SUMMARY: Can you have swap partitions starting at cylinder 0?

From: Chris Metcalf (metcalf@catfish.LCS.MIT.EDU)
Date: Wed Jan 08 1992 - 19:41:06 CST

In article <> I wrote:
>My understanding has always been that
>if you had a swap partition at the beginning of a disk, you would
>eventually get your disk label (and partition info) swapped on, and die
>horribly, so I've always avoided this configuration.

It turns out that since SunOS 4.0 (at least), swapping has been handled
carefully by the kernel: the first block of every partition is
skipped, so it's no longer possible to clobber the label. (However,
if you use cylinder zero as a raw device, e.g. for a database, it *is*
still possible to wipe out the label.)

The following piece of kernel comment from swap_init() bears this out:

     * To prevent swap I/O requests from crossing the boundary
     * between swap areas, we erect a "fence" between areas by
     * not allowing the first page of each swap area to be used.
     * (This also prevents us from scribbling on the disk label
     * if the swap partition is the first partition on the disk.)
     * This may not be strictly necessary, since swap_blksize also
     * prevents requests from crossing the boundary.

Thanks to the following for sending me facts (and opinions):

        Tad Guy <> (Alexander Dupuy)
        Thomas Weihrich <> (Raymond Essick) (Chinson Yi) (Mike Sullivan { Nowhere Man })

			Chris Metcalf, MIT Laboratory for Computer Science // (617) 253-7766

This archive was generated by hypermail 2.1.2 : Fri Sep 28 2001 - 23:06:33 CDT