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I love ZFS but this is something that just works in btrfs; mirror just means all blocks live in two physical locations. You certainly can do that even with an

I love ZFS but this is something that just works in btrfs; mirror just means all blocks live in two physical locations. You certainly can do that even with an odd number of drives. However ZFS is more rigid and doesn’t allow flowing blocks like this, nor dynamic defragmentation.

deagle50 5y ago
Would you use raid 5/6 in btrfs?
- NavinF 5y ago
  The btrfs raid5/6 write hole is still around, if anyone’s wondering. Though it was only recently that btrfs started warning users that it would eat their data: https://www.phoronix.com/scan.php?page=news_item&px=Btrfs-Warning-RAID5-RAID6 https://www.phoronix.com/scan.php?page=news_item&px=Btrfs-Wa...
  - mrlonglong 5y ago
    I'll certainly attest to that, btrfs did that to me. Moved over to ZFS and all that went away.
NavinF 5y ago
What are you on about? The submission is about raidz. Adding drives to a mirror has worked in zfs since prehistoric times. “zpool attach test_pool sda sdc” will mirror sda to sdc. If sda was already mirrored with sdb, you now have a triple-mirror with sda, sdb, and sdc.
- wtallis 5y ago
  > If sda was already mirrored with sdb, you now have a triple-mirror with sda, sdb, and sdc. You probably already know this, but btrfs offers a different option. Its "RAID1" mode would not result in the three drives storing your data in triplicate but rather still stores data in duplicate but with a usable capacity that's 1.5x that of a single driver's capacity. To get the ZFS behavior you describe, btrfs offers a "RAID1c3" mode, but that's a much less interesting feature.
  - NavinF 5y ago
    Huh I didn’t know that. Seems like a niche feature, but it’s still pretty neat!
    - rodgerd 5y ago
      It's a neat feature for SOHO-type users, and takes advantage of the fact that the RAID layer knows about the FS. The first implementation that I know of is Drobo's filesystem. Asymmetric RAID is probably the only config that I'd like to see in ZFS - being able to have (say) an 8TB drive and two 4TB drives, and have 8TB of mirrored space is quite nice when you're just buying commodity kit and don't want to have to retire/match drives all the time when growing mirrors.
      - wtallis 5y ago
        Yep, SOHO use cases are where this kind of capability is really useful. ZFS is great when you buy drives by the dozen, but even with this feature it's still inconveniently inflexible if you only have the budget to be upgrading or expanding one drive at a time.
    - mgerdts 5y ago
      Sounds like the way that Windows Storage Spaces works as well.
- FullyFunctional 5y ago
  I am aware of course. I was explaining why adding a single drive does make sense for RAID10 just fine and, as an example, btrfs has it (existence proof), but ZFS doesn't support it. Your example doesn't expand the storage of the vdev, which is what this entire discussion is about, it merely adds a mirror.
  - zaarn 5y ago
    You can definitely expand and shrink a zpool based on mirrors. You can remove top-level vdevs, provided the pool is fine, and expand existing pools by adding new mirrors.
    - wtallis 5y ago
      Resizing a pool that uses mirrored vdevs isn't the question; resizing vdevs is.
      - NavinF 5y ago
        In what situation would increasing the size of a vdev be better than increasing the size of a pool? The submission increases the size of a raidz vdev and that's incredibly useful, but when would that make sense for a mirror?
        wtallis 5y ago
        > In what situation would increasing the size of a vdev be better than increasing the size of a pool? When you want to add a single drive to expand your available storage capacity, without reducing the degree of redundancy you're already using. That use case should have been obvious given how much discussion it's already received in this thread, so I think you have some kind of blind spot about anything small-scale. This is why it's good to actually understand the competition, even after deciding which tradeoffs are right for you.
      - zaarn 5y ago
        You can now with this feature. If you're using RAID1 vdevs, you could always remove and add mirrors. Now you can at least add a device to a RAIDZn vdev.
chasil 5y ago
Like Btrfs, ZFS stores multiple copies of metatdata blocks when possible. Forcing ZFS to store multiple data blocks also can be done. This is great for ZFS on flash drives. https://docs.oracle.com/cd/E19253-01/819-5461/gevpg/index.html https://docs.oracle.com/cd/E19253-01/819-5461/gevpg/index.ht...
- FullyFunctional 5y ago
  That not the same thing. Bill Tallis below explained it better than I did though.

deagle50 5y ago

Would you use raid 5/6 in btrfs?

