[CentOS] Extremely slow disk transfers with SATA disks

Les Mikesell lesmikesell at gmail.com
Fri Mar 24 14:02:24 UTC 2006

On Fri, 2006-03-24 at 01:31, vanja at pobox.com wrote:

> So, that 200GB Maxtor has no problems writing (300MB file gets written in few seconds, when it's read from another 80GB SATA 
> disk), but when I read something from that disk - I get 2MB/sec. I have no trouble reading from 80GB SATA disk, so I guess 
> it will be cable/hardware related.

Writing is always buffered and unless you do some kind of
fsync you don't know when it is committed back to disk.  Your
timing will stop when it goes to the RAM buffer.  Reading
is buffered too, but the first read obviously has to wait
for the disk.

> I have no idea why hdparm reports 50MB/sec rates on 200GB disk, but I'll just assume that hdparm doesn't use "real life" way 
> of testing speed :)

Change your tests to use large enough samples that you
can't buffer a significant part in RAM.  Anyway the usual
problem with IDE/SATA vs. SCSI is not sustained transfer
in a single operation, it is that scsi will accept and
buffer multiple commands while most ide/sata drives
(the WD raptor being an exception) can only have a
single command waiting for completion - and with
IDE that is per controller.

  Les Mikesell
   lesmikesell at gmail.com

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