[K12OSN] SATA vs. SCSI

Chris Kacoroski ckacoroski at nsd.org
Mon Sep 20 18:17:29 UTC 2004



Ken Meyer wrote:

> 1) RAID 0 is a very bad deal.  It does not provide very much performance
> increase (apparently in part due to the caching algorithms embedded in the
> typical drive's firmware), AND you have doubled your jeopardy because the
> failure of EITHER drive will eat your lunch (See Comment One below, copied
> from the S/R site untouched by human-driven keyboards).
> 
Agreed.  I only use this for applications that need lots of very fast 
tmp disk space.  Most folks use Raid 1 (mirrors).


> 2) The superiority of SCSI, given similar physical characteristics (RPM, bit
> density, et al) of the drives, has been most evident in servers having
> multi-user, random access to many small files, and that this advantage has
> been due to "elevator queuing" in which the drive actively manages the order
> of retrieving data.  ATA drives never implemented this feature because the
> advantage in single-user system was small or even negative.  However, SATA
> drives are encroaching on this capability, called TCQ (Tagged Command
> Queuing) by WD and generically, and NCQ (Native...) by Maxtor and Seagate.
SATA should have this in a year or so.

> 3) A question.  It has been my understanding that writing data to, say, a
> RAID 5 array was SLOWER than to a single drive, since space had to be
> allocated for pieces to be put on each of the drives, but that reading might
> be faster (ATA or SCSI, regardless of TCQ or not).  So, does one get better
> performance by putting all drives in an independent configuration, maybe as
> a single volume, than as a RAID array -- reliability and the "redundancy
> tax" aside?
On older arrays there definitely is a RAID 5 write hit.  On many of the 
newer ones (e.g. Seek Systems, EonStor) they get around this by caching 
tricks such as immediately writing all data to a temp location on the 
raid and then when they get an entire stripe of data, rewriting it 
during a pause properly to the Raid5, etc.  I am now using Seek Raid 5 
arrays to support enterprise oracle DB as they are a lot cheaper than 
mirroring everything (the traditional way).

cheers,

ski

-- 
"When we try to pick out anything by itself, we find it
   connected to the entire universe"		John Muir

Chris "Ski" Kacoroski, ckacoroski at nsd.org, 425-489-6263





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