Re: raid0 slower than devices it is assembled of?
From: Adam Kropelin (akropel1_at_rochester.rr.com)
Date: 12/16/03
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Date: Tue, 16 Dec 2003 16:11:31 -0500 To: jw schultz <jw@pegasys.ws>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
On Mon, Dec 15, 2003 at 08:01:56PM -0800, jw schultz wrote:
> On Mon, Dec 15, 2003 at 02:34:54PM +0100, Witold Krecicki wrote:
> > Those are results of hdparm -tT on drives:
> > <cite>
> > /dev/md/1:
> > Timing buffer-cache reads: 128 MB in 0.40 seconds =323.28 MB/sec
> > Timing buffered disk reads: 64 MB in 1.75 seconds = 36.47 MB/sec
> > /dev/sda:
> > Timing buffer-cache reads: 128 MB in 0.41 seconds =309.23 MB/sec
> > Timing buffered disk reads: 64 MB in 1.46 seconds = 43.87 MB/sec
> > /dev/sdb:
> > Timing buffer-cache reads: 128 MB in 0.41 seconds =315.32 MB/sec
> > Timing buffered disk reads: 64 MB in 1.23 seconds = 52.04 MB/sec
> > </cite>
>
> No Linux [R]AID improves sequential performance. How would
> reading 65KB from two disks in alternation be faster than
> reading continuously from one disk?
Never say never:
/dev/sda:
Timing buffer-cache reads: 128 MB in 3.34 seconds = 38.38 MB/sec
Timing buffered disk reads: 64 MB in 8.60 seconds = 7.44 MB/sec
/dev/sdb:
Timing buffer-cache reads: 128 MB in 3.40 seconds = 37.64 MB/sec
Timing buffered disk reads: 64 MB in 8.60 seconds = 7.44 MB/sec
<...plus four more just like them...>
/dev/md0:
Timing buffer-cache reads: 128 MB in 3.35 seconds = 38.17 MB/sec
Timing buffered disk reads: 64 MB in 4.05 seconds = 15.79 MB/sec
md0 is a simple RAID0 of all six disks.
Yes, these disks are dirt slow to begin with (Andrew Morton once
mentioned he had pencils that wrote faster than my disks) but apparently
md manages to get some parallelism going, even for sequential reads.
(This is 2.6.0-test11-bk8, FWIW.)
--Adam
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- Previous message: edjard_at_ufam.edu.br: "Re: Re: Re: More questions about 2.6 /proc/meminfo was: (Mem: and Swap: lines in /proc/meminfo)"
- In reply to: jw schultz: "Re: raid0 slower than devices it is assembled of?"
- Next in thread: jw schultz: "Re: raid0 slower than devices it is assembled of?"
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