----- Message d'origine -----
De: "Ross S. W. Walker" <rwalker@medallion.com>
Date: Mardi, Novembre 13, 2007 9:12 am
Objet: Re: [CentOS] Need advice on storage
À: centos@centos.org

>
> 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
>

Hi Ross,

  Good technical explanation.  I'm finally planning to go RAID 10 on a VTrak 15100 (It has battery backup).  As i said in another post, i found the VTrak to have a free SCSI bus and free SATA slots.  This unit has 2 independant SCSI buses and uses SATA drives.  It remains to be seen if the controller is non blocking when using 2 servers.

  When we'll have the budget to replace the server, i'll probably go with something like a Tyan TA26 (S3992 ServerWorks chipset) with SAS 15K Drives and Adaptec 3405 if CentOS has good support for it.

  Communigate stores messages in mbox files so we have a lot of them (about 40-45 users with a huge Public folders holding all our projects infos).  The actual dataset is about 65 Gigs (and growing!). So the random seek is important.

Thanks for your comment!

Guy Boisvert