Hi, I think commit 1e87901e18 was wrong. Starting with that commit the REQ_FLUSH and REQ_FUA bits get stripped away if the queue does not advertise REQ_FLUSH or REQ_FUA support.
But the REQ_FLUSH bit is also tested for when not merging requests (blk_queue_bio()) or when it comes to the elevator (blk_flush_plug_list()). So, since this patch the elevator reorders write requests on queues that do not have REQ_FLUSH or REQ_FUA set. While on queues that have REQ_FLUSH or REQ_FUA set, the elevator does not reorder writes across FLUSHes. The Documentation/block/writeback_cache_control.txt file says: --snip-- Implementation details for filesystems -------------------------------------- Filesystems can simply set the REQ_FLUSH and REQ_FUA bits and do not have to worry if the underlying devices need any explicit cache flushing and how the Forced Unit Access is implemented. The REQ_FLUSH and REQ_FUA flags may both be set on a single bio. --snap-- I have the impression every file system lets IO drain, and issues a flush afterwards with the blkdev_issue_flush() function. BTW that function turns into a non-obvious no-op as soon as the queue does not have the REQ_FUA or REQ_FLUSH bits set. It does not look like it is a no-op by intention. The file systems seem to be all fine, only in DRBD we have a mode were we depend on REQ_FUA/REQ_FLUSH requests being real boundaries for reordering of writes. This is broken since the mentioned commit as we recently found out. I suggest that either this commit gets reverted, or the documentation is updated. I am ready to prepare such a patch, but I need directions how it should be fixed. Best regards, Phil -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
