Piergiorgio Sartor wrote:
Hi,
But unless your drive firmware is broken the drive with only ever give
the correct data or an error. Smart has a counter for blocks that have
gone bad and will be fixed pending a write to them:
Current_Pending_Sector.
The only way the drive should be able to give you bad data is if
multiple bits toggle in such a way that the ECC still fits.
Not really, I've disks which are *perfect* in smart sense
and nevertheless I had mistmatch count.
This was a SW problem, I think now fixed, in RAID-10 code.
IIRC there still is an error in raid-1 code, in that data is written to
multiple drives without preventing modification of the memory between
writes. As I understand Neil's explanation, this happens (a) when memory
is being changed rapidly and frequently via memory mapped files, or (b)
writing via O_DIRECT, or (c) when raid-1 is being used for swap. I'm not
totally sure why the last one, but I have always seem mismatches on swap
in a system which is actually swapping. What is more troubling is that
if I do a hibernate, which writes to swap, and then force a boot from
other media to a Live-CD, doing a check of the swap array occasionally
shows a mismatch. That doesn't give me a secure feeling, although I have
never had an issue in practice, I was just curious.
This means that, yes, there could be mismatches, without
any warning, from other sources than disks.
And these could be anywhere in the system.
I already mentioned, time ago, a cabling problem which was
leading to a similar result: wrong data on different disks,
without any warning or error from the HW layer.
That is why it is important to know *where* the mismatch
occurs and, if possible, in which device component.
If it is an empty part of the FS, no problem, if it
belongs to a specific file, then it would be possible
to restore/recreate it.
Of course, a tool will be needed telling which file is
using a certain block of the device.
There are tools which claim to do that, or list blocks used in a given
file, which is not nearly as useful, but easier to do.
--
Bill Davidsen <davidsen@xxxxxxx>
"We can't solve today's problems by using the same thinking we
used in creating them." - Einstein
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