amendment for "Bad block HOWTO" page

Hi,

I found that the information given on the "Bad block HOWTO" page for 
forcing a bad sector to be replaced doesn't work on a disk with a 4 kiB 
physical sector size. At least that is the case on my Seagate Barracuda 
7200.14 3 TB drive. This is what I found:

Running a short self-test on this drive revealed a bad sector:

# 1  Short offline       Completed: read failure       70%     10174 
      4130265680

I double-checked this with dd:

# dd if=/dev/sde of=/dev/null bs=512 skip=4130265680 count=1
dd: error reading ‘/dev/sde’: Input/output error
0+0 records in
0+0 records out
0 bytes (0 B) copied, 6.80211 s, 0.0 kB/s

So indeed, this sector is bad and I want to replace it. This is what I 
did, following the description on your (otherwise excellent) HOWTO page:

# dd if=/dev/zero of=/dev/sde bs=512 seek=4130265680 count=1
dd: error writing ‘/dev/sde’: Input/output error
1+0 records in
0+0 records out
0 bytes (0 B) copied, 22.6075 s, 0.0 kB/s

The operation failed! Doing another read operation as shown above 
confirmed that the bad sector was indeed not replaced. So on a hunch I 
decided to try using a 4 kiB buffer size. This means that the value for 
the skip parameter needs to be adjusted. There are eight logical 
512-byte sectors in one 4096-byte physical sector, so the new skip value 
should be 4130265680 / 8 = 516283210. Using this, the new command becomes:

# dd if=/dev/zero of=/dev/sde bs=4096 seek=516283210  count=1
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.000359402 s, 11.4 MB/s

And now the operation succeeded! Double-checking confirmed that the bad 
sector was indeed replaced now:

# dd if=/dev/sde of=/dev/null bs=512 skip=4130265680 count=1
1+0 records in
1+0 records out
512 bytes (512 B) copied, 0.000310025 s, 1.7 MB/s

So it looks like replacing a bad sector only works if you overwrite the 
entire physical sector in one single write operation. Overwriting only 
part of the physical sector has no effect.


Peter van Hoof.

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