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


On Tue, 2012-01-24 at 23:15 -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.  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.  It would also be
> possible to trigger on the size of the device so that the 32MB USB stick
> doesn't sit busy for a minute with readahead that is useless.
> Cheers, Andreas

If the reason for not setting a larger readahead value is just that it
might increase memory pressure and thus decrease performance, is it
possible to use a suitable metric from the VM in order to set the value
automatically according to circumstances?


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