Re: [PATCH v2] writeback: Do not sync data dirtied after sync start

[Jan Kara <jack@suse.cz>]

On Sat 28-09-13 08:31:19, Wu Fengguang wrote:
> Hi Jan,
> 
> On Thu, Sep 26, 2013 at 09:23:58PM +0200, Jan Kara wrote:
> > When there are processes heavily creating small files while sync(2) is
> > running, it can easily happen that quite some new files are created
> > between WB_SYNC_NONE and WB_SYNC_ALL pass of sync(2). That can happen
> > especially if there are several busy filesystems (remember that sync
> > traverses filesystems sequentially and waits in WB_SYNC_ALL phase on one
> > fs before starting it on another fs). Because WB_SYNC_ALL pass is slow
> > (e.g. causes a transaction commit and cache flush for each inode in
> > ext3), resulting sync(2) times are rather large.
> 
> This is a very good change. An old problem that may worth noting here
> is that inode_dirtied_after() has a workaround for the inodes whose
> ->dirtied_when is never updated due to being constantly redirtied.
> That workaround still leaves a small time window that sync() may skip
> a should-be-synced inode. Since the problem existed before this patch
> so I'm fine with this change.
  Thanks for review. Do you mean the situation when jiffies in
inode->dirtied_when essentially wrap around on 32-bit systems wrt current
time? Yes, that is still a problem for which I don't know a better fix than
the current workaround.

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR