ivoras’ FreeBSD blog Just another FreeBSD Developer’s weblog

August 24, 2007

RAID flash

Filed under: FreeBSD — ivoras @ 11:00 am

I’ve just had a sort-of shock about how cheap the USB flash drives have become (vs. how pricey they used to be a long time ago). I didn’t look at their prices for a long time (since I didn’t need any new flash drives) so I was pleasantly surprised with the per/MB prices that have become “normal”. On the other hand, capacity of consumer USB drives hasn’t gone up much – it seems 8 GB is the top of the range now, and speed is still not great – it seems 8-10 MB/s is the norm. But it’s relatively cheap technology now.

It seems that now is a good time to start experimenting with flash memory even in production, especially where seek times are important (flash drives have no seek latency that’s present in mechanical drives). Having no seek time has a nice side-effect when drives are used in RAID1 with gmirror, which can be configured to split large read requests over the mirrored drives. With mechanical drives this mode couldn’t produce significant performance because the drive heads still needed to seek through the unread portions (for sequential requests) but the situation is ideal for seek-less drives.

I think the ideal solution here would be RAID 1+0 with four drives. Admittedly, this would only give 16 GB of storage space (if each individual drive is 8 GB), but the (read) performance should scale linearly to ~~ 40 MB/s (once it would double in gmirror/RAID1, then again in gstripe/RAID0), which is decent. 16 GB is relatively small, but flash memory is much cheaper than server memory (e.g. FB-DIMM) and most databases are relatively small, so it might not be affordable to keep the whole database in RAM any more.

Only one question remains now – how many IOPS can be had from flash memory (unconfirmed information says: around 2000), and does having all flash drives on the same host USB bus (e.g. one USB hub with 4 drives, connected to the motherboard port) introduce too much latency?

1 Comment

  1. Consumer Electronics Reviews…

    I couldn’t understand some parts of this article, but it sounds interesting…

    Trackback by Consumer Electronics Reviews — November 28, 2007 @ 7:18 am

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