linux-kernel - Re: [PATCH 3/6] direct-io: add support for write stream IDs

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Date:	Thu, 16 Apr 2015 23:20:45 -0700
From:	Ming Lin <mlin@...nel.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	Ming Lin <mlin@...nel.org>, Jens Axboe <axboe@...nel.dk>,
	Jens Axboe <axboe@...com>, lkml <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org, ming.l@....samsung.com,
	"Kwan (Hingkwan) Huen-SSI" <kwan.huen@....samsung.com>
Subject: Re: [PATCH 3/6] direct-io: add support for write stream IDs

On Sat, Apr 11, 2015 at 4:59 AM, Dave Chinner <david@...morbit.com> wrote:
> On Fri, Apr 10, 2015 at 04:50:05PM -0700, Ming Lin wrote:
>> On Wed, Mar 25, 2015 at 7:26 AM, Jens Axboe <axboe@...nel.dk> wrote:
>> > On 03/24/2015 08:43 PM, Dave Chinner wrote:
>> >>
>> >> On Tue, Mar 24, 2015 at 09:27:00AM -0600, Jens Axboe wrote:
>> >>>
>> >>> Get the streamid from the file, if any, and set it on the bio.
>> >>>
>> >>> Signed-off-by: Jens Axboe <axboe@...com>
>> >>> ---
>> >>>   fs/direct-io.c | 4 ++++
>> >>>   1 file changed, 4 insertions(+)
>> >>>
>> >>> diff --git a/fs/direct-io.c b/fs/direct-io.c
>> >>> index e181b6b2e297..5d2750346451 100644
>> >>> --- a/fs/direct-io.c
>> >>> +++ b/fs/direct-io.c
>> >>> @@ -77,6 +77,7 @@ struct dio_submit {
>> >>>         int reap_counter;               /* rate limit reaping */
>> >>>         sector_t final_block_in_request;/* doesn't change */
>> >>>         int boundary;                   /* prev block is at a boundary */
>> >>> +       int streamid;                   /* Write stream ID */
>> >>>         get_block_t *get_block;         /* block mapping function */
>> >>>         dio_submit_t *submit_io;        /* IO submition function */
>> >>>
>> >>> @@ -372,6 +373,8 @@ dio_bio_alloc(struct dio *dio, struct dio_submit
>> >>> *sdio,
>> >>>
>> >>>         sdio->bio = bio;
>> >>>         sdio->logical_offset_in_bio = sdio->cur_page_fs_offset;
>> >>> +
>> >>> +       bio_set_streamid(bio, sdio->streamid);
>> >>>   }
>> >>>
>> >>>   /*
>> >>> @@ -1205,6 +1208,7 @@ do_blockdev_direct_IO(int rw, struct kiocb *iocb,
>> >>> struct inode *inode,
>> >>>         sdio.blkbits = blkbits;
>> >>>         sdio.blkfactor = i_blkbits - blkbits;
>> >>>         sdio.block_in_file = offset >> blkbits;
>> >>> +       sdio.streamid = iocb->ki_filp->f_streamid;
>> >>
>> >>
>> >> If iocb->ki_filp->f_streamid is not set, then it should fall back to
>> >> whatever is set on the inode->i_streamid.
>>
>> Why should do the fall back?
>
> Because then you have a method of using streams with applications
> that aren't aware of streams.
>
> Or perhaps you have a file you know has different access patterns to
> the rest of the files in a directory, and you don't want to have to
> set the stream on every process that opens and uses that file. e.g.
> database writeahead log files (sequential write, never read) vs
> database index/table files (random read/write).....
>
>> > Good point, agree. Will make that change.
>>
>> That change causes problem for direct IO, for example
>>
>> process 1:
>> fd = open("/dev/nvme0n1", O_DIRECT...);
>> //set stream_id 1
>> fadvise(fd, 1, 0, POSIX_FADV_STREAMID);
>> pwrite(fd, ....);
>>
>> process 2:
>> fd = open("/dev/nvme0n1", O_DIRECT...);
>> //should be legacy stream_id 0
>> pwrite(fd, ....);
>>
>> But now process 2 also see stream_id 1, which is wrong.
>
> It's not wrong, your behaviour model is just different You have
> defined a process/fd based stream model and not considered
> considered that admins and applications might want to use a file
> based stream model instead, so applications don't need to even be
> aware that write streams are in use...

The stream must be opened, otherwise device will return error if application
write to a not-opened stream.

Device has limited number of streams, for example, 16 streams.
There are 2 APIs to open/close the stream.

process 1:
fd = open("/dev/nvme0n1", O_DIRECT...);
stream_id = open_stream("/dev/nvme0n1", ....);
fadvise(fd, stream_id, 0, POSIX_FADV_STREAMID);
pwrite(fd, ....);
close_stream("/dev/nvme0n1", stream_id);

process 2:
fd = open("/dev/nvme0n1", O_DIRECT...);
//should be legacy stream_id 0
pwrite(fd, ....);

Now process 2 gets error because the "stream_id" was already closed.

Thanks,
Ming
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