Re: Reordering of ublk IO requests

[Ming Lei <ming.lei@redhat.com>]

On Fri, Nov 18, 2022 at 10:41:31AM +0100, Andreas Hindborg wrote:
> 
> Ming Lei <ming.lei@redhat.com> writes:
> 
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> >
> >
> > 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?


Thanks, 
Ming