Messages in this thread | | | Date | Sun, 24 Apr 2011 11:15:31 +0800 | From | Wu Fengguang <> | Subject | Re: [PATCH 5/6] writeback: sync expired inodes first in background writeback |
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> One of the many requirements for writeback is that if userspace is > continually dirtying pages in a particular file, that shouldn't cause > the kupdate function to concentrate on that file's newly-dirtied pages, > neglecting pages from other files which were less-recently dirtied. > (and dirty nodes, etc).
Sadly I do find the old pages that the flusher never get a chance to catch and write them out.
In the below case, if the task dirties pages fast enough at the end of file, writeback_index will never get a chance to wrap back. There may be various variations of this case.
file head [ *** ==>***************]==> old pages writeback_index fresh dirties
Ironically the current kernel relies on pageout() to catch these old pages, which is not only inefficient, but also not reliable. If a full LRU walk takes an hour, the old pages may stay dirtied for an hour.
We may have to do (conditional) tagged ->writepages to safeguard users from losing data he'd expect to be written hours ago.
Thanks, Fengguang
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