This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

Unix, Xenix and ODT General FAQ

Why does fsck want a scratch file?

This is an ancient post that has little or no relevance to modern systems. Fsck doesn't even have a "scratch" file option today, though with terabyte disks it is possible to imagine a system with not enough ram for fsck. However, modern fsck's know how to use swap, so this gets a bit harder, but see Running fsck on Large Paritions in FreeBSD.

There is an archaic limit to how large a filesystem fsck can check using available memory (archaic because it hasn't kept up with the growth in system memory). The exact limit is not something that appears to be documented anywhere, and may also vary between versions and different filesystem types. When this size is exceeded, fsck will want to use a scratch file to hold information while it's running. Before I continue, please read the man page for the -t option to fsck, and pay particular attention to the warning about following it with a space. Failure to do so may destroy data.

You may have been prompted at the time you installed the OS to create a scratch division if your root filesystem was too large for fsck to check. If so, you might wish to edit /etc/default/filesys to specify that this should be used if the system has to check the root filesystem after a crash. Add -t /dev/scratch (or whatever you called the scratch filesystem) in the fsckflags= entry for /dev/root.

For any filesystem other than root, you can generally use a temporary file on your root filesystem as a scratch file. fsck will create it and delete it automatically, once you've told it what file to use. I usually use /tmp/scratch.

If, however, you find you need to fsck /dev/root, which is too big to check without a scratch file and you don't have a scratch filesystem, you still have some choices. A blank (but formatted) floppy diskette will often do the trick. If you're running fsck in single-user mode and you can guarantee that no swapping has taken place and no swapping will take place while you're running fsck, you could use /dev/swap.

For Unix 3.2v4.2 and ODT 3.0, see uod418a, which provides a new fsck which may eliminate the need for a scratch file.

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