On Fri, Nov 18, 2022 at 12:49:15PM +0100, Andreas Hindborg wrote: > > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > CAUTION: This email originated from outside of Western Digital. Do not click on > > links or open attachments unless you recognize the sender and know that the > > content is safe. > > > > > > On Fri, Nov 18, 2022 at 10:41:31AM +0100, Andreas Hindborg wrote: > >> > >> Ming Lei <ming.lei@xxxxxxxxxx> writes: > >> > >> > CAUTION: This email originated from outside of Western Digital. Do not click on > >> > links or open attachments unless you recognize the sender and know that the > >> > content is safe. > >> > > >> > > >> > On Fri, Nov 18, 2022 at 01:35:29PM +0900, Damien Le Moal wrote: > >> >> On 11/18/22 13:12, Ming Lei wrote: > >> >> [...] > >> >> >>> You can only assign it to zoned write request, but you still have to check > >> >> >>> the sequence inside each zone, right? Then why not just check LBAs in > >> >> >>> each zone simply? > >> >> >> > >> >> >> We would need to know the zone map, which is not otherwise required. > >> >> >> Then we would need to track the write pointer for each open zone for > >> >> >> each queue, so that we can stall writes that are not issued at the write > >> >> >> pointer. This is in effect all zones, because we cannot track when zones > >> >> >> are implicitly closed. Then, if different queues are issuing writes to > >> >> > > >> >> > Can you explain "implicitly closed" state a bit? > >> >> > > >> >> > From https://zonedstorage.io/docs/introduction/zoned-storage, only the > >> >> > following words are mentioned about closed state: > >> >> > > >> >> > ```Conversely, implicitly or explicitly opened zoned can be transitioned to the > >> >> > closed state using the CLOSE ZONE command.``` > >> >> > >> >> When a write is issued to an empty or closed zone, the drive will > >> >> automatically transition the zone into the implicit open state. This is > >> >> called implicit open because the host did not (explicitly) issue an open > >> >> zone command. > >> >> > >> >> When there are too many implicitly open zones, the drive may choose to > >> >> close one of the implicitly opened zone to implicitly open the zone that > >> >> is a target for a write command. > >> >> > >> >> Simple in a nutshell. This is done so that the drive can work with a > >> >> limited set of resources needed to handle open zones, that is, zones that > >> >> are being written. There are some more nasty details to all this with > >> >> limits on the number of open zones and active zones that a zoned drive may > >> >> have. > >> > > >> > OK, thanks for the clarification about implicitly closed, but I > >> > understand this close can't change the zone's write pointer. > >> > >> You are right, it does not matter if the zone is implicitly closed, I > >> was mistaken. But we still have to track the write pointer of every zone > >> in open or active state, otherwise we cannot know if a write that arrive > >> to a zone with no outstanding IO is actually at the write pointer, or > >> whether we need to hold it. > >> > >> > > >> >> > >> >> > > >> >> > zone info can be cached in the mapping(hash table)(zone sector is the key, and zone > >> >> > info is the value), which can be implemented as one LRU style. If any zone > >> >> > info isn't hit in the mapping table, ioctl(BLKREPORTZONE) can be called for > >> >> > obtaining the zone info. > >> >> > > >> >> >> the same zone, we need to sync across queues. Userspace may have > >> >> >> synchronization in place to issue writes with multiple threads while > >> >> >> still hitting the write pointer. > >> >> > > >> >> > You can trust mq-dealine, which guaranteed that write IO is sent to ->queue_rq() > >> >> > in order, no matter MQ or SQ. > >> >> > > >> >> > Yes, it could be issue from multiple queues for ublksrv, which doesn't sync > >> >> > among multiple queues. > >> >> > > >> >> > But per-zone re-order still can solve the issue, just need one lock > >> >> > for each zone to cover the MQ re-order. > >> >> > >> >> That lock is already there and using it, mq-deadline will never dispatch > >> >> more than one write per zone at any time. This is to avoid write > >> >> reordering. So multi queue or not, for any zone, there is no possibility > >> >> of having writes reordered. > >> > > >> > oops, I miss the single queue depth point per zone, so ublk won't break > >> > zoned write at all, and I agree order of batch IOs is one problem, but > >> > not hard to solve. > >> > >> The current implementation _does_ break zoned write because it reverses > >> batched writes. But if it is an easy fix, that is cool :) > > > > Please look at Damien's comment: > > > >>> That lock is already there and using it, mq-deadline will never dispatch > >>> more than one write per zone at any time. This is to avoid write > >>> reordering. So multi queue or not, for any zone, there is no possibility > >>> of having writes reordered. > > > > For zoned write, mq-deadline is used to limit at most one inflight write > > for each zone. > > > > So can you explain a bit how the current implementation breaks zoned > > write? > > Like Damien wrote in another email, mq-deadline will only impose > ordering for requests submitted in batch. The flow we have is the > following: > > - Userspace sends requests to ublk gendisk > - Requests go through block layer and is _not_ reordered when using > mq-deadline. They may be split. > - Requests hit ublk_drv and ublk_drv will reverse order of _all_ > batched up requests (including split requests). For ublk-zone, ublk driver needs to be exposed as zoned device by calling disk_set_zoned() finally, which definitely isn't supported now, so mq-deadline at most sends one write IO for each zone after ublk-zone is supported, see blk_req_can_dispatch_to_zone(). > - ublk_drv sends request to ublksrv in _reverse_ order. > - ublksrv sends requests _not_ batched up to target device. > - Requests that enter mq-deadline at the same time are reordered in LBA > order, that is all good. > - Requests that enter the kernel in different batches are not reordered > in LBA order and end up missing the write pointer. This is bad. Again, please read Damien's comment: >> That lock is already there and using it, mq-deadline will never dispatch >> more than one write per zone at any time. Anytime, there is at most one write IO for each zone, how can the single write IO be re-order? Thanks, Ming
