Re: Found on Slashdot: Wear Leveling, RAID Can Wipe Out SSD Advantage

I saw the Slashdot article last evening and most of the stuff there I felt was FUD. First off the spoke often of $100,000 and $150,000 RAID controllers with 60 drives. "I have never and will never build a 60 drive RAID set" (not even on Frankie).

IBM's SSDs are the first with SAS connectivity and as Jim said are built with nearly 2x overcapacity and built in wear leveling at the device. I find it difficult to believe that would eat up the device's performance as the only time it would come into play is when the device is written to. Since you have to write somewhere anyway how is that going to slow it down? Now if the RAID card or the O/S did the wear leveling that's another problem. We don't do that.

IBM also limits SSDs to a total of 8 per drawer and per RAID card. This is in deference to the extreme performance of the devices relative to the RAID cards. IBM Thinks these things through quite thoroughly. This doesn't mean they are perfect of course but reports from the field are that these babies really fly.

- Larry

At an IBM pre-POWER7 release meeting Larry and I asked the question
about the SSD degradation. The answer was simple, we (IBM) engineered
it with sufficient overcapacity that the lifespan of the drive will
equal that of a physical drive. They also have sufficient performance
testing to be confident in the stated performance of the RAID arrays as
long as they are built as recommended.

While I think it is too early to tell if the SSDs are in fact living up
the the high praise they are getting, it is also a bit early to start
worrying about the ill effects of the technology. It is true that SSDs
are considerably higher in cost per storage unit than traditional DASD,
but then years ago when I (and many others on the list as well) started
in IT, mass storage was Tape, only transactional data that was less than
several months old was kept on actual DASD storage. Mechanical DASD is
now the new tape, relatively cheap, easy to manage, and reliable. Makes
you wonder what's next.

Jim Oberholtzer
CEO/Chief Technical Architect
Agile Technology Architects -- LLC



David Gibbs wrote:

I recently saw this on Slashdot ... and since IBM i supports both RAID and SSD's, I thought it might be interesting ...

http://hardware.slashdot.org/story/10/03/06/1650232/Wear-Leveling-RAID-Can-Wipe-Out-SSD-Advantage

This article discusses using solid state disks in enterprise storage networks. A couple of problems noted by the author: wear leveling can eat up most of a drive's bandwidth and make write performance no faster than a hard drive, and using SSDs with RAID controllers brings up its own set of problems. 'Even the highest-performance RAID controllers today cannot support the IOPS of just three of the fastest SSDs. I am not talking about a disk tray; I am talking about the whole RAID controller. If you want full performance of expensive SSDs, you need to take your $50,000 or $100,000 RAID controller and not overpopulate it with too many drives. In fact, most vendors today have between 16 and 60 drives in a disk tray and you cannot even populate a whole tray. Add to this that some RAID vendor's disk trays are only designed for the performance of disk drives and you might find that you need a disk tray per SSD drive at a huge cost.'