Re: [RFC PATCH 00/17] btrfs zoned block device support

[Hannes Reinecke]

On 08/09/2018 08:04 PM, 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. 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.
> 
> 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 README.md of tcmu-runner [2] for howtos to generate a zoned block
> device on tcmu-runner.
> 
> [1] https://github.com/open-iscsi/tcmu-runner
> [2] https://github.com/open-iscsi/tcmu-runner/blob/master/README.md
> 
> 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
>   btrfs: do sequential allocation on HMZONED drives
>   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
> 
And unfortunately this series fails to boot for me:

BTRFS error (device nvme0n1p5): zoned devices mixed with regular devices
BTRFS error (device nvme0n1p5): failed to init hmzoned mode: -22
BTRFS error (device nvme0n1p5): open_ctree failed

Needless to say, /dev/nvme0n1p5 is _not_ a zoned device.
Nor has the zoned device a btrfs superblock ATM.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		               zSeries & Storage
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