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Re: Performance problem - reads slower than writes

To: Brian Candler <brian@xxxxxxxxxxxxxx>
Subject: Re: Performance problem - reads slower than writes
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Wed, 1 Feb 2012 14:59:15 +1100
Cc: Christoph Hellwig <hch@xxxxxxxxxxxxx>, xfs@xxxxxxxxxxx
In-reply-to: <20120131215210.GB47420@xxxxxxxxxxxxxx>
References: <20120130220019.GA45782@xxxxxxxx> <20120131020508.GF9090@dastard> <20120131103126.GA46170@xxxxxxxx> <20120131145205.GA6607@xxxxxxxxxxxxx> <20120131215210.GB47420@xxxxxxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Tue, Jan 31, 2012 at 09:52:10PM +0000, Brian Candler wrote:
> On Tue, Jan 31, 2012 at 09:52:05AM -0500, Christoph Hellwig wrote:
> > You don't just read a single file at a time but multiple ones, don't
> > you?
> 
> It's sequential at the moment, although I'll do further tests with the -c
> (concurrency) option to bonnie++
> 
> > Try playing with the following tweaks to get larger I/O to the disk:
> > 
> >  a) make sure you use the noop or deadline elevators
> >  b) increase /sys/block/sdX/queue/max_sectors_kb from its low default
> >  c) dramatically increase 
> > /sys/devices/virtual/bdi/<major>:<minor>/read_ahead_kb
> 
> Thank you very much: I will do further tests with these.
> 
> Is the read_ahead_kb knob aware of file boundaries? That is, is there any
> risk that if I set it too large it would read useless blocks past the end of
> the file?

Yes, readahead only occurs within the file, and won't readahead past
EOF.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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