THIS DOCUMENT SHOWS HOW TO IDENTIFY THE FILE ASSOCIATED WITH AN UNREADABLE DISK SECTOR, AND HOW TO FORCE THAT SECTOR TO REALLOCATE. Assumptions: Linux OS, ext2 or ext3 file system. Bruce Allen Thanks to Sergey Vlasov, Theodore Ts'o, Michael Bendzick, and others for explaining this to me. I would like to add text showing how to do this for other file systems, in particular ReiserFS, XFS, and JFS: please email me if you can provide this information. NOTE: Starting with GNU coreutils release 5.3.0, dd on Linux includes options 'iflag=direct' and 'oflag=direct'. Using these with the dd commands below should be helpful, because adding these flags should avoid any interaction with the block buffering IO layer in Linux and permit direct reads/writes from the raw device. Use 'dd --help' to see if your version of dd supports these options. If not, build the latest code from fttp://alpha.gnu.org/gnu/coreutils. In this example, the disk is failing self-tests at Logical Block Address LBA = 0x016561e9 = 23421417. The LBA counts sectors in units of 512 bytes, and starts at zero. ----------------------------------------------------------------------------------------------- root]# smartctl -l selftest /dev/hda: SMART Self-test log structure revision number 1 Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error # 1 Extended offline Completed: read failure 90% 217 0x016561e9 ----------------------------------------------------------------------------------------------- Note that other signs that there is a bad sector on the disk can be found in the non-zero value of the Current Pending Sector count: ----------------------------------------------------------------------------------------------- root]# smartctl -A /dev/hda ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0033 100 100 005 Pre-fail Always - 0 196 Reallocated_Event_Count 0x0032 100 100 000 Old_age Always - 0 197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always - 1 198 Offline_Uncorrectable 0x0008 100 100 000 Old_age Offline - 1 ----------------------------------------------------------------------------------------------- First Step: We need to locate the partition on which this sector of the disk lives: ----------------------------------------------------------------------------------------------- root]# fdisk -lu /dev/hda Disk /dev/hda: 123.5 GB, 123522416640 bytes 255 heads, 63 sectors/track, 15017 cylinders, total 241254720 sectors Units = sectors of 1 * 512 = 512 bytes Device Boot Start End Blocks Id System /dev/hda1 * 63 4209029 2104483+ 83 Linux /dev/hda2 4209030 5269319 530145 82 Linux swap /dev/hda3 5269320 238227884 116479282+ 83 Linux /dev/hda4 238227885 241248104 1510110 83 Linux ----------------------------------------------------------------------------------------------- The partition /dev/hda3 starts at LBA 5269320 and extends past the 'problem' LBA. The 'problem' LBA is offset 23421417 - 5269320 = 18152097 sectors into the partition /dev/hda3. To verify the type of the file system and the mount point, look in /etc/fstab: ----------------------------------------------------------------------------------------------- root]# grep hda3 /etc/fstab /dev/hda3 /data ext2 defaults 1 2 ----------------------------------------------------------------------------------------------- You can see that this is an ext2 file system, mounted at /data. Second Step: we need to find the blocksize of the file system (normally 4096 bytes for ext2): ----------------------------------------------------------------------------------------------- root]# tune2fs -l /dev/hda3 | grep Block Block count: 29119820 Block size: 4096 ----------------------------------------------------------------------------------------------- In this case the block size is 4096 bytes. Third Step: we need to determine which File System Block contains this LBA. The formula is: b = (int)((L-S)*512/B) where: b = File System block number B = File system block size in bytes L = LBA of bad sector S = Starting sector of partition as shown by fdisk -lu and (int) denotes the integer part. In our example, L=23421417, S=5269320, and B=4096. Hence the 'problem' LBA is in block number b = (int)18152097*512/4096 = (int)2269012.125 so b=2269012. Note: the fractional part of 0.125 indicates that this problem LBA is actually the second of the eight sectors that make up this file system block. Fourth Step: we use debugfs to locate the inode stored in this block, and the file that contains that inode: ----------------------------------------------------------------------------------------------- root]# debugfs debugfs 1.32 (09-Nov-2002) debugfs: open /dev/hda3 debugfs: icheck 2269012 Block Inode number 2269012 41032 debugfs: ncheck 41032 Inode Pathname 41032 /S1/R/H/714197568-714203359/H-R-714202192-16.gwf ----------------------------------------------------------------------------------------------- In this example, you can see that the problematic file (with the mount point included in the path) is: /data/S1/R/H/714197568-714203359/H-R-714202192-16.gwf To force the disk to reallocate this bad block we'll write zeros to the bad block, and sync the disk: ----------------------------------------------------------------------------------------------- root]# dd if=/dev/zero of=/dev/hda3 bs=4096 count=1 seek=2269012 root]# sync ----------------------------------------------------------------------------------------------- NOTE: THIS LAST STEP HAS PERMANENTLY AND IRRETREVIABLY DESTROYED SOME OF THE DATA THAT WAS IN THIS FILE. DON'T DO THIS UNLESS YOU DON'T NEED THE FILE OR YOU CAN REPLACE IT WITH A FRESH OR CORRECT VERSION. Now everything is back to normal: the sector has been reallocated. Compare the output just below to similar output near the top of this article: ----------------------------------------------------------------------------------------------- root]# smartctl -A /dev/hda ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0033 100 100 005 Pre-fail Always - 1 196 Reallocated_Event_Count 0x0032 100 100 000 Old_age Always - 1 197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always - 0 198 Offline_Uncorrectable 0x0008 100 100 000 Old_age Offline - 1 ----------------------------------------------------------------------------------------------- Note: for some disks it may be necessary to update the SMART Attribute values by using smartctl -t offline /dev/hda The disk now passes its self-tests again: ----------------------------------------------------------------------------------------------- root]# smartctl -t long /dev/hda [wait until test completes, then] root]# smartctl -l selftest /dev/hda SMART Self-test log structure revision number 1 Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error # 1 Extended offline Completed without error 00% 239 - # 2 Extended offline Completed: read failure 90% 217 0x016561e9 # 3 Extended offline Completed: read failure 90% 212 0x016561e9 # 4 Extended offline Completed: read failure 90% 181 0x016561e9 # 5 Extended offline Completed without error 00% 14 - # 6 Extended offline Completed without error 00% 4 - ----------------------------------------------------------------------------------------------- and no longer shows any offline uncorrectable sectors: ----------------------------------------------------------------------------------------------- root]# smartctl -A /dev/hda ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0033 100 100 005 Pre-fail Always - 1 196 Reallocated_Event_Count 0x0032 100 100 000 Old_age Always - 1 197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always - 0 198 Offline_Uncorrectable 0x0008 100 100 000 Old_age Offline - 0 ----------------------------------------------------------------------------------------------- A SECOND EXAMPLE On this drive, the first sign of trouble was this email from smartd: To: ballen Subject: SMART error (selftest) detected on host: medusa-slave166.medusa.phys.uwm.edu This email was generated by the smartd daemon running on host: medusa-slave166.medusa.phys.uwm.edu in the domain: master001-nis The following warning/error was logged by the smartd daemon: Device: /dev/hda, Self-Test Log error count increased from 0 to 1 Running smartctl -a /dev/hda confirmed the problem: Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error # 1 Extended offline Completed: read failure 80% 682 0x021d9f44 Note that the failing LBA reported is 0x021d9f44 (base 16) = 35495748 (base 10) ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0033 100 100 005 Pre-fail Always - 0 196 Reallocated_Event_Count 0x0032 100 100 000 Old_age Always - 0 197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always - 3 198 Offline_Uncorrectable 0x0008 100 100 000 Old_age Offline - 3 and one can see above that there are 3 sectors on the list of pending sectors that the disk can't read but would like to reallocate. The device also shows errors in the SMART error log: Error 212 occurred at disk power-on lifetime: 690 hours After command completion occurred, registers