Re: bad sector and unused area.

NeilBrown wrote:

On Fri, 09 Jul 2021, Wakko Warner wrote:

quoted

Reindl Harald wrote:

quoted

Am 08.07.21 um 23:48 schrieb Wakko Warner:

quoted

I have a raid5 of 3 disks.

2 of them have bad sectors.  Sector 1110 and 1118.

I'm curious to know if these sectors actually contain any data or if they
can just be overwritten.

the RAID layer don't know anything about data by definition, it even don't
care what filesystem is running on top

So did you actually read what I said?

    Data Offset : 262144 sectors
   Super Offset : 0 sectors

This is /dev/sda1 which starts at sector 128 of /dev/sda.  Sector 1110 on
/dev/sda is bad.  That would place it between super offset and data offset. 
Thus, there wouldn't be a filesystem there.  I'm asking if it contains any
"data" that might be "used" by "something".  My /dev/md0 data starts at
offset 262144 of /dev/sda1 which is well beyond sector 1110 of /dev/sda.

If you had an unusually large bitmap, it might use those bad sectors,
but unlikely.
If you reshaped the array to more devices, it would reduce the Data
Offset, so might start writing in that space.

Otherwise you should be safe.

Ok thanks.  I did do a superblock backup on my 3 drives a few months ago.  I
checked offset 72 to offset 262143, it was all 0s.  I assume whatever is
there was on the disk and not wiped when I created the array.

I also dumped the same sectors from each of the disks.  interestingly, what
is stored there is the same on all 3 drives except for the areas that were
bad.

This array has never been reshaped.  It also has never had a disk replaced.

I already DD'd zero to the drive, but only the sectors that were not
readable.  Didn't see any reason not to.  I found it interesting that the
reallocated sectors didn't go up.  The drives have 13 years of power on
time.

This is mdstat:
md0 : active raid5 sda1[0] sdb1[1] sdc1[3]
      488018688 blocks super 1.1 level 5, 64k chunk, algorithm 2 [3/3] [UUU]
      bitmap: 1/2 pages [4KB], 65536KB chunk

And that is the only array on this machine.

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