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Re: Bad RAID1 read performance

From: Dimitri <dimitrik(dot)fr(at)gmail(dot)com>
To: "Albert Cervera Areny" <albert(at)sedifa(dot)com>
Cc: pgsql-performance(at)postgresql(dot)org, "Luke Lonergan" <llonergan(at)greenplum(dot)com>
Subject: Re: Bad RAID1 read performance
Date: 2007-05-30 18:19:56
Message-ID: 5482c80a0705301119x5543a151q44b029a9743c933e@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-performance
As there is no 'continuous space' option on ext3/ext2 (or probably "-f
fragment_size" may do a trick?) - I think after some filesystem
activity you simply loose continuous space allocation and rather
expected sequential reading may be transformed into random seeking of
'logically' sequentual blocks...

Rgds,
-Dimitri

On 5/30/07, Albert Cervera Areny <albert(at)sedifa(dot)com> wrote:
> Hardware isn't very good I believe, and it's about 2-3 years old, but the
> RAID
> is Linux software, and though not very good the difference between reading
> and writing should probably be greater... (?)
>
> Would you set 512Kb readahead on both drives and RAID? I tried various
> configurations and none seemed to make a big difference. It seemed correct
> to
> me to set 512kb per drive and 1024kb for md0.
>
> A Dimecres 30 Maig 2007 16:09, Luke Lonergan va escriure:
> > This sounds like a bad RAID controller - are you using a built-in hardware
> > RAID?  If so, you will likely want to use Linux software RAID instead.
> >
> > Also - you might want to try a 512KB readahead - I've found that is
> optimal
> > for RAID1 on some RAID controllers.
> >
> > - Luke
> >
> > On 5/30/07 2:35 AM, "Albert Cervera Areny" <albert(at)sedifa(dot)com> wrote:
> > > Hi,
> > > after doing the "dd" tests for a server we have at work I obtained:
> > > Read: 47.20 Mb/s
> > > Write: 39.82 Mb/s
> > > Some days ago read performance was around 20Mb/s due to no readahead in
> > > md0 so I modified it using hdparm. However, it seems to me that being it
> > > a RAID1 read speed could be much better. These are SATA disks with 3Gb
> of
> > > RAM so I did 'time bash -c "dd if=/dev/zero of=bigfile bs=8k
> count=786432
> > > && sync"'. File system is ext3 (if read many times in the list that XFS
> > > is faster), but I don't want to change the file system right now.
> > > Modifing the readahead from the current 1024k to 2048k doesn't make any
> > > difference. Are there any other tweaks I can make?
> > >
> > >
> > > ---------------------------(end of broadcast)---------------------------
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> > >
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> >
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>
> --
> Albert Cervera Areny
> Dept. Informàtica Sedifa, S.L.
>
> Av. Can Bordoll, 149
> 08202 - Sabadell (Barcelona)
> Tel. 93 715 51 11
> Fax. 93 715 51 12
>
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