NavinF 5y ago
The btrfs raid5/6 write hole is still around, if anyone’s wondering. Though it was only recently that btrfs started warning users that it would eat their data: https://www.phoronix.com/scan.php?page=news_item&px=Btrfs-Warning-RAID5-RAID6 https://www.phoronix.com/scan.php?page=news_item&px=Btrfs-Wa...
- mrlonglong 5y ago
  I'll certainly attest to that, btrfs did that to me. Moved over to ZFS and all that went away.

NavinF 5y ago

What are you on about? The submission is about raidz. Adding drives to a mirror has worked in zfs since prehistoric times. “zpool attach test_pool sda sdc” will mirror sda to sdc. If sda was already mirrored with sdb, you now have a triple-mirror with sda, sdb, and sdc.

wtallis 5y ago
> If sda was already mirrored with sdb, you now have a triple-mirror with sda, sdb, and sdc. You probably already know this, but btrfs offers a different option. Its "RAID1" mode would not result in the three drives storing your data in triplicate but rather still stores data in duplicate but with a usable capacity that's 1.5x that of a single driver's capacity. To get the ZFS behavior you describe, btrfs offers a "RAID1c3" mode, but that's a much less interesting feature.
- NavinF 5y ago
  Huh I didn’t know that. Seems like a niche feature, but it’s still pretty neat!
  - rodgerd 5y ago
    It's a neat feature for SOHO-type users, and takes advantage of the fact that the RAID layer knows about the FS. The first implementation that I know of is Drobo's filesystem. Asymmetric RAID is probably the only config that I'd like to see in ZFS - being able to have (say) an 8TB drive and two 4TB drives, and have 8TB of mirrored space is quite nice when you're just buying commodity kit and don't want to have to retire/match drives all the time when growing mirrors.
    - wtallis 5y ago
      Yep, SOHO use cases are where this kind of capability is really useful. ZFS is great when you buy drives by the dozen, but even with this feature it's still inconveniently inflexible if you only have the budget to be upgrading or expanding one drive at a time.
  - mgerdts 5y ago
    Sounds like the way that Windows Storage Spaces works as well.
FullyFunctional 5y ago
I am aware of course. I was explaining why adding a single drive does make sense for RAID10 just fine and, as an example, btrfs has it (existence proof), but ZFS doesn't support it. Your example doesn't expand the storage of the vdev, which is what this entire discussion is about, it merely adds a mirror.
- zaarn 5y ago
  You can definitely expand and shrink a zpool based on mirrors. You can remove top-level vdevs, provided the pool is fine, and expand existing pools by adding new mirrors.
  - wtallis 5y ago
    Resizing a pool that uses mirrored vdevs isn't the question; resizing vdevs is.
    - NavinF 5y ago
      In what situation would increasing the size of a vdev be better than increasing the size of a pool? The submission increases the size of a raidz vdev and that's incredibly useful, but when would that make sense for a mirror?
      - wtallis 5y ago
        > In what situation would increasing the size of a vdev be better than increasing the size of a pool? When you want to add a single drive to expand your available storage capacity, without reducing the degree of redundancy you're already using. That use case should have been obvious given how much discussion it's already received in this thread, so I think you have some kind of blind spot about anything small-scale. This is why it's good to actually understand the competition, even after deciding which tradeoffs are right for you.
    - zaarn 5y ago
      You can now with this feature. If you're using RAID1 vdevs, you could always remove and add mirrors. Now you can at least add a device to a RAIDZn vdev.

chasil 5y ago

Like Btrfs, ZFS stores multiple copies of metatdata blocks when possible. Forcing ZFS to store multiple data blocks also can be done. This is great for ZFS on flash drives. https://docs.oracle.com/cd/E19253-01/819-5461/gevpg/index.html https://docs.oracle.com/cd/E19253-01/819-5461/gevpg/index.ht...

FullyFunctional 5y ago
That not the same thing. Bill Tallis below explained it better than I did though.