Re: [RFC] ext4 metadata checksumming design

[Coly Li]

On 2011å08æ17æ 11:25, Darrick J. Wong Wrote:
> Hi all,
> 
> I've created a page on the ext4 wiki outlining the patchset that I'm working on
> to add metadata checksumming to ext4.  The page can be found at this address:
> https://ext4.wiki.kernel.org/index.php/Ext4_Metadata_Checksums
> 

Hi Darrick,

I just go through the proposal, have on objection for most of the text. Only some things want to confirm,
1) If a metadata_csum enabled file system is metadata_csum disabled, it should be better to mark the block group or
inode whether the existing (disabled) checksum is valid or not. So if people re-enable metadata_csum, we can save quite
a lot of time to re-build check sums for all metadata objects.
2) In no-journal mode, every time when we modify the metadata objects, we may have to hold a lock, calculate the check
sum, and release the lock, which may introduce performance regression. I hope this is only my unnecessary over worry.

BTW, an engineer in Taobao kernel team, is trying to count different meta data objects I/O in run time now. One of the
first efforts, is trying to unify a set of routines to read or dirty meta data object blocks. A.k.a something (might)
like ext4_read_ext_block(), ext4_read_idx_block(), ... etc. Then the counting routines can be added inside the meta data
object blocks I/O routines. So far, it seems the modification is not trivial, needs more study on the code. Anyway,
since you mentioned on the wiki page, just let you know what we are doing now :-)

P.S. The idea of meta data I/O counting is to help us understanding the I/O characteristic of our online servers running
Ext4 file systems, which is the basic material for further I/O performance optimization.

Thanks.
-- 
Coly Li
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