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SubjectRe: Soft metadata updates paper w/code
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Colin Plumb wrote:
>
> Looking at reiserfs, I found a University of Michigan tech report
> on "soft updates" a technique to remove all synchronous writes
> from a file system yet maintain fsck-less crashes.
>
> (By always marking data in use before it's used and ensuring that
> data is not used before it's marked free, a crash can leave some

If you first mark a block as used, and only then write the block,
the filesystem won't be corrupt when the system crashes inbetween.
However, your DATA will be corrupt.

I think it's better to have fsck detect a broken filesystem, than to
have a file silently contain bad data. As far as I know, Linus also
thinks this way.

You also eliminate the possibility of reordering writes at the
driver or drive level. (e.g. "tagged queuing" is forbidden...)

All this would be very hard to debug: You'd have to try throwing
the switch on your system quite a few times to be sure it works.

Roger.



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