> 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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