Re: Performance problem - reads slower than writes

Dave Chinner
2012-02-01T03:59:20Z

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@fromorbit.com