SAS disks -- 6g vs. 15k spindle speed

[Holt Satterfield]

I purchased an HP DL380 G6 along with 146GB 6G DP 10k drives. I already own an HP DL380 G5 with 72GB SP 15k drives. From what I've read, it seems that spindle speed (15K) trumps throughput speed (6G). But, I'm not sure. I'm thinking I should place the 72GB 15K drives on the G6 since I'm making it my database server and want better disk times. What's preferable? Thanks, Disk Masters.

[Patrick Terlisten]

Hello Holt,

the 6G SAS link is capable to transfer round about 600 MB/s. Each disk has it's own SAS link, either to a controller or SAS expander. You disk has a rotation speed of 15k UPM. Faster rotation results in more IOs per second. And IOs per second multiplied by the IO size results in MB/s. And MB/s is throughput. :) More disks = more IOs per Second = more throughput.

I don't think that changing the disks to models with 6G SAS interface will boost you DB server. I would prefer to add more disks to level the load over more disks. For example: Depending on the workload, four 72 GB 3G 15k SAS disks can be faster then two 146 GB 6G 10k SAS disks.

Hope this helps.

Regards,
Patrick

[Jan Soska]

Hello,
I would add a note:
10K 2,5" disk should have ~ the same performance as 15K 3,5" disk.

But - as stated above, more spindles means more IOPS...

Jan

[Cass Witkowski]

Unless you are accessing large data files in a serial manner the transfer speed does not matter much.

The transfer speed of reading the data physically off of the disk runs around 117 to 121 MB/S So having a connection bus that runs at 600 MB/s versus 300 MB/s May not buy you much. If your typical transfer size is 8KB then the time it takes to transfer data is insignificant to the rotational latentcy of the disk.

10K RPM has a rotation latentcy of 3 ms
15K RPM has a rotation latentcy of 2 ms

Transfering 8KB at 117 MB/s is .07 ms

So you can see that the RPM of the disk has two orders of magnitude effect on the I/O rate compared to transfer time.

What I haven't talked about is the other time involed ina disk I/O including head seek time.