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Eliminate/Minimize Disk Queue Length - SATA vs SATA II III and RAID0

February 28, 2013 3:49:24 PM


My goal is to select drive options for a new PC build, that will eliminate disk queue length. Or minimize it to 2 or less.

My current single 7200rpm SATA I 1.5GB drive had a disk queue length of 20. I am building a new PC, and want the drive/RAID to fast enough to avoid disk queue length for the same situation, or keep it to 2 or less.

The disk queue happened during writing very small sizes of data, at a very high frequency (for a stock charting program - writing tiny pieces of data, but at a rapid rate.). So a large bandwidth is not needed, but writing speed is critical. Reading speed is not as important.
So, from what I have read, the priority is on the highest possible "random write" speed.

Here's all the possible relevant info, via Piriform Speccy, for the system that had the 20 disk queue length.:
Drive is Hitachi HTS722016K9SA00. 16 heads. 16383 cylinders. 48-bit LBA. Sata II 3GB. Transfer mode SATA I
Attached to a XP Pro 32-bit machine, with Core2Duo P8400 (2.26Ghz, 4MB, 45nm), running 1066MHz ram. In a Thinkpad T500 20283HU.

New PC will have:
Win7 H.P. 64-bit, i5 3570k (quad 3.4Ghz, 6MB, 22nm), running 1600Mhz ram, on a Z75/Z77 board. The i5 may be overclocked to 4.0Ghz. (The i5 will allow 1600ram speeds, and allow 6GB SATA III speeds)

1) - Given the same exact circumstances that created the disk queue length of 20 for the 'Transfer Mode SATA I' drive described..
What approximate MB/s random write speed could keep the queue under 2?

I'm trying to figure this out myself also.
Looking for a general "random write" speed (MB/s) for a SATA1 1.5GB 7200rpm drive. Once I find that I can compare.

-a 120 SATA III 6GB SSD can have a random write speed of 150MB/s or 72,000 IOPS
-a 120GB SATA II 3GB SSD can reach a random write of ??? MB/s or 50,000 IOPS
-my 120GB SATA 1.5GB shows a random write speed of 1 MB/s
(Using CrystalDiskMark 3 - settings 4k QueueDepth of 32) (4k is at 1 MB/s,, and 512k is 25 MB/s)

So a 120GB SATA III 6GB SSD, can process tiny-sized random writes in less than 100th of the time that my 120GB SATA1 1.5GB can? And even at 512k sizes, random writes would be completed in about 1/6 of the time?
Is this right??

Also, I read that SSDs are parallel devices and can I/O up to 10 items (I/Os) at a time (some models have less)... whereas a rotational drive can only I/O one at a time. This aspect alone would half the Disk Queue Length of 20 that my 7200rpm drive experienced.
The OWC Mercury Pro SSD 120GB has 16 channels.
This article compares real-world IOPS of 7200rpm, 15000rpm, and a SATA II 3GB SSD, with a graph of the results.
The 120GB SATA 3GB performed 44 times as many I/Os per seconds, compared to the fastest 7200rpm drive. 81,000 compared to 184.
(based on transfer request size of 8 kB, 35/65% of random/sequential distribution; 65%/35% of read/write distribution and 64 outstanding IOps).

Can anyone who understands drive stats tell me if the general numbers I'm using above are actually correct?