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    SubjectRe: [PATCH] 64 bit scsi read/write


    On Sun, 15 Jul 2001, Chris Wedgwood wrote:

    > On Sat, Jul 14, 2001 at 10:11:30PM +0200, Daniel Phillips wrote:
    >
    > Atomic commit. The superblock, which references the updated
    > version of the filesystem, carries a sequence number and a
    > checksum. It is written to one of two alternating locations. On
    > restart, both locations are read and the highest numbered
    > superblock with a correct checksum is chosen as the new filesystem
    > root.
    >
    > Yes... and which ever part of the superblock contains the sequence
    > number must be written atomically.
    >
    > The point is, you _NEED_ to be sure that data written before the
    > superblock (or indeed anywhere further up the tree, you can make
    > changes in theory which don't require super-block updates) are written
    > firmly to the platters before any thing which refers to it is updated.
    >
    > Alan was saying with IDE you cannot reliably do this, I assume you can
    > with SCSI was my point.

    In the IBM solution to this (1977-78, VM/CMS) the critical data was
    written at the begining and the end of the block. If the two data items
    didn't match then the block was rejected.

    john alvord
    >
    >
    >
    > --cw
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