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Thread: RAID1C34 new for BTRFS

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    Ascendant GreyGeek's Avatar
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    Jun 6th 2020
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    08:52 PM

    Cool RAID1C34 new for BTRFS

    linux v5.5 (Jan 2020)

    • new block group profiles: RAID1 with 3- and 4- copies
      • RAID1 in btrfs has always 2 copies, now add support for 3 and 4
      • this is an incompat feature (named RAID1C34)
      • recommended use of RAID1C3 is replacement of RAID6 profile on metadata, this brings a more reliable resiliency against 2 device loss/damage

    There are the following block group types available:
    <tbody> </tbody>
    Profile Redundancy Space utilization Min/max devices
    Copies Parity Striping
    single 1 100% 1/any
    DUP 2 / 1 device 50% 1/any (see note 1)
    RAID0 1 to N 100% 2/any
    RAID1 2 50% 2/any
    RAID1C3 3 33% 3/any
    RAID1C4 4 25% 4/any
    RAID10 2 1 to N 50% 4/any
    RAID5 1 1 2 to N-1 (N-1)/N 2/any (see note 2)
    RAID6 1 2 3 to N-2 (N-2)/N 3/any (see note 3)

    Warning: It’s not recommended to create filesystems with RAID0/1/10/5/6 profiles on partitions from the same device. Neither redundancy nor performance will be improved.
    Note 1: DUP may exist on more than 1 device if it starts on a single device and another one is added. Since version 4.5.1, mkfs.btrfs will let you create DUP on multiple devices without restrictions.
    Note 2: It’s not recommended to use 2 devices with RAID5. In that case, parity stripe will contain the same data as the data stripe, making RAID5 degraded to RAID1 with more overhead.
    Note 3: It’s also not recommended to use 3 devices with RAID6, unless you want to get effectively 3 copies in a RAID1-like manner (but not exactly that).
    Note 4: Since kernel 5.5 it’s possible to use RAID1C3 as replacement for RAID6, higher space cost but reliable.
    Last edited by GreyGeek; Mar 30th 2020 at 12:25 AM.

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