Re: [PATCH 00/23] per device dirty throttling -v8

[Arjan van de Ven <arjan@infradead.org>]

On Sat, 2007-08-04 at 17:48 -0400, Theodore Tso wrote:
> On Sat, Aug 04, 2007 at 01:13:19PM -0700, Arjan van de Ven wrote:
> > there is another trick possible (more involved though, Al will have to
> > jump in on that one I suspect): Have 2 types of "dirty inode" states;
> > one is the current dirty state (meaning the full range of ext3
> > transactions etc) and "lighter" state of "atime-dirty"; which will not
> > do the background syncs or journal transactions (so if your machine
> > crashes, you lose the atime update) but it does keep atime for most
> > normal cases and keeps it standard compliant "except after a crash".
> 
> That would make us standards compliant (POSIX explicitly says that
> what happens after a unclean shutdown is Unspecified) and it would
> make things a heck of a lot faster.  However, there is a potential
> problem which is that it will keep a large number of inodes pinned in
> memory, which is its own problem.  So there would have to be some way
> to force the atime updates to be merged when under memory pressure,
> and and perhaps on some much longer background interval (i.e., every
> hour or so).

on the journalling side this would be one transaction (not 5 milion)
and... since inodes are grouped on disk, you can even get some better
coalescing this way... 

Wonder if we could do inode-grouping smartly; eg if we HAVE to write
inode X, also write out the atime-dirty inodes in range X-Y to X+Y
(where Y is some tunable) in the same IO..


-- 
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