On Tue, Jul 18, 2006 at 06:58:56PM +1000, Neil Brown wrote: > On Tuesday July 18, nathans@xxxxxxx wrote: > > On Mon, Jul 17, 2006 at 01:32:38AM +0800, Federico Sevilla III wrote: > > > On Sat, Jul 15, 2006 at 12:48:56PM +0200, Martin Steigerwald wrote: > > > > I am currently gathering information to write an article about journal > > > > filesystems with emphasis on write barrier functionality, how it > > > > works, why journalling filesystems need write barrier and the current > > > > implementation of write barrier support for different filesystems. > > "Journalling filesystems need write barrier" isn't really accurate. > They can make good use of write barrier if it is supported, and where > it isn't supported, they should use blkdev_issue_flush in combination > with regular submit/wait. blkdev_issue_flush() causes a write cache flush - just like a barrier typically causes a write cache flush up to the I/O with the barrier in it. Both of these mechanisms provide the same thing - an I/O barrier that enforces ordering of I/Os to disk. Given that filesystems already indicate to the block layer when they want a barrier, wouldn't it be better to get the block layer to issue this cache flush if the underlying device doesn't support barriers and it receives a barrier request? FWIW, Only XFS and Reiser3 use this function, and only then when issuing a fsync when barriers are disabled to make sure a common test (fsync then power cycle) doesn't result in data loss... > > Noone here seems to know, maybe Neil &| the other folks on linux-raid > > can help us out with details on status of MD and write barriers? > > In 2.6.17, md/raid1 will detect if the underlying devices support > barriers and if they all do, it will accept barrier requests from the > filesystem and pass those requests down to all devices. > > Other raid levels will reject all barrier requests. Any particular reason for not supporting barriers on the other types of RAID? Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
