How can I make my HW RAID reads faster? (request for FAQ)

[Ben Ruset]

is this something specific to XFS or would it work on RAID'ed ext3 file 
systems?

Sitsofe Wheeler wrote:
> Matthias Saou wrote:
> 
> (Matthias: Thanks for freshrpms - it's been very handy over the years)
> 
>> Looking for information on XFS filesystem options, I found this very
>> instructive thread :
>>
>> http://oss.sgi.com/archives/xfs/2007-09/msg00322.html
>>
>> I read about this simple change in one of the posts, which improved
>> performance a lot for the person making the benchmark, so I ran it :
>> # blockdev --setra 16384 /dev/sdb
>>
>> I now get 500-520MB/s read speed with hardware RAID5 where I used to
>> get 30-35MB/s... pretty amazing, I must admit.
>>
>> Looking at the man page, this just sets a read-ahead value... I'm
>> puzzled yet very happy!
>>
>> Would any of the others who reported slow read speeds with their PERC5s
>> and MD1000s care to also try it and report back to the list?
> 
> This small piece of wisdom/open secret for 2.6 kernels keeps having to be
> dug up over and over again (along with noatime
> (http://lwn.net/Articles/244829/ ) and stripe alignment
> (http://insights.oetiker.ch/linux/raidoptimization.html )). You can see me
> finding the similar results earlier last year on
> http://lists.us.dell.com/pipermail/linux-poweredge/2007-August/032471.html
> (the pity is that it doesn't seem to have helped anyone else). It would be
> good if this sort of thing made it into a FAQ because it is such a common
> tip (it should be the among the first thing that people try when running
> all the standard hardware RAID cards).
> 
> Obviously one needs to be aware of drawbacks of doing this (might slow down
> random I/O of lots of small files, focuses on one benchmark) but it is
> still handy to know and compared to other tips it is relatively easy to
> try.
>