mirror of
https://github.com/kdave/btrfs-progs
synced 2024-12-13 09:57:03 +00:00
66 lines
3.0 KiB
ReStructuredText
66 lines
3.0 KiB
ReStructuredText
Since version 5.12 btrfs supports so called *zoned mode*. This is a special
|
|
on-disk format and allocation/write strategy that's friendly to zoned devices.
|
|
In short, a device is partitioned into fixed-size zones and each zone can be
|
|
updated by append-only manner, or reset. As btrfs has no fixed data structures,
|
|
except the super blocks, the zoned mode only requires block placement that
|
|
follows the device constraints. You can learn about the whole architecture at
|
|
https://zonedstorage.io .
|
|
|
|
The devices are also called SMR/ZBC/ZNS, in *host-managed* mode. Note that
|
|
there are devices that appear as non-zoned but actually are, this is
|
|
*drive-managed* and using zoned mode won't help.
|
|
|
|
The zone size depends on the device, typical sizes are 256MiB or 1GiB. In
|
|
general it must be a power of two. Emulated zoned devices like *null_blk* allow
|
|
to set various zone sizes.
|
|
|
|
Requirements, limitations
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
* all devices must have the same zone size
|
|
* maximum zone size is 8GiB
|
|
* mixing zoned and non-zoned devices is possible, the zone writes are emulated,
|
|
but this is namely for testing
|
|
* the super block is handled in a special way and is at different locations than on a non-zoned filesystem:
|
|
* primary: 0B (and the next two zones)
|
|
* secondary: 512GiB (and the next two zones)
|
|
* tertiary: 4TiB (4096GiB, and the next two zones)
|
|
|
|
Incompatible features
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The main constraint of the zoned devices is lack of in-place update of the data.
|
|
This is inherently incompatibile with some features:
|
|
|
|
* nodatacow - overwrite in-place, cannot create such files
|
|
* fallocate - preallocating space for in-place first write
|
|
* mixed-bg - unordered writes to data and metadata, fixing that means using
|
|
separate data and metadata block groups
|
|
* booting - the zone at offset 0 contains superblock, resetting the zone would
|
|
destroy the bootloader data
|
|
|
|
Initial support lacks some features but they're planned:
|
|
|
|
* only single profile is supported
|
|
* fstrim - due to dependency on free space cache v1
|
|
|
|
Super block
|
|
^^^^^^^^^^^
|
|
|
|
As said above, super block is handled in a special way. In order to be crash
|
|
safe, at least one zone in a known location must contain a valid superblock.
|
|
This is implemented as a ring buffer in two consecutive zones, starting from
|
|
known offsets 0B, 512GiB and 4TiB.
|
|
|
|
The values are different than on non-zoned devices. Each new super block is
|
|
appended to the end of the zone, once it's filled, the zone is reset and writes
|
|
continue to the next one. Looking up the latest super block needs to read
|
|
offsets of both zones and determine the last written version.
|
|
|
|
The amount of space reserved for super block depends on the zone size. The
|
|
secondary and tertiary copies are at distant offsets as the capacity of the
|
|
devices is expected to be large, tens of terabytes. Maximum zone size supported
|
|
is 8GiB, which would mean that eg. offset 0-16GiB would be reserved just for
|
|
the super block on a hypothetical device of that zone size. This is wasteful
|
|
but required to guarantee crash safety.
|