Btw, something semi-related I've been looking at recently: Currently O_DIRECT writes bypass all kernel caches, but there they do use the disk caches. We currenly don't have any barrier support for them at all, which is really bad for data integrity in virtualized environments. I've started thinking about how to implement this. The simplest scheme would be to mark the last request of each O_DIRECT write as barrier requests. This works nicely from the FS perspective and works with all hardware supporting barriers. It's massive overkill though - we really only need to flush the cache after our request, and not before. And for SCSI we would be much better just setting the FUA bit on the commands and not require a full cache flush at all. The next scheme would be to simply always do a cache flush after the direct I/O write has completed, but given that blkdev_issue_flush blocks until the command is done that would a) require everyone to use the end_io callback and b) spend a lot of time in that workque. This only requires one full cache flush, but it's still suboptimal. I have prototypes this for XFS, but I don't really like it. The best scheme would be to get some highlevel FUA request in the block layer which gets emulated by a post-command cache flush. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
