2.5" drive performance specs

Garret,

If you compare 10k/2.5" vs 15k/3.5" stats from common drive manufacturer's spec sheets, the OEMs also rate 15k/3.5" drives with better performance specs.

There are two primary metrics to look at:

• Avg. Latency: Time to rotate the drive halfway. The rotational speed of the drive (e.g 10k/15k) is the primary factor for this metric.
• Avg. Seek time: Time to move the head a third of the way across the platter.

When you add these two times together, you get the Service Time for the drive -- that is, the average time (in ms) it will take for that drive to complete a single, random small-block IO, with no drive queueing. From the service time (amount of time required for a single IO), we can estimate the number of IO's that the drive can do in a second -- the "rule of thumb" IOPS for that drive.

Consider the 10k/2.5" and 15k/3.5" drives from HGST; you can find their specs at these links:

10k/2.5" -- Ultrastar C10K600 (SAS Hard Drive) | HGST Storage

15k/3.5" -- Ultrastar 15K600 (SAS Hard Drive) | HGST Storage

The 10k/2.5" drive has a latency of 3ms and a seek time of 3.7ms -- for a total service time of 6.7ms.

The 15k/3.5" drive has a latency of 2ms and a seek time of 3.4ms -- for a total service time of 5.4ms.

We can derive the rule of thumb IOPS per drive by dividing 1 second (1000ms) by the service time of the drive.

For the 10k/2.5" drive -- 1000 / 6.7ms = ~150 IOPS

For the 15k/3.5" drive -- 1000 / 5.4ms = ~185 IOPS

Both drives can certainly do more than this, but it will mean that IOs are queuing at the drives, which will result in elongated response times.

You also mentioned that you're comparing a config with 10k/2.5" vs another config with larger 15k/3.5" drives. The fact that the 3.5" drives are larger (capacity-wise) means that, per drive, the IO Density (IOPS per GB) is actually lower on the 15k drives.

If we take those same rule of thumb IOPS above, and calculate the IO density:

For a 300GB/10k/2.5" drive -- 150 / 300 = 0.5 IOPS per GB

For a 600GB/15k/3.5" drive -- 180 / 600 = 0.3 IOPS per GB

So if these two configs you're comparing have the same usable capacity available in the FC/SAS tier, then the 10k tier actually has more performance capability than the 15k tier.

Personally... I'm a fan of 10k/2.5" drives, and I think we'll see their use continue to increase.

Hope that helps...

- Sean

Sean, great stuff... thanks

Thanks for the response Sean.

In our configs we have all 10K drives so we are comparing a 2.5" 10K to 3.5" 10K for the SAS tier.  There would also be the comparison of 2.5" 7200 NL-SAS vs 3.5" 7200 SATA for the SATA tier.  Obviously our configurations are quite different to account for the max size of 1TB 2.5" 7200 vs 2TB SATA drives.  I definitely agree with you that I like 10K 2.5" drives but I was confused as the specs of a 2.5" 10K and a 3.5" 10K were listed at the same.

Somehow the overall price per TB comes out to about the same.  It seems like a no brainer to go with the 2.5" since you can get a larger max configuration of 3200 drives compared to 1920 3.5" drives.  Even with the smaller 1TB drives the maximum capacity is larger due to the ammount of additional 2.5" drives that can be used.

Interesting.  I have seen that before with Dell but wasn't aware that EMC did that on their VMAX's at all since EMC is pretty strict on what drives they use.