You did what with a mirror?

August 11, 2011

In days gone by if someone mixed a SAS (fast) drive with a SATA (slow) drive in a mirror us tech guys would generally smile to ourselves, pat the customer comfortingly on the head and make for the nearest exit as quickly as possible muttering under our breath.

It did not make sense to mix fast and slow drives into a RAID 1 because both reads and writes were pretty much limited to the performance of the slowest drive and you had seriously wasted your money on an expensive drive.

So along comes SSD (and big ones at that). Now in MLC format they are getting bigger and somewhat cheaper, and they generally have very good read speeds. Their write speeds are nothing to “write” home about (pun intended) but in a workstation environment or small server you can live with that.

Now Adaptec’s engineers have either had an epihpeny or a complete brain fade … I thought the latter until I looked a little closer at a new innovation in our latest firmwares for series 2, 5 and 6 series controllers.

A hybrid mirror of SSD and Sata hard drive … (or SAS hard drive for that matter)

Now how does that work?

Pretty simply really (in fact most good ideas as simple ones). Data is written to both drives as per a normal mirror. Because the SSD is at least as fast or faster than it’s partner SATA drive the user loses nothing in write performance compared to the standard SATA mirror he was generally intending to run. However, and this is the big innovation, all reads come from the SSD.

Now in mirrors gone by the theory was that reads came evenly from both drives, but the reality is that reads more often come from only one drive. So why not just force the mirror to read only from the fastest drive. When dealing with SAS/SATA combinations the performance improvement would have only been marginal to say the least, but with SSD its a different story.

SSD read speeds are way up and above those of SATA drives, and it makes sense to take advantage of that.

So that’s the way it works. Writes to both disks, reads only from the SSD. Of course if the SSD fails (heaven forbid I just said that), then the reads will switch over to the SATA drive and all will continue to work as normally expected in a mirrored environment.

Now that’s a pretty cool use of SSD … all the performance with safety without having to buy two drives.

Bring on those ever-increasing drive sizes in SSD!



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