SnapRAID / MergerFS - Revision history

Andrew: /* Identifying/Fixing a bad block */

2025-12-19T07:18:10Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2025-11-19T06:46:44Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2025-08-21T05:28:46Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2025-08-21T04:53:10Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2024-11-25T05:16:03Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2022-04-26T09:00:48Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2022-04-26T09:00:14Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2022-04-26T08:56:49Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2022-04-26T08:45:59Z

‎Identifying/Fixing a bad block

Andrew: /* Identifying/Fixing a bad block */

2022-04-26T08:31:50Z

‎Identifying/Fixing a bad block

@@ Line 40: / Line 40: @@
 * Get the sector size for the disk with <code>fdisk -l /dev/sdX | grep Units</code> (we'll call this '''S'''). In my case this is 512.
 * Identify the starting sector for the /dev/sdXY volume (eg. /dev/sda1) with <code>fdisk -l /dev/sdX</code> (we'll call this '''T'''). In my case this is 2048.
-* Calculate the offset to go from disk to volume location with <code>(T * S / B)</code> (we'll call this '''O'''). In my case this is 256.
+* Calculate the offset to go from disk to volume location with <code>(T * S / B)</code> (we'll call this '''F'''). In my case this is 256.
-* Run ddrescuelog to list out the bad block locations (using the block size B) and convert to a volume location (by subtracting the offset O):
+* Run ddrescuelog to list out the bad block locations (using the block size B) and convert to a volume location (by subtracting the offset F):
 <pre>
-ddrescuelog -b B --list-blocks=- sdX.map | xargs -n 1 -O +
+ddrescuelog -b B --list-blocks=- sdX.map | xargs -n 1 expr -F +
 </pre>
 * Let's call each of these bad volume blocks '''BB'''

@@ Line 40: / Line 40: @@
 * Get the sector size for the disk with <code>fdisk -l /dev/sdX | grep Units</code> (we'll call this '''S'''). In my case this is 512.
 * Identify the starting sector for the /dev/sdXY volume (eg. /dev/sda1) with <code>fdisk -l /dev/sdX</code> (we'll call this '''T'''). In my case this is 2048.
-* Run ddrescuelog to list out the bad block locations (using the block size B):
+* Calculate the offset to go from disk to volume location with <code>(T * S / B)</code> (we'll call this '''O'''). In my case this is 256.
 <pre>
-ddrescuelog -b B --list-blocks=- sdX.map
+ddrescuelog -b B --list-blocks=- sdX.map | xargs -n 1 -O +
 </pre>
-* For each of these, convert to a block location in the volume (rather than the disk) by subtracting <code>(T * S / B)</code>. In my case that's 256. Let's call each of these bad volume blocks '''BB'''
+* Let's call each of these bad volume blocks '''BB'''
 * Start debugfs for the volume with :
 <pre>

@@ Line 102: / Line 102: @@
 * Write random data over the bad file, which should force the pending sector to be marked bad and reallocated from spare space on the disk:
 <pre>
-shred –v /path/to/bad/file
+shred -n 1 –v /path/to/bad/file
 </pre>
 * Check that the number of failing sectors has decreased:
 <pre>
-smartctl –a /dev/sdX | grep Pending
+smartctl -a /dev/sdX | grep Pending
 </pre>
@@ Line 113: / Line 113: @@
 <pre>
 rm /path/to/bad/file
 </pre>

@@ Line 45: / Line 45: @@
 </pre>
 * For each of these, convert to a block location in the volume (rather than the disk) by subtracting <code>(T * S / B)</code>. In my case that's 256. Let's call each of these bad volume blocks '''BB'''
-* Start debugfs for the volume with <code>debugfs /dev/sdXY</code>
+* Start debugfs for the volume with :
 * For each '''BB''' run:
 <pre>

@@ Line 50: / Line 50: @@
 testb BB
 </pre>
-* If debugfs returns "Block BB not in use", then that block isn't part of a file and can safely be overwritten. If it returns an inode number (we'll call it '''I''') then you can convert that inode number to a file path with:
+* If debugfs returns "Block BB not in use", then that block isn't part of a file and can safely be overwritten. If it returns "Block BB marked in use" then we need to get an inode number (we'll call it '''I''') and convert that inode number to a file path with:
 <pre>
 ncheck I
 </pre>

