote:
> On Tue, Jan 24, 2012 at 11:15:13PM -0700, Andreas Dilger wrote:
> > On 2012-01-24, at 8:29 PM, Wu Fengguang wrote:
> > > On Tue, Jan 24, 2012 at 09:39:36PM +0100, Jan Kara wrote:
> > >> On Tue 24-01-12 15:13:40, Jeff Moyer wrote:
> > >>>> Maybe 128 KB is a too small default these days but OTOH noone prevents you
> > >>>> from raising it (e.g. SLES uses 1 MB as a default).
> > >>> 
> > >>> For some reason, I thought it had been bumped to 512KB by default.  Must
> > >>> be that overactive imagination I have...  Anyway, if all of the distros
> > >>> start bumping the default, don't you think it's time to consider bumping
> > >>> it upstream, too?  I thought there was a lot of work put into not being
> > >>> too aggressive on readahead, so the downside of having a larger
> > >>> read_ahead_kb setting was fairly small.
> > >> 
> > >>  Yeah, I believe 512KB should be pretty safe these days except for
> > >> embedded world. OTOH average desktop user doesn't really care so it's
> > >> mostly servers with beefy storage that care... (note that I wrote we raised
> > >> the read_ahead_kb for SLES but not for openSUSE or SLED (desktop enterprise
> > >> distro)).
> > > 
> > > Maybe we don't need to care much about the embedded world when raising
> > > the default readahead size? Because even the current 128KB is too much
> > > for them, and I see Android setting the readahead size to 4KB...
> > > 
> > > Some time ago I posted a series for raising the default readahead size
> > > to 512KB. But I'm open to use 1MB now (shall we vote on it?).
> > 
> > I'm all in favour of 1MB (aligned) readahead.
> 
> 1MB readahead aligned to i*1MB boundaries? I like this idea. It will
> work well if the filesystems employ the same alignment rule for large
> files.
  Yeah. Clever filesystems (e.g. XFS) can be configured to align files e.g.
to raid stripes AFAIK so for them this could be worthwhile.

> > I think the embedded folks
> > already set enough CONFIG opts that we could trigger on one of those
> > (e.g. CONFIG_EMBEDDED) to avoid stepping on their toes.
> 
> Good point. We could add a configurable CONFIG_READAHEAD_KB=128 when
> CONFIG_EMBEDDED is selected.
  Sounds good.

								Honza
-- 
Jan Kara <jack at suse.cz>
SUSE Labs, CR