On Tue, Aug 23 2022 at 8:18P -0400,
Pankaj Raghav <p.raghav@xxxxxxxxxxx> wrote:
> Only zoned devices with power-of-2(po2) number of sectors per zone(zone
> size) were supported in linux but now non power-of-2(npo2) zone sizes
> support has been added to the block layer.
>
> Filesystems such as F2FS and btrfs have support for zoned devices with
> po2 zone size assumption. Before adding native support for npo2 zone
> sizes, it was suggested to create a dm target for npo2 zone size device to
> appear as a po2 zone size target so that file systems can initially
> work without any explicit changes by using this target.
>
> The design of this target is very simple: remap the device zone size to
> the zone capacity and change the zone size to be the nearest power of 2
> value.
>
> For e.g., a device with a zone size/capacity of 3M will have an equivalent
> target layout as follows:
>
> Device layout :-
> zone capacity = 3M
> zone size = 3M
>
> |--------------|-------------|
> 0 3M 6M
>
> Target layout :-
> zone capacity=3M
> zone size = 4M
>
> |--------------|---|--------------|---|
> 0 3M 4M 7M 8M
>
> The area between target's zone capacity and zone size will be emulated
> in the target.
> The read IOs that fall in the emulated gap area will return 0 filled
> bio and all the other IOs in that area will result in an error.
> If a read IO span across the emulated area boundary, then the IOs are
> split across them. All other IO operations that span across the emulated
> area boundary will result in an error.
>
> The target can be easily created as follows:
> dmsetup create <label> --table '0 <size_sects> po2zone /dev/nvme<id>'
>
> Note that the target does not support partial mapping of the underlying
> device.
>
> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
> Suggested-by: Johannes Thumshirn <johannes.thumshirn@xxxxxxx>
> Suggested-by: Damien Le Moal <damien.lemoal@xxxxxxx>
> Suggested-by: Hannes Reinecke <hare@xxxxxxx>
This target needs more review from those who Suggested-by it.
And the header and docs needs to address:
1) why is a partial mapping of the underlying device disallowed?
2) why is it assumed all IO is read-only? (talk to me and others like
we don't know the inherent limitations of this class of zoned hw)
On a code level:
1) are you certain you're properly failing all writes?
- are writes allowed to the "zone capacity area" but _not_
allowed to the "emulated zone area"? (if yes, _please document_).
2) yes, you absolutely need to implement the .status target_type hook
(for both STATUS and TABLE).
3) really not loving the nested return (of DM_MAPIO_SUBMITTED or
DM_MAPIO_REMAPPED) from methods called from dm_po2z_map(). Would
prefer to not have to do a depth-first search to see where and when
dm_po2z_map() returns a DM_MAPIO_XXX unless there is a solid
justification for it. To me it just obfuscates the DM interface a
bit too much.
Otherwise, pretty clean code and nothing weird going on.
I look forward to seeing your next (final?) revision of this patchset.
Thanks,
Mike
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