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SSD is Something to See

June 30, 2009

Nigel Griffiths has another great demonstration video, this one a discussion of the recently announced SSD drives. I'll give you some highlights here, but watching the entire clip is well worth the 22 minutes.

Nigel mentions that these drives evenly spread data throughout the memory cells on the device, and all of the cells can be accessed at once. He also states that you shouldn't attempt to defragment an SSD because doing so unnecessarily consumes some of the finite number of writes and shortens the drive's lifespan.

These disks currently ship with 69 GB usable out of a 128-GB capacity. This spare capacity allows the disk to actively search for suspect memory cells, mark them as bad and move data away from them without losing any capacity, since you have only half of the possible disk space available for your use.  Compared to traditional drives, SSDs can access every memory cell at once, so rotational latency and seek times aren't issues. Nigel shows random I/O per second (IOPS) in the range of 28,000, as opposed to 125-200 IOPS with a traditional disk drive.

He also notes that SSDs use less power than a traditional disk. In his example he compares 330 watts for five SSDs vs. 7,700 watts in a single DS4700 system. Disk per disk, he says SSDs require about half of the power of traditional disk subsystems.

In the demo Nigel covers the different disk carriers, machines and expansion drawers that the drives can be used in, going from 2.5-inch small form factor disks for blades to 3.5-inch SAS carriers. He also displays various pictures--including some of a dismantled SSD--to provide a more complete visual perspective.

To take advantage of SSD, you must update your machine firmware and make sure that HMC, AIX and VIOS are at supported levels. In the demo Nigel connects an SDD directly to his LPAR, and then runs lsconf to show the disk in his machine. Then, after running cfgmgr, you see that the disk doesn't immediately appear. Nigel goes into the menus--DIAG > task selection > RAID array manager > IBM SAS disk array manager--to create an array candidate pdisk and format to 528-byte sectors.

Once you take these steps, you'll be able to select the solid state drive that will format the disk. At this point, you can see a pdisk, which can put into an array. From the same IBM SAS disk array manager menu, you can create an SAS disk array by selecting the controller, RAID level, stripe size and the pdisk you just created. You can then you can go back and list the array, which will show you the hdisk and pdisk mappings that you just created.

If this doesn't seem clear, watch the demo. It should make more sense once you see it performed on a live system.

Now when he runs lsconf, the hdisk appears, and it can be used as would any other disk -- you can add it to a volume group, create filesystems, etc.

Nigel runs some tests to demonstrate SSD speed. To prepare for testing, he unmounts his test /SSD filesystem and remounts it with the cio option. By running a tool called ndisk64, he performs some random I/O tests, runs iostat and can see 11,000 IOPS and 44 MB/sec.

For the second test, he uses the raw device and gets 27,000 IOPS to the disk. He then mentions the rumors he's heard about even faster times being recorded in the labs.

So why do we care about SSD? Customers with intense performance requirements can put heavily accessed data onto an SSD. Nigel also brings up using SSD in conjunction with active memory sharing and paging spaces, as SSD will provide much faster access than regular disks. (However, I've also recently heard arguments that the slower write times vs. read times might be an issue with this application. I'll dig a bit further into any downfalls with this use for SSD.)

He covers many other demos covering topics like active memory sharing, the Integrated Virtualization Manager, partition mobility, WPARs, nmon, etc. An index is available so you can browse around for the topics that interest you.

YouTube also has some relevant material. Search on "ssd vs. hdd laptop" and watch demonstrations where people put two identical laptops (save for the disks) side by side and boot them. Of course, these disks aren't the same as those in your Power server, but you can still see the performance gains--those laptops with SSD are much faster than those with traditional drives.

As the technology evolves and the prices come down, I expect more customers to examine SSD's benefits, in both their servers and their personal machines.

Posted June 30, 2009| Permalink

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