If you are seeing iowaits that high then something needs to be done.

You CAN gain performance by striping, but the downtime due to disk failure can make your decision making skills look flawed.

For a mail server I highly recommend RAID10 and drives with high RPM so you get better random io performance as 99% of io on a mail home server will be random io and while regular SATA drives would be ok for a file server which is mostly sequential they will stack up io waits on a busy mail or database server. The raptors are costly because they run at 10k rpm and have low seek times, this gives them much better random io performance, but sequential io is the same with regular SATA drives. In fact sequential io performance is almost identical between 7200rpm SATA and 15000rpm SAS the real difference is in random io where the 15k drives are 2x faster (1.2MB of 4k random ios a second versus 640KB of 4k random ios a second, unbuffered).

If you have an external enclosure then that will help, but make sure the array is compatible with the drives you want. SATA disks cannot go into a SCSI array, though some SAS enclosures and controllers allow you to mix SAS and SATA drives (LSI is one).

Performance can trump size here so if the choice is between 200GB 7200 rpm SATA drives or 72GB 15000rpm SAS drives, if your data fits onto the 72GB drives (probably 140GB if in a 4 drive RAID10) go with the 72GB drives (taking growth into consideration too).

Hardware RAID can help too if you utilize onboard battery backed up write-back cache. The more write-back cache the better, just make sure it is battery backed up (BBU).

-Ross


-----Original Message-----
From: centos-bounces@centos.org <centos-bounces@centos.org>
To: CentOS mailing list <centos@centos.org>
Sent: Tue Nov 13 01:58:27 2007
Subject: Re: [CentOS] Need advice on storage



----- Message d'origine -----
De: Kenneth Price <kprice@nowyouknow.net>
Date: Lundi, Novembre 12, 2007 10:58 pm
Objet: Re: [CentOS] Need advice on storage
À: CentOS mailing list <centos@centos.org>
>
> You've already received a few responses about reconfiguring your
> entireserver, so I'll avoid that route and simply answer your
> question straight.
>
> "No"
>
> There will not be a "significant" performance boost striping
> only 2 drives.
> The 10K RPM Raptors alone will definitely increase performance,
> but there
> is little/no real advantage striping them together. Getting any
> significant performance increase from a RAID array really
> requires at
> *least* 4 drives ... the more the merrier ... although I'll
> probably catch
> some flack from that statement. :-)
>
> A previous poster mentioned increasing your RAM from 1Gb to 2Gb.
> That was
> an excellent idea and should be done in parallel with whatever drive
> changes you make.
>
> Regards,
> Ken
>
>

Thanks Ken.

That's a good thing to know that software striping won't really boost performance, i then won't waste money on that avenue.  I cannot explain why as i don't know about Linux drivers architecture and general system low level behavior but maybe somebody on this list could give us a clue.

As i said on my previous post, i found that the server is inside a rack that has a VTRAK 15100 SCSI to SATA Enclosure with a free SCSI bus (the unit has 2 independant bus) and hard disk slots.  I think we have a spare Adaptec 39160 so if it's the case, i could go with this solution and buy 4 x WD 250 Gigs "YS" (RAID) serie drives @ about 75$ each and put all this in RAID 10.  The unit supports NCQ.  It remains to be seen how well it can perform even if they are talking about "up to 200 MBps".

Regards,

Guy Boisvert

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