I was going to call this “size does matter” … but out of respect for the young lady who vets these posts I won’t go there
So this is all about the issue of choosing the right size drives to suit your performance requirements. Now a lot of people just choose the number of drives based on capacity, and what overall size they need, but there are a few considerations that should be looked at when it comes to size of drive and performance. I’ll break this into two sections – spinning media and SSD:
Spinning Media …
As a general statement (bound to cause me some grief), the performance of spinning media is pretty much the same across a family of drives, no matter what the size. If you look at something like Seagate’s Constellation 2 SATA drives, they quote seek, read and write speeds of exactly the same numbers for 250GB up to 2TB drives. So while they get bigger, they don’t get faster.
So why does this matter. Well, in spinning media RAID arrays, generally the number of spindles has a major impact on the performance of the array. For example, a mirror (2 drives of 6TB drives – 6GTB capacity) will not be anywhere near as fast in streaming data as 8 x 1TB drives in a RAID 5 (7TB capacity or thereabouts). The additional spindles in the RAID 5 allow for small reads and writes from each drive, speeding things up considerably.
To do some really rough maths on streaming data – if a drive does 120MB/sec read speed, then theoretically the best speed you can get from the mirror is 240MB/sec and from the RAID5 is would be 840MB/sec. That’s all in a perfect world of course, but you get the idea.
Of course, the RAID configuration matters and needs to suit the data type you are building for, but in general, with spinning media, you can say that more spindles equals more performance. Yes, there are power usage considerations, and cost considerations (though not a great deal), and those all need to be taken into account, but I’m talking about performance here, so stay focused on that side of the equation.
On the SSD side of the equation, there is in fact a big difference between the performance numbers of a small drive vs those of a large drive. Yes there are cost differences as well, but let’s look at the numbers …
Looking at Intel’s DC3500 SSD (a very, very good product imho), there is not a great deal of difference in the IOPs speed from the 80GB to the 800GB drives (70K to 75K respectively), but in the streaming speed there are some pretty dramatic differences. The 80GB drive claims a sequential read speed of 340MB/sec, while the 800GB drives claims a sequential read speed of 500MB/sec. The write speed difference is even more dramatic, with the 80GB drive writing at a claimed 100MB/sec, and the 800GB drive writing at a claimed 450MB/sec.
So why does this matter? Continue reading