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    SubjectRe: (reiserfs) Re: LVM / Filesystems / High availability
    Theodore Y. Ts'o wrote:
    > Different people use LVM in different ways. Some people think of
    > LVM as something you can do by creating a fake block device "out of
    > bits of real devices", as you put it. Other people put stronger
    > requirements on "LVM", which effectively requires that the filesystem
    > be involved at a very real level.

    There are two levels of LVM on HP-UX:

    1. The basic LVM functionality is to provide a virtual block device
    (logical volumes) and also provides some RAID levels. Resizing of
    filesystem requires unmounting and using the extendfs utility.

    2. The optional on-line LVM provides on-line resizing functionality.
    This requires tight integration with the fs code. IMO on HP-UX
    this requires usage of the vxfs (veritas) file system and is not
    possible with hfs. The fsadm utility is used to perform on-line
    operations on a file system.

    3. Additional HA functionality (like shared volumes).

    The basic LVM functionality should be rather easy. The implementation
    of Heinz Mauelshausen does already include most of the functionality
    and the user space utilities. Merging the existing MD code (or the
    functionality) should get us there. All we'd need as a beginning
    is a tool per filesystem which allows offline fs resizing.

    > In general, existing implementations either don't deal at all, or they
    > deal by having the filesystem use a structured block address scheme one
    > way or another. For example, when we add extent maps to ext2, I plan to
    > leave room for so that block numbers can be 8 bytes long, instead of 4
    > bytes. The low 4 bytes will be the original block address, as before.
    > The next 2 bytes will eventually contain the LVM index, where each
    > additional logical volume will contain its own superblock and UUID, plus
    > an indication that this is a subsidiary volume (and the UUID of the
    > master volume). The master volume is the first volume created in the
    > LVM set, and it will contain a table mapping LVM index numbers to UUIDs'
    > of each subsidary volume. (The last 2 bytes of the 8 byte address will
    > be reserved for future expansion.)
    > This makes it easy to remove a LVM in the middle of the filesystem; you
    > simply move the blocks of that particular LVM to other LVM's, and then
    > remove the LVM from the master disk's LVM table.

    This still requires off-line resizing (as the re-sizing of ext2
    is not trivial to do safe)? For "online" functionality, ext2 would
    need to be able to use or free an extend on the fly. Otherwise
    using a logical volume should be fine.


    Michael Marxmeier Marxmeier Software GmbH
    E-Mail: Besenbruchstrasse 9
    Voice : +49 202 2431440 42285 Wuppertal, Germany
    Fax : +49 202 2431420

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