On Sun, 4 Jan 2009, Rob Landley wrote:
On Sunday 04 January 2009 17:30:52 Theodore Tso wrote:
On Sun, Jan 04, 2009 at 11:40:52PM +0100, Pavel Machek wrote:
Not neccessarily.
If I have a bit of precious data and lot of junk on the card, I want
to copy out the precious data before the card dies. Reading the whole
media may just take too long.
That's probably very true for rotating harddrives after headcrash...
For a small amount data, maybe; but the number of seeks is often far
more destructive than the amount of time the disk is spinning. And in
practice, what generally happens is the user starts looking around to
make sure there wasn't anything else on the disk worth saving, and now
data is getting copied off based on human reaction time. So that's
why I normally advise users that doing a full image copy of the disk
is much better than, say, "cp -r /home/luser /backup", or cd'ing
around a filesystem hierarchy and trying to save files one by one.
That would be true if the disk hardware wasn't doing a gazillion retries to
read a bad sector internally (taking 5 seconds to come back and report
failure), and then the darn scsi layer added another gazillion retries on top
of that, and the two multiply together to make it so slow that that when you
leave the thing copying the disk overnight it's STILL not done 24 hours later.
Going in and cherry picking individual files looks kind of appealing in that
situation.
I've also had cases where one particular spot on the drive is bad. any
attempt to read that sector fails and causes the drive to error out until
a reboot. grabbing individual files I could skip the file(s) in the
affected portion and retreive everything else on the drive (or in some
cases raid array with multiple failures)
David Lang
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