On Mon, 29 Aug 2016, Mike Snitzer wrote: > On Sat, Aug 27 2016 at 11:09am -0400, > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > > > > > On Fri, 26 Aug 2016, Mike Snitzer wrote: > > > > > On Thu, Aug 25 2016 at 4:13pm -0400, > > > Jens Axboe <axboe@xxxxxxxxx> wrote: > > > > > > > On 08/25/2016 12:34 PM, Mikulas Patocka wrote: > > > > > > > > > >Device mapper can't split the bio in generic_make_request - it frees the > > > > >md->queue->bio_split bioset, to save one kernel thread per device. Device > > > > >mapper uses its own splitting mechanism. > > > > > > > > > >So what is the final decision? - should device mapper split the big bio or > > > > >should bcache not submit big bios? > > > > > > > > > >I think splitting big bios in the device mapper is better - simply because > > > > >it is much less code than reworking bcache to split bios internally. > > > > > > > > > >BTW. In the device mapper, we have a layer dm-io, that was created to work > > > > >around bio size limitations - it accepts unlimited I/O request and splits > > > > >it to several bios. When bio size limitations are gone, we could simplify > > > > >dm-io too. > > > > > > > > The patch from Ming Lei was applied for 4.8 the other day. > > > > > > See linux-block commit: > > > 4d70dca4eadf2f block: make sure a big bio is split into at most 256 bvecs > > > http://git.kernel.dk/cgit/linux-block/commit/?h=for-linus&id=4d70dca4eadf2f95abe389116ac02b8439c2d16c > > > > But this patch won't work for device mapper, blk_bio_segment_split is > > called from blk_queue_split and device mapper doesn't use blk_queue_split > > (it can't because it frees queue->bio_split). > > > > We still need these two patches: > > https://www.redhat.com/archives/dm-devel/2016-May/msg00211.html > > https://www.redhat.com/archives/dm-devel/2016-May/msg00210.html > > I'm left wondering whether it'd be better to revert commit dbba42d8a9e > ("dm: eliminate unused "bioset" process for each bio-based DM device") That would create one more process per dm device. There is no need to create another process if the bio can be split in the device mapper. > And also look for other areas of DM that could leverage the block core's > relatively new splitting capabilities. > > Otherwise, we'll just be sprinking more BIO_MAX_PAGES-based splitting > code in DM targets because DM is so "special" that it doesn't need the > block core's splitting (even though it clearly would now benefit). It would be possible to split bios on BIO_MAX_PAGES boundary in the dm core. But I checked all the dm targets and I found problem only in dm-crypt and dm-log-writes. So there is no need to split bio bios for the other targets. > Mike Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
