fsck strangeness

[CyberLeo Kitsana]

Ian Smith wrote:
> My knowledge of this is thin, despite reading McKusick's paper through
> several times, but we're told that background fsck runs on a snapshot of
> the fs concerned.  How any bg fsck corrections are woven back into the
> live fs later is still a mystery to me, but that's because I still have
> an only barely superficial understanding of how snapshots work ..

Background FSCK only repairs a small subset of filesystem
incosistencies. Specifically, those inconsistencies that softupdates
allows to occur, such as data blocks allocated out of the bitmap, but
not actually assigned to any inode. Background FSCK only needs to find
these (by looking at a fully consistent and unchanging snapshot of the
filesystem) and deallocate them in the live filesystem, a simple
operation given that it's guaranteed nothing will be using a block that
is both marked used and not assigned to anything.

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