Re: fsck command line API

["Pali Rohár" <pali.rohar@gmail.com>]

On Saturday 30 December 2017 08:20:45 Theodore Ts'o wrote:
> On Sat, Dec 30, 2017 at 01:03:52PM +0100, Pali Rohár wrote:
> > 
> > Hi! Thank you for detailed information. I have just one more question,
> > what should filesystem specific fsck do if is started by -a or -p (or
> > with -y) during boot and filesystem structures indicates that last time
> > it was successfully (clean) unmounted? Should it scan whole disk and
> > check all data (files/directories/structures) for consistency? Or should
> > trust for "clean" state and stop? And if stop, how to tell that
> > filesystem fsck to really scan whole disk? For example scanning 2TB disk
> > is really time consuming, specially at boot time.
> 
> What e2fsck (fsck.extN) does is check to see if the file system has
> the "errors/corruptions were detected by the kernel" bit set.  If so,
> it will do a full check.  Otherwise, if time-based or mount-based
> criteria is enabled, and exceeded, then e2fsck will do a full check.
> Otherwise, it will stop.
> 
> The other thing e2fsck for ext4 file systems will do is to replay the
> journal.  This is useful because fsck will run fsck in parallel, while
> mount -a mounts file systems serially.  So running the journal in
> parallel when you have multiple disk spindles can be a big win.  This
> may be less of a big deal these days since systemd will run mounts in
> parallel.
> 
> As far as time-based or mount-based full checks (see tune2fs for
> discussion on this topic), we don't enable this by default any more in
> e2fsprogs.  That's precisely because doing full check for 10 TB disk
> takes and 60TB RAID arrays takes a long time.  The idea of doing mount
> based checks goes back to the BSD days, because disks and memory are
> really crappy, and so checking to find problems before they become
> catastrophic data loss events made sense.  These days for big disks,
> the cost/benefit ratio doesn't work out as well.
> 
> Also, if you have snapshot support in your file system, you can simply
> create a snapshot, and run the fsck on the snapshot.  An example of
> how to do this can be found here:
> 
> https://git.kernel.org/pub/scm/fs/ext2/e2fsprogs.git/tree/contrib/e2croncheck
> 
> Cheers,
> 
> 					- Ted

Ok, thank you for explanation. In case for a new UDF fsck tool it would
also make sense then to not do full check at boot time if filesystem is
marked as clean.

-- 
Pali Rohár
pali.rohar@gmail.com