From mboxrd@z Thu Jan  1 00:00:00 1970
From: Justin Piszcz <jpiszcz@lucidpixels.com>
Subject: Re: Help: very slow software RAID 5.
Date: Thu, 20 Sep 2007 04:25:44 -0400 (EDT)
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To: "Dean S. Messing" <deanm@sharplabs.com>
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On Wed, 19 Sep 2007, Dean S. Messing wrote:

>
> Justin Piszcz wrote:
> : Dean Messing wrote:
> : > Jusin Piszcz wrote:
> : > :
> <snip>
>
> : > I discovered your January correspondence to the list about this.  Yes,
> : > the read-ahead length makes a dramtic difference---for sequential data
> : > reading.  However, ...
>
> <snip>
>
> : > : Then re-benchmark.
> : >
> : > Large Read-ahead nearly double the speed of 'hdparm -t'.  (So does
> : > simply using the "--direct" flag.  Can you explain this?)  Also, in
> : > your opinion, is it wise to use such large read-aheads for a RAID 5
> : > intended for the use to which I plan to put it?
> :
>
> : > Did you try increasing the readahead and benchmarking?  You can set it
> : > back to what it was when you are done.
>
> Sorry if my earlier reply was not clear.
>
> Yes, I did increase r.a. (to 16384 and 32768) and re-ran hdparm each time.
> Result:
>
> r.a. =   512:  hdparm -t                -->  ~67 MB/s
> "   "    "    hdparm -t --direct       --> ~121 MB/s
>
> r.a. =  4096:  hdparm -t                --> ~106 MB/s
>
> r.a. =  8192:  hdparm -t                --> ~113 MB/s
>
> r.a. = 16384:  hdparm -t                --> ~120 MB/s
>
> r.a. = 32768:  hdparm -t                --> ~121 MB/s
>
>
> As I said:  it nearly doubled the speed give by 'hdparm -t'
>
> But (as I said) so did using the "--direct" flag, which inhibits
> "page cache" whatever that is.
>
> Also (as I asked) what is the downside?  From what I have read,
> random access reads will take a hit.  Is this correct?
>
> Thanks very much for your help!
>
> Dean
>
>

Hmm good question, in all of my benchmarking I made sure the majority of 
the benchmarks increased in overall speed with bonnie++ I did not notice 
any degradation myself-- although perhaps I was not benchmarking for that 
workload.

Justin.