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    SubjectRe: [RFC PATCH 00/17] btrfs zoned block device support
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    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.
    >
    Thanks for doing all the work.
    However, the patches don't apply cleanly to current master branch.
    Can you please rebase them?

    Thanks.

    Cheers,

    Hannes
    --
    Dr. Hannes Reinecke zSeries & Storage
    hare@suse.com +49 911 74053 688
    SUSE LINUX GmbH, Maxfeldstr. 5, 90409 Nürnberg
    GF: F. Imendörffer, J. Smithard, D. Upmanyu, G. Norton
    HRB 21284 (AG Nürnberg)

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