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Please assume I'm using a recent version of Debian Linux, on ext4, on a 2 TB Seagate Momentus hard drive, with all the default settings.

A) What does e2fsck do during a normal bootup? (And what is this operation called?)

B) What does e2fsck do after an unclean shutdown? [Edit: According to a post by Ted Ts'o, it does a "journal replay". This normally only takes a few seconds.]

C) What does e2fsck do once every 30 mounts or so? [Edit: According to Mr. Ts'o, it does a "filesystem consistency check". This normally takes at least a few minutes.]

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A> Ordinarily, e2fsck will simply check a partition to determine whether it believes itself to be cleanly unmounted. If it was, move on to case C.

B> If the partition seems to have been dirtily unmounted or otherwise claims to need repair, it will then perform a full check of the filesystem (depending on your particular system's startup options). This "checking and fixing" is called a "filesystem check" (which is what fsck is short for).

C> Just to be safe, a partition will automatically flag itself as dirty so that upon next inspection, a full check will be run just in case the filesystem's built-in sanity checking has failed or missed some bit rot or other filesystem corruption. This interval can be adjusted with tune2fs (or similar tools for filesystems not in the ext2fs family) utility. This is also simply called a "filesystem check".

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  • I thought case B takes just a few seconds, but case C takes five or ten minutes. Am I wrong? Mar 24, 2017 at 20:31
  • It depends. Generally, case "C" is the partition just saying "hey, pretend this is case 'B', boss."
    – DopeGhoti
    Mar 24, 2017 at 20:43
  • I still believe that case B is much quicker than case C. I've found a source to support my claim, and have edited a link to it into my question. I would downvote you, but I don't have enough reputation points. Mar 24, 2017 at 21:38
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    This answer basically only addresses ext2, not ext3 or ext4, since it doesn't mention the journal (which exists precisely to provide a faster alternative to case B). When there's a journal, an unclean shutdown usually leads to journal replay instead of a full scan.
    – user41515
    Apr 8, 2017 at 21:32

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