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Lying drives

Lying drives

Posted May 21, 2008 20:50 UTC (Wed) by ncm (subscriber, #165)
Parent article: Barriers and journaling filesystems

Then, of course, we have drives that claim to support barrier operations, but lie about it whenever traffic goes up. They look way better in benchmarks. Most likely whatever drive you have on your own system is one of those. Drives that don't lie are low-volume products, so cost a lot more.

My conclusion is that it doesn't matter much what sort of barriers the system asks of a drive; the best you can do is have the OS send blocks to the drive in the right order. Then, your only hope is battery backup, which actually helps only if your system stops writing to disk at least a few seconds before the drive itself loses power. Distressingly many UPS-backed machines don't actually do a controlled shutdown when the battery gets low.

Note that if the head's writing when the power drops, that sector (and possibly several following it) is toast. The notion of using the motor as a generator to keep the electronics sane for a few milliseconds is a fun idea, but not actually implemented in real drives.

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