67 lines
3.0 KiB
ReStructuredText
67 lines
3.0 KiB
ReStructuredText
Since version 5.12 btrfs supports so called *zoned mode*. This is a special
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on-disk format and allocation/write strategy that's friendly to zoned devices.
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In short, a device is partitioned into fixed-size zones and each zone can be
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updated by append-only manner, or reset. As btrfs has no fixed data structures,
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except the super blocks, the zoned mode only requires block placement that
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follows the device constraints. You can learn about the whole architecture at
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https://zonedstorage.io .
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The devices are also called SMR/ZBC/ZNS, in *host-managed* mode. Note that
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there are devices that appear as non-zoned but actually are, this is
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*drive-managed* and using zoned mode won't help.
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The zone size depends on the device, typical sizes are 256MiB or 1GiB. In
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general it must be a power of two. Emulated zoned devices like *null_blk* allow
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to set various zone sizes.
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Requirements, limitations
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^^^^^^^^^^^^^^^^^^^^^^^^^
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* all devices must have the same zone size
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* maximum zone size is 8GiB
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* minimum zone size is 4MiB
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* mixing zoned and non-zoned devices is possible, the zone writes are emulated,
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but this is namely for testing
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* the super block is handled in a special way and is at different locations than on a non-zoned filesystem:
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* primary: 0B (and the next two zones)
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* secondary: 512GiB (and the next two zones)
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* tertiary: 4TiB (4096GiB, and the next two zones)
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Incompatible features
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^^^^^^^^^^^^^^^^^^^^^
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The main constraint of the zoned devices is lack of in-place update of the data.
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This is inherently incompatible with some features:
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* NODATACOW - overwrite in-place, cannot create such files
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* fallocate - preallocating space for in-place first write
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* mixed-bg - unordered writes to data and metadata, fixing that means using
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separate data and metadata block groups
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* booting - the zone at offset 0 contains superblock, resetting the zone would
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destroy the bootloader data
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Initial support lacks some features but they're planned:
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* only single profile is supported
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* fstrim - due to dependency on free space cache v1
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Super block
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^^^^^^^^^^^
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As said above, super block is handled in a special way. In order to be crash
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safe, at least one zone in a known location must contain a valid superblock.
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This is implemented as a ring buffer in two consecutive zones, starting from
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known offsets 0B, 512GiB and 4TiB.
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The values are different than on non-zoned devices. Each new super block is
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appended to the end of the zone, once it's filled, the zone is reset and writes
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continue to the next one. Looking up the latest super block needs to read
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offsets of both zones and determine the last written version.
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The amount of space reserved for super block depends on the zone size. The
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secondary and tertiary copies are at distant offsets as the capacity of the
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devices is expected to be large, tens of terabytes. Maximum zone size supported
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is 8GiB, which would mean that e.g. offset 0-16GiB would be reserved just for
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the super block on a hypothetical device of that zone size. This is wasteful
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but required to guarantee crash safety.
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