Re: [PATCH v4 6/6] io_uring: add support for zone-append

[Damien Le Moal]

On 2020/09/29 3:58, Kanchan Joshi wrote:
[...]
> ZoneFS is better when it is about dealing at single-zone granularity,
> and direct-block seems better when it is about grouping zones (in
> various ways including striping). The latter case (i.e. grouping
> zones) requires more involved mapping, and I agree that it can be left
> to application (for both ZoneFS and raw-block backends).
> But when an application tries that on ZoneFS, apart from mapping there
> would be additional cost of indirection/fd-management (due to
> file-on-files).

There is no indirection in zonefs. fd-to-struct file/inode conversion is very
fast and happens for every system call anyway, regardless of what the fd
represents. So I really do not understand what your worry is here. If you are
worried about overhead/performance, then please show numbers. If something is
wrong, we can work on fixing it.

> And if new features (zone-append for now) are available only on
> ZoneFS, it forces application to use something that maynot be most
> optimal for its need.

"may" is not enough to convince me...

> Coming to the original problem of plumbing append - I think divergence
> started because RWF_APPEND did not have any meaning for block device.
> Did I miss any other reason?

Correct.

> How about write-anywhere semantics (RWF_RELAXED_WRITE or
> RWF_ANONYMOUS_WRITE flag) on block-dev.

"write-anywhere" ? What do you mean ? That is not possible on zoned devices,
even with zone append, since you at least need to guarantee that zones have
enough unwritten space to accept an append command.

> Zone-append works a lot like write-anywhere on block-dev (or on any
> other file that combines multiple-zones, in non-sequential fashion).

That is an over-simplification that is not helpful at all. Zone append is not
"write anywhere" at all. And "write anywhere" is not a concept that exist on
regular block devices anyway. Writes only go to the offset that the user
decided, through lseek(), pwrite() or aio->aio_offset. It is not like the block
layer decides where the writes land. The same constraint applies to zone append:
the user decide the target zone. That is not "anywhere". Please be precise with
wording and implied/desired semantic. Narrow down the scope of your concept
names for clarity.

And talking about "file that combines multiple-zones" would mean that we are now
back in FS land, not raw block device file accesses anymore. So which one are we
talking about ? It looks like you are confusing what the application does and
how it uses whatever usable interface to the device with what that interface
actually is. It is very confusing.

>>> Also it seems difficult (compared to block dev) to fit simple-copy TP
>>> in ZoneFS. The new
>>> command needs: one NVMe drive, list of source LBAs and one destination
>>> LBA. In ZoneFS, we would deal with N+1 file-descriptors (N source zone
>>> file, and one destination zone file) for that. While with block
>>> interface, we do not need  more than one file-descriptor representing
>>> the entire device. With more zone-files, we face open/close overhead too.
>>
>> Are you expecting simple-copy to allow requests that are not zone aligned ? I do
>> not think that will ever happen. Otherwise, the gotcha cases for it would be far
>> too numerous. Simple-copy is essentially an optimized regular write command.
>> Similarly to that command, it will not allow copies over zone boundaries and
>> will need the destination LBA to be aligned to the destination zone WP. I have
>> not checked the TP though and given the NVMe NDA, I will stop the discussion here.
> 
> TP is ratified, if that is the problem you are referring to.

Ah. Yes. Got confused with ZRWA. Simple-copy is a different story anyway. Let's
not mix it into zone append user interface please.

> 
>> filesend() could be used as the interface for simple-copy. Implementing that in
>> zonefs would not be that hard. What is your plan for simple-copy interface for
>> raw block device ? An  ioctl ? filesend() too ? As as with any other user level
>> API, we should not be restricted to a particular device type if we can avoid it,
>> so in-kernel emulation of the feature is needed for devices that do not have
>> simple-copy or scsi extended copy. filesend() seems to me like the best choice
>> since all of that is already implemented there.
> 
> At this moment, ioctl as sync and io-uring for async. sendfile() and
> copy_file_range() takes two fds....with that we can represent copy
> from one source zone to another zone.
> But it does not fit to represent larger copy (from N source zones to
> one destination zone).

nvme passthrough ? If that does not fit your use case, then think of an
interface, its definition/semantic and propose it. But again, use a different
thread. This is mixing up zone-append and simple copy, which I do not think are
directly related.

> Not sure if I am clear, perhaps sending RFC would be better for
> discussion on simple-copy.

Separate this discussion from zone append please. Mixing up 2 problems in one
thread is not helpful to make progress.


-- 
Damien Le Moal
Western Digital Research