were: ER ST SC SN CL CH DH -- -- -- -- -- -- -- 40 51 12 46 9f 1d e2 Error: UNC 18 sectors at LBA = 0x021d9f46 = 35495750 Commands leading to the command that caused the error were: CR FR SC SN CL CH DH DC Timestamp Command/Feature_Name -- -- -- -- -- -- -- -- --------- -------------------- 25 00 12 46 9f 1d e0 00 2485545.000 READ DMA EXT Signs of trouble at this LBA may also be found in SYSLOG: [root]# grep LBA /var/log/messages | awk '{print $12}' | sort | uniq LBAsect=35495748 LBAsect=35495750 So I decide to do a quick check to see how many bad sectors there really are. Using the bash shell I check 70 sectors around the trouble area: [root]# export i=35495730 [root]# while [ $i -lt 35495800 ] > do echo $i > dd if=/dev/hda of=/dev/null bs=512 count=1 skip=$i > let i+=1 > done 35495734 1+0 records in 1+0 records out 35495735 dd: reading `/dev/hda': Input/output error 0+0 records in 0+0 records out 35495751 dd: reading `/dev/hda': Input/output error 0+0 records in 0+0 records out 35495752 1+0 records in 1+0 records out which shows that the seventeen sectors 35495735-35495751 (inclusive) are not readable. Next, we identify the files at those locations. The partitioning information on this disk is identical to the first example above, and as in that case the problem sectors are on the third partition /dev/hda3. So we have: L=35495735 to 35495751 S=5269320 B=4096 so that b=3778301 to 3778303 are the three bad blocks in the file system. [root]# debugfs debugfs 1.32 (09-Nov-2002) debugfs: open /dev/hda3 debugfs: icheck 3778301 Block Inode number 3778301 45192 debugfs: icheck 3778302 Block Inode number 3778302 45192 debugfs: icheck 3778303 Block Inode number 3778303 45192 debugfs: ncheck 45192 Inode Pathname 45192 /S1/R/H/714979488-714985279/H-R-714979984-16.gwf debugfs: quit And finally, just to confirm that this is really the damaged file: [root]# md5sum /data/S1/R/H/714979488-714985279/H-R-714979984-16.gwf md5sum: /data/S1/R/H/714979488-714985279/H-R-714979984-16.gwf: Input/output error Finally we force the disk to reallocate the three bad blocks: [root]# dd if=/dev/zero of=/dev/hda3 bs=4096 count=3 seek=3778301 [root]# sync We could also probably use: [root]# dd if=/dev/zero of=/dev/hda bs=512 count=17 seek=35495735 At this point we now have: ID# ATTRIBUTE_NAME FLAG VALUE WORST THRESH TYPE UPDATED WHEN_FAILED RAW_VALUE 5 Reallocated_Sector_Ct 0x0033 100 100 005 Pre-fail Always - 0 196 Reallocated_Event_Count 0x0032 100 100 000 Old_age Always - 0 197 Current_Pending_Sector 0x0022 100 100 000 Old_age Always - 0 198 Offline_Uncorrectable 0x0008 100 100 000 Old_age Offline - 0 which is encouraging, since the pending sectors count is now zero. Note that the drive reallocation count has not yet increased: the drive may now have confidence in these sectors and have decided not to reallocate them.. A device self test: [root#] smartctl -t long /dev/hda (then wait about an hour) shows no unreadable sectors or errors: Num Test_Description Status Remaining LifeTime(hours) LBA_of_first_error # 1 Extended offline Completed without error 00% 692 - # 2 Extended offline Completed: read failure 80% 682 0x021d9f44 [USEFUL HINTS ADDED BY OTHERS] --------------------------------------------------------------------------- From: Kay Diederichs I read your badblocks-howto at http://smartmontools.sourceforge.net/BadBlockHowTo.txt and greatly benefitted from it. One thing that's (maybe) missing is that often the "smartctl -t long" scan finds a bad sector which is _not_ assigned to any file. In that case it does not help to run debugfs, or rather debugfs reports the fact that no file owns that sector. Furthermore, it is somewhat laborious to come up with the correct numbers for debugfs, and debugfs is slow ... So what I suggest in the case of presence of Current_Pending_Sector/Offline_Uncorrectable errors is to create a huge file on that filesystem. dd if=/dev/zero of=/some/mount/point bs=4k creates the file. Leave it running until the partition/filesystem is full. This will make the disk reallocate those sectors which do not belong to a file. Check the "smartctl -a" output after that and make sure that the sectors are reallocated. If any remain, use the debugfs method. Of course the usual caveats apply - back it up first, and so on. --------------------------------------------------------------------------- This document is version $Id: BadBlockHowTo.txt,v 1.7 2005/04/26 16:56:19 ballen4705 Exp $ It is Copyright Bruce Allen (2004) and distributed under GPL2.