Wait for an IO to complete

[Valdis.Kletnieks at vt.edu]

On Sat, 29 Mar 2014 18:17:52 +0530, Sankar P said:

> However, there is no guarantee that the data will be actually written
> to disk. I have heard instances where a caching layer in the disk
> tells the filesystem that the data is written but the data was not
> written and there was a power failure and an ensuing loss of data.

Actually, let me go a step further - I know of no current disks which
support write caching that *don't* lie to the OS and say "I/O complete,
data is on the disk" when it lands in the cache (leaving you vulnerable
if there's a power hit before it flushes the cashe).

Even more evil - although you'd *think* that the solution is to just disable
the write cache so the disk can't lie and has to wait for the data to hit
the platters, that's not quite true.  There's been disks (especially on the
very low end where they Just Don't Care, and on the high end were numbers
are everything) that will *say* they disabled the write cache, but it's still
doing it inside....
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