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From: Damien Le Moal <Damien.LeMoal@wdc.com>
To: Kanchan Joshi <joshiiitr@gmail.com>
Cc: "hch@infradead.org" <hch@infradead.org>,
	Jens Axboe <axboe@kernel.dk>,
	Pavel Begunkov <asml.silence@gmail.com>,
	Kanchan Joshi <joshi.k@samsung.com>,
	"viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
	"bcrl@kvack.org" <bcrl@kvack.org>,
	Matthew Wilcox <willy@infradead.org>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-aio@kvack.org" <linux-aio@kvack.org>,
	"io-uring@vger.kernel.org" <io-uring@vger.kernel.org>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	"linux-api@vger.kernel.org" <linux-api@vger.kernel.org>,
	SelvaKumar S <selvakuma.s1@samsung.com>,
	Nitesh Shetty <nj.shetty@samsung.com>,
	Javier Gonzalez <javier.gonz@samsung.com>,
	Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
	Naohiro Aota <Naohiro.Aota@wdc.com>
Subject: Re: [PATCH v4 6/6] io_uring: add support for zone-append
Date: Tue, 29 Sep 2020 01:24:02 +0000
Message-ID: <CY4PR04MB3751BFF86D1F7F1D22A143E6E7320@CY4PR04MB3751.namprd04.prod.outlook.com> (raw)
In-Reply-To: <CA+1E3rJANOsPOzjtJHSViVMq+Uc-sB0iZoExxBG++v2ghaL4uA@mail.gmail.com>

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

  reply index

Thread overview: 52+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <CGME20200724155244epcas5p2902f57e36e490ee8772da19aa9408cdc@epcas5p2.samsung.com>
2020-07-24 15:49 ` [PATCH v4 0/6] zone-append support in io-uring and aio Kanchan Joshi
     [not found]   ` <CGME20200724155258epcas5p1a75b926950a18cd1e6c8e7a047e6c589@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 1/6] fs: introduce FMODE_ZONE_APPEND and IOCB_ZONE_APPEND Kanchan Joshi
2020-07-24 16:34       ` Jens Axboe
2020-07-26 15:18       ` Christoph Hellwig
2020-07-28  1:49         ` Matthew Wilcox
2020-07-28  7:26           ` Christoph Hellwig
     [not found]   ` <CGME20200724155324epcas5p18e1d3b4402d1e4a8eca87d0b56a3fa9b@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 2/6] fs: change ki_complete interface to support 64bit ret2 Kanchan Joshi
2020-07-26 15:18       ` Christoph Hellwig
     [not found]   ` <CGME20200724155329epcas5p345ba6bad0b8fe18056bb4bcd26c10019@epcas5p3.samsung.com>
2020-07-24 15:49     ` [PATCH v4 3/6] uio: return status with iov truncation Kanchan Joshi
     [not found]   ` <CGME20200724155341epcas5p15bfc55927f2abb60f19784270fe8e377@epcas5p1.samsung.com>
2020-07-24 15:49     ` [PATCH v4 4/6] block: add zone append handling for direct I/O path Kanchan Joshi
2020-07-26 15:19       ` Christoph Hellwig
     [not found]   ` <CGME20200724155346epcas5p2cfb383fe9904a45280c6145f4c13e1b4@epcas5p2.samsung.com>
2020-07-24 15:49     ` [PATCH v4 5/6] block: enable zone-append for iov_iter of bvec type Kanchan Joshi
2020-07-26 15:20       ` Christoph Hellwig
     [not found]   ` <CGME20200724155350epcas5p3b8f1d59eda7f8fbb38c828f692d42fd6@epcas5p3.samsung.com>
2020-07-24 15:49     ` [PATCH v4 6/6] io_uring: add support for zone-append Kanchan Joshi
2020-07-24 16:29       ` Jens Axboe
2020-07-27 19:16         ` Kanchan Joshi
2020-07-27 20:34           ` Jens Axboe
2020-07-30 16:08             ` Pavel Begunkov
2020-07-30 16:13               ` Jens Axboe
2020-07-30 16:26                 ` Pavel Begunkov
2020-07-30 17:16                   ` Jens Axboe
2020-07-30 17:38                     ` Pavel Begunkov
2020-07-30 17:51                       ` Kanchan Joshi
2020-07-30 17:54                         ` Jens Axboe
2020-07-30 18:25                           ` Kanchan Joshi
2020-07-31  6:42                             ` Damien Le Moal
2020-07-31  6:45                               ` hch
2020-07-31  6:59                                 ` Damien Le Moal
2020-07-31  7:58                                   ` Kanchan Joshi
2020-07-31  8:14                                     ` Damien Le Moal
2020-07-31  9:14                                       ` hch
2020-07-31  9:34                                         ` Damien Le Moal
2020-07-31  9:41                                           ` hch
2020-07-31 10:16                                             ` Damien Le Moal
2020-07-31 12:51                                               ` hch
2020-07-31 13:08                                                 ` hch
2020-07-31 15:07                                                   ` Kanchan Joshi
2020-08-05  7:35                                                 ` Damien Le Moal
2020-08-14  8:14                                                   ` hch
2020-08-14  8:27                                                     ` Damien Le Moal
2020-08-14 12:04                                                       ` hch
2020-08-14 12:20                                                         ` Damien Le Moal
2020-09-07  7:01                                                     ` Kanchan Joshi
2020-09-08 15:18                                                       ` hch
2020-09-24 17:19                                                         ` Kanchan Joshi
2020-09-25  2:52                                                           ` Damien Le Moal
2020-09-28 18:58                                                             ` Kanchan Joshi
2020-09-29  1:24                                                               ` Damien Le Moal [this message]
2020-09-29 18:49                                                                 ` Kanchan Joshi
2020-07-31  9:38                                       ` Kanchan Joshi
2020-07-31  7:08                               ` Kanchan Joshi
2020-07-30 15:57       ` Pavel Begunkov

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