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UltraCapacitors and flash back RAID cards

CeBit 2009 New tech from Adaptec
Mon Mar 09 2009, 14:10

ADAPTEC WAS SHOWING three technologies off at CeBIT, but no really new products. The nice thing is that the technologies are going to be in near future products.

Note the power spikes on spinup

The first one is easy enough to explain, it is called Intelligent Power Management. The idea is simple, if the RAID array is not being used, it slows it down or shuts it off. There are two states, slow and off, and they do just what you would think they do.

Adaptec uses the battery backed cache intelligently to keep drives spun down or off. If small amounts of data hits the cache, it will just sit there until the drives need to be spun up. Since the adapter is battery backed, failure isn't much of a problem, and the power saved is worth any risk.

You can program the adapter to keep spun up at peak hours, and change default states when usage is low or unlikely. Not much to say other than it makes complete sense, and should have a minimal impact of performance if done right.

One of the long running geek arguments is flash vs battery backups. SAS adapters have long used battery backups, flash never made it for one reason or other. With Zero-Maintenance Cache Protection, Adaptec has moved into the flash world.


Back in a flash

You will first see it in the 5445Z controller, and it looks like a battery pack, but is really an ultracapacitor. The idea is to keep power to the card long enough to copy the cache to flash when some microcephalic executive touring the server room pulls a 'what does this button do?' move.

The cap can support the card for a short while, so dumping that much data to flash quickly needs a wide bus. As you can see above, there are a lot of pins on the ZMCP connector, so that that is what Adaptec put on the 544Z. In June, you can buy the cards, pull them apart and see for yourself.

SSDs as cache

The last technology is a little further out, and a bit harder to explain, but it will have a large impact if done right. If you look at the numbers, on the right, you have an array of Seagate Savvio drives scoring 2058.65 on IOMeter. The left you see the same number of dirt standard IDE drives scoring 9557.28. The difference is the new technology.

If you look closely, there is an Intel ZX-25 32G SSD just below the monitor. It is being used as a cache for the SATA raid, giving it almost 5x the performance of the 15K SAS drives, one hell of a performance boost.

This is only a tech demo though, there are a lot of problems left. One is simple, there are tons of SSDs out there with different performance characteristics, and each one needs a different set of tuning parameters to make it work well, Adaptec is testing away, and the drives will either come to common standards, or Adaptec will map them all. Either way, it will be a bit.

The other problem is a bit more fundamental, flash has limited write cycles, somewhere around 100K writes before things go a little off, and instead of lossy compression, you get lossy storage. People concerned with things like correct numbers on their bank statement don't like this. If you divide the maximum write speed of an SSD by the capacity * write life, you end up with a little over six months of life at 100% writes.

To combat this, Adaptec is being intelligent about what it caches on the SSD. This may sounds like a simple thing, but it is much akin to someone giving you a block of marble and saying "Just take out everything that doesn't look like a statue", not easy. The work being done here is to figure out what to cache and when, not just where.

All of these technologies will be out in due time, some sooner than others. All look to enhance drive arrays in a meaningful way, it will just be a bit before you can buy them. µ



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