fsck on a mounted fs as read-only

Warren Block wblock at wonkity.com
Thu May 31 13:16:13 UTC 2012

On Thu, 31 May 2012, Polytropon wrote:

> On Thu, 31 May 2012 14:19:33 +0400, dmitry at zhigulinet.ru wrote:
>> Good afternoon.
>> Could not tell whether you can run fsck on checking mounted
>> file system as read-only, if prior to that with which the parameters
>> ftp # mount
>> ...
>> / dev/aacd0 on / var / ftp (ufs, NFS exported, local, read-only)
>> Launched with these parameters and this is what gives
>> ftp # fsck -yf / dev/aacd0
>> ** / Dev/aacd0 (NO WRITE)
>> ** Last Mounted on / var / ftp
>> ** Phase 1 - Check Blocks and Sizes
>> As I understand it does not fix the fsck filesystem.
> Correct. For file system modifications the file system may not
> be mounted because "lower level operations" maybe will take
> place. In your current setting, only checks will be performed,
> but _if_ something needs to be modified, it will not happen.
> The reason: It _might_ affect the file system to change, even
> if it's "just" in read-only state.
> Solution: Unmount the file system and re-run fsck.

fsck(8) can work on filesystems mounted read only.  If, say, 
graphics/cairo causes X to crash the machine, rebooting into single-user 
mode and running
# fsck -y -t ufs
will clean up the read-only mounted / also.  Agreed that this should be 
avoided if possible, but sometimes it is necessary.

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