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    SubjectRe: [PATCH 14/39] union-mount: Union mounts documentation
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    Neil Brown:
    > I wonder if the restriction is not more serious than this.
    > Given the prevalence of "copy-up", particularly of directories, I would think
    > that even off-line upgrade would not be supported.
    > If the upgrade adds a file in a directory that has already been read (and
    > hence copied-up), or changes a file that has been chmodded, then the upgrade
    > will not be completely visible, which sounds dangerous.
    :::
    > I see two block-layer solutions. The obvious is a COW block device as you
    > have mentioned. I am not convinced that it is as bad as you think.
    :::

    DM snapshot provides the COW block feature and it will match your idea
    since the size of COW device is much smaller genearally. But it doesn't
    support off-line upgrade either. If you do, it is equivalent to corrupt
    filesystem for DM snapshot device.

    Here is pros/cons of DM snapshot comparing a union.
    - the number of bytes to be copied between devices is much smaller.

    - the type of filesystem must be one and only.
    - the fs must be writable, no readonly fs, even for the lower original
    device. so the compression fs will not be usable. but if we use
    loopback mount, we may address this issue.
    for instance,
    mount /cdrom/squashfs.img /sq
    losetup /sq/ext2.img
    losetup /somewhere/cow
    dmsetup "snapshot /dev/loop0 /dev/loop1 ..."

    - it will be difficult (or needs more operations) to extract the
    difference between the original device and COW.

    - DM snapshot-merge may help a lot when users try merging. in the
    fs-layer union, users will use rsync(1).

    - in fs-based union, users can add/remove members(layer) dynamicall
    without unmounting. of course, all files on the removing layer should
    not be busy.


    Also here is my concern about UnionMount. All these issues have been
    reported before.
    - for users, the inode number may change silently. eg. copy-up.
    - link(2) may break by copy-up.
    - read(2) may get an obsoleted filedata (fstat(2) too).
    - fcntl(F_SETLK) may be broken by copy-up.
    - unnecessary copy-up may happen, for example mmap(MAP_PRIVATE) after
    open(O_RDWR).


    J. R. Okajima


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