Running e2fsck on a mounted filesystem

I know, running fsck on a mounted filesystem is utterly unrecommended. The command warns you (it actually frightens you) with the following message:

# fsck /dev/VolGroup00/LogVol00
fsck 1.41.4 (27-Jan-2009)
e2fsck 1.41.4 (27-Jan-2009)
/dev/VolGroup00/LogVol00 is mounted.  

WARNING!!!  Running e2fsck on a mounted filesystem may cause
SEVERE filesystem damage.

Do you really want to continue (y/n)? no

check aborted.

But sometimes I need to check a filesystem in a remote host, so I cannot boot from a liveCD to run fsck in the unmounted device. Looking for an option allowing me to overcome this nuisance I found the following in e2fsck’s man page:

Note  that  in general it is not safe to run e2fsck on mounted filesys-
tems.  The only exception is if the -n option is specified, and -c, -l,
or  -L  options  are not specified.

Using e2fsck instead of fsck is not a problem because it checks ext2 and ext3 filesystems and mine are ext3 (fsck checks and optionally repairs a lot of filesystem types).

Let us see what the man page says about the -n option:

-n     Open the filesystem read-only, and assume an answer of  `no'  to
       all  questions.   Allows  e2fsck  to  be used non-interactively.
       (Note: if the -c, -l, or -L options are specified in addition to
       the -n option, then the filesystem will be opened read-write, to
       permit the bad-blocks list to be  updated.   However,  no  other
       changes will be made to the filesystem.)  This option may not be
       specified at the same time as the -p or -y options.

So it seems to be safe running it with that option. But the e2fsck man page also states (dealing with the safe check):

However, even if it is safe to do
so, the results printed by e2fsck are not valid if  the  filesystem  is
mounted.    If e2fsck asks whether or not you should check a filesystem
which is mounted, the only correct answer is ``no''.  Only experts  who
really know what they are doing should consider answering this question
in any other way.

So, I use it only when I want to know if there is something wrong with the filesystem. I run it with the -f option too which forces checking even if the file system seems clean.

# e2fsck -fn /dev/VolGroup00/LogVol00 
e2fsck 1.41.4 (27-Jan-2009)
Warning!  /dev/VolGroup00/LogVol00 is mounted.
Warning: skipping journal recovery because doing a read-only filesystem check.
Pass 1: Checking inodes, blocks, and sizes
Pass 2: Checking directory structure
Pass 3: Checking directory connectivity
Pass 4: Checking reference counts
Pass 5: Checking group summary information
F10-i686-Live: 123335/475136 files (3.8% non-contiguous), 876070/1900544 blocks

If the output is like above all is OK, but if the device has errors, then you will need to run fsck with the filesystem unmounted.

speak up

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