@@ Line 78: / Line 78: @@
 for block in `ddrescuelog -b S --list-blocks=- sdX.map`
 do
-   hdparm –read-sector $block /dev/sdX > /dev/null
+   hdparm --read-sector $block /dev/sdX > /dev/null
 done
 </pre>
@@ Line 86: / Line 86: @@
 for block in `ddrescuelog -b S --list-blocks=- sdX.map`
 do
-   hdparm –write-sector $block /dev/sdX
+   hdparm --write-sector $block /dev/sdX
 done
 </pre>

@@ Line 58: / Line 58: @@
 * Check with dd that we’ve got the right block IDs. For each one of these reads we expect to see an error (and “0+0 records in”):
 <pre>
-for block in `ddrescuelog –list-blocks=- sdX.map`
+for block in `ddrescuelog -b B --list-blocks=- sdX.map`
 do
-   dd if=/dev/sdX of=/dev/null count=1 bs=512 skip=$block
+   dd if=/dev/sdX of=/dev/null count=1 bs=B skip=$block
 done
 </pre>
@@ Line 66: / Line 66: @@
 * For each of the bad blocks, write zeros over the block to force it to be reallocated from spare space on the drive. Be careful here – getting it wrong will destroy data! Also note that when reading, “skip” is used to position the input stream, but here “seek” is used to position the output stream:
 <pre>
-for block in `ddrescuelog –list-blocks=- sdX.map`
+for block in `ddrescuelog -b B --list-blocks=- sdX.map`
 do
-   dd if=/dev/zero of=/dev/sdX count=1 bs=512 seek=$block
+   dd if=/dev/zero of=/dev/sdX count=1 bs=B seek=$block
 done
 </pre>
@@ Line 74: / Line 74: @@
 * It’s possible that dd will fail to write to the block, in which case try again with hdparm:
-** First check that we’ve got the right sectors (we expect to see “SG_IO: bad/missing sense data” for each sector on stderr, so we pipe stdout to /dev/null to avoid noise):
+** First check that we’ve got the right sectors (we expect to see “SG_IO: bad/missing sense data” for each sector on stderr, so we pipe stdout to /dev/null to avoid noise). Note that we use '''S''' as the block size for ddrescuelog, since hdparm deals in sectors:
 <pre>
-for block in `ddrescuelog –list-blocks=- sdX.map`
+for block in `ddrescuelog -b S --list-blocks=- sdX.map`
 do
    hdparm –read-sector $block /dev/sdX > /dev/null
@@ Line 82: / Line 82: @@
 </pre>
-** Assuming we’ve seen the expected errors, write zeros over each of the bad sectors. Be careful here – getting it wrong will destroy data! You may be asked to add a “—yes-i-know-what-i-am-doing” flag.
+** Assuming we’ve seen the expected errors, write zeros over each of the bad sectors. Note that we use '''S''' as the block size for ddrescuelog, since hdparm deals in sectors. Be careful here – getting it wrong will destroy data! You may be asked to add a “—yes-i-know-what-i-am-doing” flag.
 <pre>
-for block in `ddrescuelog –list-blocks=- sdX.map`
+for block in `ddrescuelog -b S --list-blocks=- sdX.map`
 do
    hdparm –write-sector $block /dev/sdX
 done
 </pre>

@@ Line 44: / Line 44: @@
 ddrescuelog -b B --list-blocks=- sdX.map
 </pre>
-* For each of these, convert to a block location in the volume (rather than the disk) by subtracting <code>(T * S / B)</code>. In my case that's 256.
+* For each of these, convert to a block location in the volume (rather than the disk) by subtracting <code>(T * S / B)</code>. In my case that's 256. Let's call each of these bad volume blocks '''BB'''
 More details:
 * https://www.smartmontools.org/wiki/BadBlockHowto
 * [[Mdadm#Repairing_failing_disk_on_degraded_array]]