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Subject: Re: [PATCH v7 0/6] block: Introduce REQ_ALLOCATE flag for
 REQ_OP_WRITE_ZEROES
From:   Kirill Tkhai <ktkhai@virtuozzo.com>
To:     axboe@kernel.dk
Cc:     martin.petersen@oracle.com, bob.liu@oracle.com,
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Date:   Fri, 6 Mar 2020 12:11:14 +0300
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ping

On 13.02.2020 10:55, Kirill Tkhai wrote:
> Hi, Jens,
> 
> could you please provide some comments on this? I sent v1 two months ago,
> and it would be great to know your vision of the functionality and
> the approach and whether it is going to go to block tree.
> 
> Thanks,
> Kirill
> 
> On 13.02.2020 10:39, Kirill Tkhai wrote:
>> (was "[PATCH block v2 0/3] block: Introduce REQ_NOZERO flag
>>       for REQ_OP_WRITE_ZEROES operation";
>>  was "[PATCH RFC 0/3] block,ext4: Introduce REQ_OP_ASSIGN_RANGE
>>       to reflect extents allocation in block device internals")
>>
>> v7: Two comments changed.
>>
>> v6: req_op() cosmetic change.
>>
>> v5: Kill dm|md patch, which disables REQ_ALLOCATE for these devices.
>>     Disable REQ_ALLOCATE for all stacking devices instead of this.
>>
>> v4: Correct argument for mddev_check_write_zeroes().
>>
>> v3: Rename REQ_NOZERO to REQ_ALLOCATE.
>>     Split helpers to separate patches.
>>     Add a patch, disabling max_allocate_sectors inheritance for dm.
>>
>> v2: Introduce new flag for REQ_OP_WRITE_ZEROES instead of
>>     introduction a new operation as suggested by Martin K. Petersen.
>>     Removed ext4-related patch to focus on block changes
>>     for now.
>>
>> Information about continuous extent placement may be useful
>> for some block devices. Say, distributed network filesystems,
>> which provide block device interface, may use this information
>> for better blocks placement over the nodes in their cluster,
>> and for better performance. Block devices, which map a file
>> on another filesystem (loop), may request the same length extent
>> on underlining filesystem for less fragmentation and for batching
>> allocation requests. Also, hypervisors like QEMU may use this
>> information for optimization of cluster allocations.
>>
>> This patchset introduces REQ_ALLOCATE flag for REQ_OP_WRITE_ZEROES,
>> which makes a block device to allocate blocks instead of actual
>> blocks zeroing. This may be used for forwarding user's fallocate(0)
>> requests into block device internals. E.g., in loop driver this
>> will result in allocation extents in backing-file, so subsequent
>> write won't fail by the reason of no available space. Distributed
>> network filesystems will be able to assign specific servers for
>> specific extents, so subsequent write will be more efficient.
>>
>> Patches [1-3/6] are preparation on helper functions, patch [4/6]
>> introduces REQ_ALLOCATE flag and implements all the logic,
>> patch [5/6] adds one more helper, patch [6/6] adds loop
>> as the first user of the flag.
>>
>> Note, that here is only block-related patches, example of usage
>> for ext4 with a performance numbers may be seen in [1].
>>
>> [1] https://lore.kernel.org/linux-ext4/157599697369.12112.10138136904533871162.stgit@localhost.localdomain/T/#me5bdd5cc313e14de615d81bea214f355ae975db0
>> ---
>>
>> Kirill Tkhai (6):
>>       block: Add @flags argument to bdev_write_zeroes_sectors()
>>       block: Pass op_flags into blk_queue_get_max_sectors()
>>       block: Introduce blk_queue_get_max_write_zeroes_sectors()
>>       block: Add support for REQ_ALLOCATE flag
>>       block: Add blk_queue_max_allocate_sectors()
>>       loop: Add support for REQ_ALLOCATE
>>
>>
>>  block/blk-core.c                    |    6 +++---
>>  block/blk-lib.c                     |   17 ++++++++++-------
>>  block/blk-merge.c                   |    9 ++++++---
>>  block/blk-settings.c                |   17 +++++++++++++++++
>>  drivers/block/loop.c                |   20 +++++++++++++-------
>>  drivers/md/dm-kcopyd.c              |    2 +-
>>  drivers/target/target_core_iblock.c |    4 ++--
>>  fs/block_dev.c                      |    4 ++++
>>  include/linux/blk_types.h           |    6 ++++++
>>  include/linux/blkdev.h              |   34 ++++++++++++++++++++++++++--------
>>  10 files changed, 88 insertions(+), 31 deletions(-)
>>
>> --
>> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
>> Reviewed-by: Bob Liu <bob.liu@oracle.com>
>>
>