Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

[Alex Tomas <alex@clusterfs.com>]

Andrew Morton wrote:
>>> But under this proposal, t_sync_datalist just gets removed: the new
>>> ordered-data mode _only_ need to do the sb->inode->page walk.  So if I'm
>>> understanding you, the way in which we'd handle any such race is to make
>>> kjournald's writeback of the dirty pages block in lock_page().  Once it
>>> gets the page lock it can look to see if some other thread has mapped the
>>> page to disk.
>> if I'm right holding number of pages locked, then they won't be locked, but
>> writeback. of course kjournald can block on writeback as well, but how does
>> it find pages with *newly allocated* blocks only?
> 
> I don't think we'd want kjournald to do that.  Even if a page was dirtied
> by an overwrite, we'd want to write it back during commit, just from a
> quality-of-implementation point of view.  If we were to leave these pages
> unwritten during commit then a post-recovery file could have a mix of
> up-to-five-second-old data and up-to-30-seconds-old data.

trying to implement this I've got to think that there is one significant
difference between t_sync_datalist and sb->inode->page walk: t_sync_datalist
is per-transaction. IOW, it doesn't change once transaction is closed. in
contrast, nothing (currently) would prevent others to modify pages while
commit is in progress. I think this is serious disadvantage of the solution.

what I'd propose is sort of in-core tracker for all data-related IOs in flight
(assigned to specific transaction) and wait for their completion in commit
thread.

thanks, Alex