Changes between Version 8 and Version 9 of BadBlockHowto

Timestamp:

Mar 25, 2017, 3:13:10 PM (7 years ago)

Author:

Gabriele Pohl

Comment:

LVM repairs

BadBlockHowto

@@ -491 +491 @@
 For creating, destroying, resizing, checking and copying partitions, and the file systems on them, GNU's `parted` is worth examining. The [http://www.tldp.org/HOWTO/Large-Disk-HOWTO.html Large Disk HOWTO] is also a useful resource. 
 
+
+=== LVM repairs ===
+
+This section was written by Frederic BOITEUX. It was titled: "HOW TO LOCATE AND REPAIR BAD BLOCKS ON AN LVM VOLUME".
+
+Smartd reports an error in a short test :
+
+{{{
+# smartctl -a /dev/hdb
+...
+SMART Self-test log structure revision number 1
+Num  Test_Description    Status                  Remaining  LifeTime(hours)  LBA_of_first_error
+# 1  Short offline       Completed: read failure       90%        66         37383668
+}}}
+
+So the disk has a bad block located in `LBA block 37383668`
+
+In which physical partition is the bad block ?
+
+{{{
+# sfdisk -luS /dev/hdb  # or 'fdisk -ul /dev/hdb'
+
+Disk /dev/hdb: 9729 cylinders, 255 heads, 63 sectors/track
+Units = sectors of 512 bytes, counting from 0
+
+   Device Boot    Start       End   #sectors  Id  System
+/dev/hdb1            63    996029     995967  82  Linux swap / Solaris
+/dev/hdb2   *    996030   1188809     192780  83  Linux
+/dev/hdb3       1188810 156296384  155107575  8e  Linux LVM
+/dev/hdb4             0         -          0   0  Empty
+}}}
+
+It's in the `/dev/hdb3` partition, a LVM2 partition. From the LVM2 partition beginning, the bad block has an offset of
+{{{
+(37383668 - 1188810) = 36194858
+}}}
+
+We have to find in which LVM2 logical partition the block belongs to.
+
+In which logical partition is the bad block ?
+
+''IMPORTANT'' : LVM2 can use different schemes dividing its physical partitions to logical ones : linear, striped, contiguous or not... The following example assumes that allocation is linear !
+
+The physical partition used by LVM2 is divided in PE (Physical Extent) units of the same size, starting at `pe_start` 512 bytes blocks from the beginning of the physical partition.
+
+The `pvdisplay` command gives the size of the PE (in KB) of the LVM partition :
+
+{{{
+#  part=/dev/hdb3 ; pvdisplay -c $part | awk -F: '{print $8}'
+4096
+}}}
+
+To get its size in LBA block size (512 bytes or 0.5 KB), we multiply this number by 2 : `4096 * 2 = 8192 blocks` for each PE.
+
+To find the offset from the beginning of the physical partition is a bit more difficult : if you have a recent LVM2 version, try :
+
+{{{
+# pvs -o+pe_start $part
+}}}
+
+Either, you can look in `/etc/lvm/backup` :
+{{{
+# grep pe_start $(grep -l $part /etc/lvm/backup/*)
+                        pe_start = 384
+}}}
+
+Then, we search in which PE is the badblock, calculating the PE rank in which the faulty block of the partition is : `physical partition's bad block number / sizeof(PE)` =
+{{{
+36194858 / 8192 = 4418.3176
+}}}
+
+So we have to find in which LVM2 logical partition is used the PE number 4418 (count starts from 0) :
+{{{
+# lvdisplay --maps |egrep 'Physical|LV Name|Type'
+  LV Name                /dev/WDC80Go/racine
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    0 to 127
+  LV Name                /dev/WDC80Go/usr
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    128 to 1407
+  LV Name                /dev/WDC80Go/var
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    1408 to 1663
+  LV Name                /dev/WDC80Go/tmp
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    1664 to 1791
+  LV Name                /dev/WDC80Go/home
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    1792 to 3071
+  LV Name                /dev/WDC80Go/ext1
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    3072 to 10751
+  LV Name                /dev/WDC80Go/ext2
+    Type                linear
+    Physical volume     /dev/hdb3
+    Physical extents    10752 to 18932
+}}}
+
+So the PE #4418 is in the `/dev/WDC80Go/ext1` LVM logical partition.
+
+Size of logical block of file system on `/dev/WDC80Go/ext1` :
+
+It's a ext3 fs, so I get it like this :
+{{{
+# dumpe2fs /dev/WDC80Go/ext1 | grep 'Block size'
+dumpe2fs 1.37 (21-Mar-2005)
+Block size:               4096
+}}}
+
+bad block number for the file system :
+
+The logical partition begins on `PE 3072` :
+{{{
+ (# PE's start of partition * sizeof(PE)) + parttion offset[pe_start] =
+ (3072 * 8192) + 384 = 25166208
+}}}
+
+512b block of the physical partition, so the bad block number for the file system  is :
+{{{
+(36194858 - 25166208) / (sizeof(fs block) / 512)
+= 11028650 / (4096 / 512)  = 1378581.25
+}}}
+
+Test of the fs bad block :
+{{{
+dd if=/dev/WDC80Go/ext1 of=block1378581 bs=4096 count=1 skip=1378581
+}}}
+
+If this `dd` command succeeds, without any error message in console or syslog, then the block number calculation is probably wrong ! *Don't* go further, re-check it and if you don't find the error, please renounce !
+
+Search / correction follows the same scheme as for simple partitions :
+* find possible impacted files with `debugfs` (`icheck <fs block nb>`, then `ncheck <icheck nb>`).
+* reallocate bad block writing zeros in it, ''using the fs block size'' :
+
+{{{
+dd if=/dev/zero of=/dev/WDC80Go/ext1 count=1 bs=4096 seek=1378581
+}}}
+
+Et voilà !
+
 == Footnotes ==