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Re: One SCSI disk or RAID0 on two SATA disks
Posted by: Scott Miller
Date: November 14, 2006 08:04PM

How much of a performance boost you get with RAID 0 depends on your data. With large blocks of data you can see a boost in transfer rate, but for small blocks there's little speedup. This is on a per-transfer basis, but with lots of transfers per second you'll always see a performance improvement if you have lots of disks rather than a single disk. You're spreading the transactions out across multiple disks.

When I was a StorageWorks (back when it was Compaq, and acquired via the DEC merger) I remember seeing a study between 10,000 and 15,000 RPM SCSI drives. There is a linear correlation between overall throughput and RPM. If you look at the drive specs you'll see they've sped up every spec, not just the RPM. This is partly due to the smaller diameter of the media--even though it's housed in a 3.5" form factor it's 2.5" media. Lower rotating mass and all means less power and noise.

SCSI drives are typically sold into the server and storage systems market while SATA/ATA drives are aimed at the desktop market. SCSI does command queuing and out of order execution, something which desktop machines don't really need.

So, more than double the RPM means less than half the rotational latency. Because of the shorter stroke length, lower moving mass of the smaller heads, and more samples per second with higher RPM means substantially quicker seek and settle times. Both translate to much quicker times finding the data. Higher RPM drives also mean quicker raw data transfer rates from the head/disk, but on a 15,000 RPM drive this might be limited by the R/W chip technology more than how tightly one can smoosh the bits together on the track. Raw bit transfer might not be double, but it will be quicker on the higher RPM drive. Transfer in and out of the on-drive cache will be quicker too as from generation to generation SCSI has always led ATA technology in interface transfer rate.

There are a variety of SCSI interfaces available--if you define SCSI as a command set. Of course there is parallel SCSI, but Fibre Chanel uses the SCSI command set as does iSCSI (SCSI over TCP/IP, only used between the computer and the storage system), and there's the new SAS or Serial Attached SCSI. SAS can be thought of SCSI's answer to SATA. I don't have any experience with SAS, but I understand a SAS controller can run both SAS and SATA disk drives! Sweet if you want to trade off high speed for high density and mix them both on the same machine.

Which brings us to the great thing about SATA and ATA, low cost and/or super data density. Not only are 10,000 RPM SCSI drives about 3x more expensive than 7,200 RPM SATA/ATA, but they hold about half as much data (15,000 RPM SCSI holds about half as much as 10,000 RPM SCSI because of the smaller internal form factor).

A trick you can try--but it's dependant on the drive manufacture and how they've laid out the tracks and cylinders--is to only partition about half the available disk. You should get a shorter stroke length and thus quicker seek and settle times (maybe a 25% improvement). Test this though.

My recommendation, if you really need the speed, is to forget RAID 0 on slow disks and go right for the 15,000 RPM SCSI disks, possibly with a SAS interface and controller. If you can throw a pile of small 15,000 RPM disks at your problem you will get higher performance--for the higher $$$$ though. I don't recommend Fibre Chanel for direct attached storage, it's scary expensive. It rocks for SANs though.

Just don't do RAID 5 (stupendously slow write times).


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Re: One SCSI disk or RAID0 on two SATA disks
November 14, 2006 08:04PM

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