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From: Kanchan Joshi <joshiiitr@gmail.com>
To: Damien Le Moal <Damien.LeMoal@wdc.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 00:28:12 +0530
Message-ID: <CA+1E3rJANOsPOzjtJHSViVMq+Uc-sB0iZoExxBG++v2ghaL4uA@mail.gmail.com> (raw)
In-Reply-To: <MWHPR04MB3758A78AFAED3543F8D38266E7360@MWHPR04MB3758.namprd04.prod.outlook.com>

On Fri, Sep 25, 2020 at 8:22 AM Damien Le Moal <Damien.LeMoal@wdc.com> wrote:
>
> On 2020/09/25 2:20, Kanchan Joshi wrote:
> > On Tue, Sep 8, 2020 at 8:48 PM hch@infradead.org <hch@infradead.org> wrote:
> >>
> >> On Mon, Sep 07, 2020 at 12:31:42PM +0530, Kanchan Joshi wrote:
> >>> But there are use-cases which benefit from supporting zone-append on
> >>> raw block-dev path.
> >>> Certain user-space log-structured/cow FS/DB will use the device that
> >>> way. Aerospike is one example.
> >>> Pass-through is synchronous, and we lose the ability to use io-uring.
> >>
> >> So use zonefs, which is designed exactly for that use case.
> >
> > Not specific to zone-append, but in general it may not be good to lock
> > new features/interfaces to ZoneFS alone, given that direct-block
> > interface has its own merits.
> > Mapping one file to a one zone is good for some use-cases, but
> > limiting for others.
> > Some user-space FS/DBs would be more efficient (less meta, indirection)
> > with the freedom to decide file-to-zone mapping/placement.
>
> There is no metadata in zonefs. One file == one zone and the mapping between
> zonefs files and zones is static, determined at mount time simply using report
> zones. Zonefs files cannot be renamed nor deleted in anyway. Choosing a zonefs
> file *is* the same as choosing a zone. Zonfes is *not* a POSIX file system doing
> dynamic block allocation to files. The backing storage of files in zonefs is
> static and fixed to the zone they represent. The difference between zonefs vs
> raw zoned block device is the API that has to be used by the application, that
> is, file descriptor representing the entire disk for raw disk vs file descriptor
> representing one zone in zonefs. Note that the later has *a lot* of advantages
> over the former: enables O_APPEND use, protects against bugs with user write
> offsets mistakes, adds consistency of cached data against zone resets, and more.
>
> > - Rocksdb and those LSM style DBs would map SSTable to zone, but
> > SSTable file may be two small (initially) and may become too large
> > (after compaction) for a zone.
>
> You are contradicting yourself here. If a SSTable is mapped to a zone, then its
> size cannot exceed the zone capacity, regardless of the interface used to access
> the zones. And except for L0 tables which can be smaller (and are in memory
> anyway), all levels tables have the same maximum size, which for zoned drives
> must be the zone capacity. In any case, solving any problem in this area does
> not depend in any way on zonefs vs raw disk interface. The implementation will
> differ depending on the chosen interface, but what needs to be done to map
> SSTables to zones is the same in both cases.
>
> > - The internal parallelism of a single zone is a design-choice, and
> > depends on the drive. Writing multiple zones parallely (striped/raid
> > way) can give better performance than writing on one. In that case one
> > would want to file that seamlessly combines multiple-zones in a
> > striped fashion.
>
> Then write a FS for that... Or have a library do it in user space. For the
> library case, the implementation will differ for zonefs vs raw disk due to the
> different API (regular file vs block devicer file), but the principles to follow
> for stripping zones into a single storage object remain the same.

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).
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.

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?
How about write-anywhere semantics (RWF_RELAXED_WRITE or
RWF_ANONYMOUS_WRITE flag) on block-dev.
Zone-append works a lot like write-anywhere on block-dev (or on any
other file that combines multiple-zones, in non-sequential fashion).

> > 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.

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

> As for the open()/close() overhead for zonefs, may be some use cases may suffer
> from it, but my tests with LevelDB+zonefs did not show any significant
> difference. zonefs open()/close() operations are way faster than for a regular
> file system since there is no metadata and all inodes always exist in-memory.
> And zonefs() now supports MAR/MOR limits for O_WRONLY open(). That can simplify
> things for the user.
>
>
> --
> Damien Le Moal
> Western Digital Research



-- 
Joshi

  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 [this message]
2020-09-29  1:24                                                               ` Damien Le Moal
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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