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    SubjectRe: [PATCH] [RFC] Advanced TCA SCSI Disk Hotswap
    On Sun, 27 Oct 2002, Rob Landley wrote:

    (maybe wrap lines around column 70 ? :)


    Stephen Tweedie did something like this already (for 2.4.19-pre10),
    called "testdrive". It uses loopback over a block device.
    He says that it will need modifications to use bio in 2.5.

    See here:


    | Hmmm... Not being familiar with the SCSI layer but sticking my nose in anyway
    | on general block device/mount point hotplug issues:
    | How hard would it be to write a simple debugging function to lobotomize a
    | block device? (So that all further I/O to that sucker immediately returns an
    | error.) Not just simulating an a hot extraction (or catastrophic failure) of
    | a block device, but also something you could use to see how gracefully
    | filesystems react.
    | The reason I ask is there was a discussion a while back about the new lazy
    | unmount (umount -l /blah/foo) not always being quite enough, and that
    | sometimes what what you want is basically "umount -9 /blah/foo" (ala kill
    | -9). Close all files, reparent all process home directories and chroot mount
    | points to a dummy inode, flush all I/O, drive a stake through the
    | superblock's heart, and scatter the ashes at sea. Somebody posted a patch to
    | actually do this. (Against 2.4, i think.) I could probably dig it up if you
    | were curious. Let's see...
    | The eject command should certainly have an "umount with shotgun" option, so
    | zombie processes can't pin your CD in the drive. (Your average end-user is
    | NOT going to be able to grovel through /proc to figure out which processes
    | have an open filehandle or home directory under the cdrom mount point so it
    | can kill them and get the disk out. They're going to power cycle the machine
    | and eject it while the bios is in charge. I've done this myself a couple of
    | times when I'm in a hurry.)
    | Anyway, if the block device under the filesystem honestly does go away for
    | hotplug eject reasons, the obvious thing to do is umount -9 the sucker
    | immediately so userspace can collapse gracefully (or even conceivably
    | recover). The main difference here is that the flushing would all error out
    | and get discarded, and this wouldn't always get reported to the user, but
    | thanks to write cacheing that's the case anyway. (Use some variant of
    | O_DIRECT or fsync if you care.) The errors userspace does see switch from
    | "all my I/O failed with a media error" to "all my filehandles closed out from
    | under me" (and the directory I'm in has been deleted), but that's still
    | relatively logical behavior.
    | Does this sound like it's off in left field?
    | Rob

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