An ATA drive executes a bunch of commands in the order it gets them, which can be slow if it needs to write data on an inside track, then read from and outside track, and back and forth until the both the read and write requests are finished.
SCSI will understand that there is a proximity benefit to the commands, so it will reorder the interlaced requests and execute one before the other. Essentially putting part of one request on hold while it does the other. That's an oversimplification of the algorithm of course. Especially since a server system will probably have many more the two interlaced requests.
Did you ever try to clean two rooms at once? Put the clothes away in one, then the other. Make the bed in one, then the other. Lots of useless travel time in there. You would probably clean one and then the other. Except if you are vacuuming, then you would probably vacuum both at once, interlacing two similar actions.
SATA is catching up though. Seagate has release SATA drives that have command queueing, but I don't know how it compares to SCSI.
On Jul 20, 2004, at 10:40 AM, Jan Kirchhoff wrote:
But shouldn't a sata-based RAID10 with 8 discs do job as well? writes would be spread on 4 discs...
Has anybody experience with those external SCSI-to-SATA RAIDs?
A SCSI-solution would cost twice as much, but would it really speed things up compared to a massive use of parallel (raid0) sata-discs?
I know disc i/o is the bottleneck in our case, of course we want the fastest disc/raid-system we can possibly get for our money.
Is our thinking too simple or shouldn't it be possible to reach the speed of fast scsi-discs by simply taking 2-3 fast sata-discs in a hardware raid0?
Our goal is a raid10, so reading should be even faster.
-- Brent Baisley Systems Architect Landover Associates, Inc. Search & Advisory Services for Advanced Technology Environments p: 212.759.6400/800.759.0577
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