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[LUG] Ensuring data is on disk - 2


With apologies for breaking the original references - I hit delete too

Okay I'm getting somewhere slowly.

SCSI disks may support RAW (read after write) and Seagate SCSI disks
support Idle Read after write (iRAW).

SATA disks refer to the feature as "read verify" or "Write-read-verify",
although it is only going to be in recent drives.

In most cases these features are disabled, or run in idling mode by
default. There is a significant write hit performance in enabling it,
but if you use large external USB drives for backup only (which I
believe to be a common configuration) then I think it may make sense to

The key tools are "smartctl" and "hdparm", but neither of which are
sufficiently fresh on Debian Lenny to display the relevant attributes to
me (if my drive has them). Although both report their own inadequacies
to describe the features of my disk drives - smartctl says one drive has
SMART but can't talk to it, and the other drive has a good SMART health
(despite me having another windows open which has counted 1948 bad
blocks and is only 77% finished scanning the disk), and hdparm displays
some unknown attributes for the first mentioned disk (include vendor
extensions), and the other disk isn't recognised at all (apparently it
is a SAMSUNG  HD642JI, 640GB).

In the mean time, as suggested there are several ways to clear the OS
cache (unmount or drop_caches), and one is left relying on luck (or
power cycling) to miss the cache in the disk (mine are 8MB and 16MB so
when writing big backups luck is likely on your side....), at which
point just reading the data should be sufficient to ensure it is safely
on disk, since only a bad block should stop the process but verifying
the data is probably wise.

All this leads to the answer to a question Ben (I think) muttered
recently about who uses tape these days.... Well suddenly it looks a lot
simpler than disks.

Also one area where I think Microsoft Windows may be doing a better job
at exposing the relevant interface.

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