Ric Wheeler <[EMAIL PROTECTED]> wrote on 01/17/2008 03:18:05 PM: > Theodore Tso wrote: > > On Wed, Jan 16, 2008 at 09:02:50PM -0500, Daniel Phillips wrote: > >> Have you observed that in the wild? A former engineer of a disk drive > >> company suggests to me that the capacitors on the board provide enough > >> power to complete the last sector, even to park the head. > >> > > Even if true (which I doubt), this is not implemented. > > A modern drive can have 16-32 MB of write cache. Worst case, those > sectors are not sequential which implies lots of head movement.
We weren't actually talking about writing out the cache. While that was part of an earlier thread which ultimately conceded that disk drives most probably do not use the spinning disk energy to write out the cache, the claim was then made that the drive at least survives long enough to finish writing the sector it was writing, thereby maintaining the integrity of the data at the drive level. People often say that a disk drive guarantees atomic writes at the sector level even in the face of a power failure. But I heard some years ago from a disk drive engineer that that is a myth just like the rotational energy thing. I added that to the discussion, but admitted that I haven't actually seen a disk drive write a partial sector. Ted brought up the separate issue of the host sending garbage to the disk device because its own power is failing at the same time, which makes the integrity at the disk level moot (or even undesirable, as you'd rather write a bad sector than a good one with the wrong data). -- Bryan Henderson IBM Almaden Research Center San Jose CA Filesystems -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
