Linux System Administration Recipes A Problem-Solution Approach

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CHAPTER 4 ■ TAKING BACKUPS AND MANAGING DATA1. The first thing to try is a straightforward run of fsck:fsck /dev/sda1If you’re lucky, this may be all you need. If this succeeds, fsck will probably askyou to confirm a few fixes. To avoid being asked this, use the -y option.However, be warned that this should be used with caution, because it meansthat fsck will attempt to fix any and all errors it encounters, which may causedata loss.■ Note This is not true of all filesystems. If you’re using XFS, for example, then checking is conducted on mount,and fsck does nothing. xfs_check and xfs_repair may also be useful. However, fsck is fine with the <strong>Linux</strong>standardext3 (and ext2) filesystems.If this does work, update your backups immediately in case it wasn’t just aone-off failure!2. If you get a “bad superblock” error, you can try using an alternative block. Onan ext2 or ext3 filesystem, the superblock contains the filesystem metadata.Because it’s essential to the filesystem, backup copies of the superblock arekept, but you may need to use them explicitly.To get the superblock information for the problem filesystem, run thefollowing:dumpe2fs /dev/hda2 | grep superblockThis will work for both ext2 and ext3. Your output should look a bit like this:Primary superblock at 0, Group descriptors at 1-3Backup superblock at 32768, Group descriptors at 32769-32771Backup superblock at 98304, Group descriptors at 98305-98307Backup superblock at 163840, Group descriptors at 163841-163843[ etc ... ]Pass in one of the backup superblocks as an option to fsck:fsck -b 32768 /dev/hda2If this runs successfully, try mounting the filesystem with mount. If you stillhave problems, try passing the backup superblock into mount explicitly:mount sb=32768 /dev/hda2 /mnt3. The next stage, if steps 1 and 2 have both failed, is to use dd to generate a bitfor-bitcopy of the failed disk. To do this, you’ll need a spare drive at least aslarge as the failed one because dd copies even empty space over.For a spare drive mounted at /mnt/recovery (this must not be the drive youbooted from) and a failed drive connected at /dev/hda, use the following:114Download at WoweBook.Com
CHAPTER 4 ■ TAKING BACKUPS AND MANAGING DATAsudo dd if=/dev/hda of=/mnt/recovery/hdaimage.ddThis may take a while to run!By default, dd will abort on error. Avoid this with the noerror option:sudo dd conv=noerror if=/dev/hda of=/mnt/recovery/hdaimage.ddAnother option is the GNU utility ddrescue, with which you can skip the dodgyareas (dd will keep trying to get at bad areas of disk). This is the gddrescuepackage (and the utility is called gddrescue) in Debian/Ubuntu. Note that thisis not the same as the package/utility dd_rescue, which is a slightly less usefulprogram.gddrescue -n /dev/hda /mnt/recovery/hdaimage.raw rescued.logThis command will grab most of the error-free areas quickly, after which youcan rerun it with -r 1 instead of -n to get as much as possible of the badpatches. This is especially useful if you’re worried about how long your diskwill continue to spin, because you can get a copy of any good dataimmediately.Once you have your bit-for-bit copy, you can run fsck on it:fsck /mnt/recovery/hdaimage.ddNext, if it’s a single-partition drive, you can mount the image as a loopbackdevice:mount -o loop /mnt/recovery/hdaimage.dd /mnt/hdaimageIdeally, your data will now be available at /mnt/hdaimage. However, if yourfailed disk had multiple partitions, you’ll need to do a little more work. Findout where the partitions are with this:fdisk -lu /mnt/recovery/hdaimage.ddwhich will list the start and end cylinders of each partition and the units inwhich they’re measured. If the second partition starts at cylinder 80300 andthe units are 512 bytes, then that partition starts at 80300 × 512 = 41,113,600bytes. In this case, the command you want looks like this:mount -o loop,offset=41113600 /mnt/recover/hdaimage.raw /mnt/hdaimageFinally, you can use dd again to write the image back onto another disk, forexample, /dev/hdb:dd if=/mnt/recovery/hdaimage.raw of=/dev/hdb■ Note Another emergency rescue tip for a bad disk: put it in the freezer for a few hours (in a watertight sealedbag). This may sound odd, but cooling the disk can give it a brief lease of new life—long enough for you to grabyour data from it.115Download at WoweBook.Com
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Contents■About the Author .......
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■ CONTENTS3-7. Writing a Nagios P
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■ CONTENTS■Chapter 8: Using the
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■ CONTENTSAbout the Author■Juli
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■ CONTENTSAcknowledgmentsMany tha
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■ CONTENTSIntroductionThis book i
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■ INTRODUCTIONDownloading the Cod
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C H A P T E R 1■ ■ ■Saving Yo
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CHAPTER 1 ■ SAVING YOURSELF EFFOR
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C H A P T E R 2■ ■ ■Centraliz
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CHAPTER 2 ■ CENTRALIZING YOUR NET
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CHAPTER 7 ■ WORKING WITH APACHE
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CHAPTER 7 ■ WORKING WITH APACHEAl
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CHAPTER 7 ■ WORKING WITH APACHE
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C H A P T E R 8■ ■ ■Making Be
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CHAPTER 8 ■ USING THE COMMAND LIN
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C H A P T E R 9■ ■ ■Working w
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CHAPTER 9 ■ WORKING WITH TEXT IN
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C H A P T E R 10■ ■ ■Things G
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CHAPTER 10 ■ THINGS GO IN, THINGS
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C H A P T E R 11■ ■ ■Tracking
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CHAPTER 11 ■ TRACKING DOWN BUGSTh
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CHAPTER 11 ■ TRACKING DOWN BUGS01
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CHAPTER 11 ■ TRACKING DOWN BUGS
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CHAPTER 11 ■ TRACKING DOWN BUGSTh
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CHAPTER 11 ■ TRACKING DOWN BUGSAn
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CHAPTER 11 ■ TRACKING DOWN BUGS#
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C H A P T E R 12■ ■ ■Managing
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CHAPTER 12 ■ MANAGING TIME AND PE
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APPENDIX■ ■ ■Perl TipsSee rec
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APPENDIX ■ PERL TIPS• Date::Par
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APPENDIX ■ PERL TIPSPerl Syntax N
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■ ■ ■Index■Symbols$CDPATH,
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■ INDEXenscript option (printing)
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■ INDEX■NNagiosadding hosts to,
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■ INDEXsyslogcentralized logging
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