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Date:   Fri, 10 Aug 2018 15:28:21 +0800
From:   Qu Wenruo <>
To:     Naohiro Aota <>, David Sterba <>,
Cc:     Chris Mason <>, Josef Bacik <>,, Hannes Reinecke <>,
        Damien Le Moal <>,
        Bart Van Assche <>,
        Matias Bjorling <>
Subject: Re: [RFC PATCH 00/17] btrfs zoned block device support

On 8/10/18 2:04 AM, Naohiro Aota wrote:
> This series adds zoned block device support to btrfs.
> A zoned block device consists of a number of zones. Zones are either
> conventional and accepting random writes or sequential and requiring that
> writes be issued in LBA order from each zone write pointer position.

Not familiar with zoned block device, especially for the sequential case.

Is that sequential case tape like?

> This
> patch series ensures that the sequential write constraint of sequential
> zones is respected while fundamentally not changing BtrFS block and I/O
> management for block stored in conventional zones.
> To achieve this, the default dev extent size of btrfs is changed on zoned
> block devices so that dev extents are always aligned to a zone. Allocation
> of blocks within a block group is changed so that the allocation is always
> sequential from the beginning of the block groups. To do so, an allocation
> pointer is added to block groups and used as the allocation hint.  The
> allocation changes also ensures that block freed below the allocation
> pointer are ignored, resulting in sequential block allocation regardless of
> the block group usage.

This looks like it would cause a lot of holes for metadata block groups.
It would be better to avoid metadata block allocation in such sequential
(And that would need the infrastructure to make extent allocator

> While the introduction of the allocation pointer ensure that blocks will be
> allocated sequentially, I/Os to write out newly allocated blocks may be
> issued out of order, causing errors when writing to sequential zones. This
> problem s solved by introducing a submit_buffer() function and changes to
> the internal I/O scheduler to ensure in-order issuing of write I/Os for
> each chunk and corresponding to the block allocation order in the chunk.
> The zones of a chunk are reset to allow reusing of the zone only when the
> block group is being freed, that is, when all the extents of the block group
> are unused.
> For btrfs volumes composed of multiple zoned disks, restrictions are added
> to ensure that all disks have the same zone size. This matches the existing
> constraint that all dev extents in a chunk must have the same size.
> It requires zoned block devices to test the patchset. Even if you don't
> have zone devices, you can use tcmu-runner [1] to emulate zoned block
> devices. It can export emulated zoned block devices via iSCSI. Please see
> the of tcmu-runner [2] for howtos to generate a zoned block
> device on tcmu-runner.
> [1]
> [2]
> Patch 1 introduces the HMZONED incompatible feature flag to indicate that
> the btrfs volume was formatted for use on zoned block devices.
> Patches 2 and 3 implement functions to gather information on the zones of
> the device (zones type and write pointer position).
> Patch 4 restrict the possible locations of super blocks to conventional
> zones to preserve the existing update in-place mechanism for the super
> blocks.
> Patches 5 to 7 disable features which are not compatible with the sequential
> write constraints of zoned block devices. This includes fallocate and
> direct I/O support. Device replace is also disabled for now.
> Patches 8 and 9 tweak the extent buffer allocation for HMZONED mode to
> implement sequential block allocation in block groups and chunks.
> Patches 10 to 12 implement the new submit buffer I/O path to ensure sequential
> write I/O delivery to the device zones.
> Patches 13 to 16 modify several parts of btrfs to handle free blocks
> without breaking the sequential block allocation and sequential write order
> as well as zone reset for unused chunks.
> Finally, patch 17 adds the HMZONED feature to the list of supported
> features.
> Naohiro Aota (17):
>   btrfs: introduce HMZONED feature flag
>   btrfs: Get zone information of zoned block devices
>   btrfs: Check and enable HMZONED mode
>   btrfs: limit super block locations in HMZONED mode
>   btrfs: disable fallocate in HMZONED mode
>   btrfs: disable direct IO in HMZONED mode
>   btrfs: disable device replace in HMZONED mode
>   btrfs: align extent allocation to zone boundary

According to the patch name, I though it's about extent allocation, but
in fact it's about dev extent allocation.
Renaming the patch would make more sense.

>   btrfs: do sequential allocation on HMZONED drives

And this is the patch modifying extent allocator.

Despite that, the support zoned storage looks pretty interesting and
have something in common with planned priority-aware extent allocator.


>   btrfs: split btrfs_map_bio()
>   btrfs: introduce submit buffer
>   btrfs: expire submit buffer on timeout
>   btrfs: avoid sync IO prioritization on checksum in HMZONED mode
>   btrfs: redirty released extent buffers in sequential BGs
>   btrfs: reset zones of unused block groups
>   btrfs: wait existing extents before truncating
>   btrfs: enable to mount HMZONED incompat flag
>  fs/btrfs/async-thread.c     |   1 +
>  fs/btrfs/async-thread.h     |   1 +
>  fs/btrfs/ctree.h            |  36 ++-
>  fs/btrfs/dev-replace.c      |  10 +
>  fs/btrfs/disk-io.c          |  48 +++-
>  fs/btrfs/extent-tree.c      | 281 +++++++++++++++++-
>  fs/btrfs/extent_io.c        |   1 +
>  fs/btrfs/extent_io.h        |   1 +
>  fs/btrfs/file.c             |   4 +
>  fs/btrfs/free-space-cache.c |  36 +++
>  fs/btrfs/free-space-cache.h |  10 +
>  fs/btrfs/inode.c            |  14 +
>  fs/btrfs/super.c            |  32 ++-
>  fs/btrfs/sysfs.c            |   2 +
>  fs/btrfs/transaction.c      |  32 +++
>  fs/btrfs/transaction.h      |   3 +
>  fs/btrfs/volumes.c          | 551 ++++++++++++++++++++++++++++++++++--
>  fs/btrfs/volumes.h          |  37 +++
>  include/uapi/linux/btrfs.h  |   1 +
>  19 files changed, 1061 insertions(+), 40 deletions(-)

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