Re: [PATCH v2 3/4] qcow2: add zoned emulation capability

[Sam Li]

Damien Le Moal <dlemoal@kernel.org> 于2023年8月29日周二 14:06写道：
>
> On 8/28/23 20:55, Sam Li wrote:
> >>> +            /* close one implicitly open zones to make it available */
> >>> +            for (int i = s->zoned_header.zone_nr_conv;
> >>> +            i < bs->bl.nr_zones; ++i) {
> >>> +                uint64_t *wp = &s->wps->wp[i];
> >>> +                if (qcow2_get_zs(*wp) == BLK_ZS_IOPEN) {
> >>> +                    ret = qcow2_write_wp_at(bs, wp, i, BLK_ZS_CLOSED);
> >>
> >> I'm wondering if it's correct to store the zone state persistently in
> >> the qcow2 file. If the guest or QEMU crashes, then zones will be left in
> >> states like EOPEN. Since the guest software will have forgotten about
> >> explicitly opened zones, the guest would need to recover zone states.
> >> I'm not sure if existing software is designed to do that.
> >>
> >> Damien: Should the zone state be persistent?
>
> Yes and no. Yes you need to preserve/maintain zone states but not as is.
> With a real drive, if you power cycle the device, you get the following states
> changes:
>
>  Before         | After power cycle
> ----------------+-------------------
>  EMPTY          | EMPTY
>  FULL           | FULL
>  IMP. OPEN      | CLOSED
>  EXP. OPEN      | CLOSED
>  CLOSED         | CLOSED
>  READ=ONLY      | READ-ONLY
>  OFFLINE        | OFFLINE
>
> So any open (implicit or explicit) zone will show up as closed after power
> cycle. That is, the number of "active" zones does not change.
> For the qcow2 emulation, as long as you do not also emulate read-only and
> offline zones, you actually do not need to save the zone state in the zone
> metadata. On startup, you can infer the state from the zone write pointer:
>
> zone wp == zone start -> EMPTY
> zone wp >= zone capacity -> FULL
> zone wp > zone start -> CLOSED
>
> And make sure that all closed zones are counted as the initial number of 
> active
> zones. The initial number of open zones will always be 0.
>
> So it is easy :)

Thanks for the explanations!

Read-only and offline are device internal events. Does qcow2 emulation
need to emulate that?

Current NVMe ZNS emulation in QEMU has a nvme_offline_zone() function.
Does it suggest keeping the offline state persistent?
https://github.com/qemu/qemu/blob/master/hw/nvme/ctrl.c#L3740

Sam