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SubjectRe: Soft metadata updates paper w/code
Rogier Wolff wrote:
>
> 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.

Not necessarily. If the block that is written contains some sort
of checksum as the last piece of information that is written in
the block, the validity of the block can be easily checked. Thus
if the whole block (together with the checksum) is written and then
the machine crashes, the block will still contain valid data. And
if the machine crashes before the checksum is written, the block
will be invalid and will be reported as such when the file is
accessed.

Andrej


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