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). - 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. So, ublk_drv is not functional for zoned storage as is. Either we have to fix up the ordering in userspace in ublksrv, and that _will_ have a performance impact. Or we fix the bug in ublk_drv that causes batched requests to be _reversed_. Thanks, Andreas
