Re: ext4 features (checksums)

From: Bill Davidsen
Date: Wed Jul 05 2006 - 07:57:16 EST


Helge Hafting wrote:
On Tue, Jul 04, 2006 at 01:19:11PM +0200, Krzysztof Halasa wrote:
Neil Brown <neilb@xxxxxxx> writes:

With checksums - the filesystem is in a better position to:
- be selective about what is checksummed - no point checksumming
blocks that aren't part of any file. Some blocks (highlevel
metadata) might always be checksummed, while other blocks
(regular data) might not if a 'fast' option was chosen.
The same applies to RAID - for example, why "synchronise" unused area?

Indeed. RAID usually avoid checksumming unused area, it sums on write
and you don't write "unused" stuff.

Not syncing unused area is possible, if there was a way for raid resync
to ask the fs what blocks are not in use. I.e. get the
free block list in disk block order. Then raid resync could skip those.

Current RAID code supports having a bitmap of dirty stripes, and can just sync those during recovery. I'm sure Neil could explain it better, but this is available without worrying about fs type. Now. Today.

--
Bill Davidsen <davidsen@xxxxxxx>
Obscure bug of 2004: BASH BUFFER OVERFLOW - if bash is being run by a
normal user and is setuid root, with the "vi" line edit mode selected,
and the character set is "big5," an off-by-one errors occurs during
wildcard (glob) expansion.

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