Messages in this thread | | | Date | Mon, 30 Mar 2009 14:36:18 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 5/7] vfs: Add wbcflush sysfs knob to disable storage device writeback cache flushes |
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On Mon, Mar 30 2009, Fernando Luis Vázquez Cao wrote: > Add a sysfs knob to disable storage device writeback cache flushes. > > Signed-off-by: Fernando Luis Vazquez Cao <fernando@oss.ntt.co.jp> > --- > > diff -urNp linux-2.6.29-orig/block/blk-barrier.c linux-2.6.29/block/blk-barrier.c > --- linux-2.6.29-orig/block/blk-barrier.c 2009-03-24 08:12:14.000000000 +0900 > +++ linux-2.6.29/block/blk-barrier.c 2009-03-30 17:08:28.000000000 +0900 > @@ -318,6 +318,9 @@ int blkdev_issue_flush(struct block_devi > if (!q) > return -ENXIO; > > + if (blk_queue_nowbcflush(q)) > + return -EOPNOTSUPP; > + > bio = bio_alloc(GFP_KERNEL, 0); > if (!bio) > return -ENOMEM; > diff -urNp linux-2.6.29-orig/block/blk-core.c linux-2.6.29/block/blk-core.c > --- linux-2.6.29-orig/block/blk-core.c 2009-03-24 08:12:14.000000000 +0900 > +++ linux-2.6.29/block/blk-core.c 2009-03-30 17:08:28.000000000 +0900 > @@ -1452,7 +1452,8 @@ static inline void __generic_make_reques > goto end_io; > } > if (bio_barrier(bio) && bio_has_data(bio) && > - (q->next_ordered == QUEUE_ORDERED_NONE)) { > + (blk_queue_nowbcflush(q) || > + q->next_ordered == QUEUE_ORDERED_NONE)) { > err = -EOPNOTSUPP; > goto end_io; > }
This (and the above hunk) should be changed. -EOPNOTSUPP means the target does not support barriers, that is a different thing to flushes not being needed. A file system issuing a barrier and getting -EOPNOTSUPP back will disable barriers, since it now thinks that ordering cannot be guaranteed.
A more appropriate change would be to successfully complete a flush without actually sending it down to the device if blk_queue_nowbcflush() is true. Then blkdev_issue_flush() would just work as well. It also needs to take stacking into account, or stacked drivers will have to propagate the settings up the stack. If you allow simply the barrier to be passed down, you get that for free.
> +static struct queue_sysfs_entry queue_wbcflush_entry = { > + .attr = {.name = "wbcflush", .mode = S_IRUGO | S_IWUSR }, > + .show = queue_wbcflush_show, > + .store = queue_wbcflush_store, > +}; > +
Naming is also pretty bad, perhaps something like "honor_cache_flush" would be better, or perhaps "cache_flush_needed". At least something that is more descriptive of this setting actually controls, wbcflush does not do that.
-- Jens Axboe
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