On Mar 24, 2006 11:13 -0800, Mingming Cao wrote:
> There are reasons for zeroing indirect blocks on truncate:
>
> * There are limits to the size of a single journal transaction
> (1/4 of the journal size). When truncating a large fragmented
> file, it may require modifying so many block bitmaps and group
> descriptors that it forces a journal transaction to close out,
> stalling the unlink operation.
> * Because of this per-transaction limit, truncate needs to zero
> the [dt]indirect blocks starting from the end of the file, in
> case it needs to start a new transaction in the middle of the
> truncate (ext3 guarantees that a partially-completed truncate
> will be consistent/completed after a crash).
> * The read/write of the file's [dt]indirect blocks from the end of
> the file to the beginning can take a lot of time, as it does
> this in single-block chunks and the blocks are not contiguous.
See my recent post on how this performance problem could be fixed.
Cheers, Andreas
--
Andreas Dilger
Principal Software Engineer
Cluster File Systems, Inc.
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