6 ms·

ZFS can scale a lot further than Ceph. The limits were designed to be large enough to be never encountered in practice. Not just large enough to be never encoun

ZFS can scale a lot further than Ceph. The limits were designed to be large enough to be never encountered in practice. Not just large enough to be never encountered by the people working on it, but large enough that it is not possible to fit a filesystem that needs more on earth, no matter how good your technology is. The only limits that can be reached is that the maximum size of a single file is 2^64 bytes, or 16 exabytes, and the maximum amount of files in a single directory is 2^48, or 281 trillion. The other limits are large enough that to reach them would generally require more energy than it would take to literally boil the oceans.

tutfbhuf 6y ago
I mean practically speaking. Lets say I actually were to buy 100 x 10TB drives. I know that I can span a ceph cluster across those drives (e.g. CERN has created some 30PB cluster in the past). Can I use 100 drives in a RAID-Z? I'm asking because the main advantage I can see for a zettabyte file system (ZFS) is at larger scale, thus I'm interested in how to actually use it e.g. for a server setup of a company that needs to save data on many disks.
- gh02t 6y ago
  ZFS can use that many drives, but it's not a distributed filesystem so it's intended to use within a single server. For 100 disks (or some arbitrarily large number) I'd assume you want something distributed, in which case ZFS could serve as the underlying FS on each node with e.g. Gluster or Ceph. ZFS as the backing store for Gluster is fairly common, for example.
  - deleted 6y ago
    [deleted]
  - secabeen 6y ago
    I have two systems with 140 drives in a single ZFS pool. (14 VDEVS of 10 drives each). They work great, and we haven't had any issues with them.
    - tutfbhuf 6y ago
      What type of connection do these system have (e.g. 10GbE) and how much latency? Would it be possible to span a ZFS Pool across multiple regions (us, europe, ...)?
      - secabeen 6y ago
        The two systems are in different buildings on campus. Each system (with 140) drives is a single unit, with 60-drive JBODs running on SAS-external connections. If you want to span a pool across multiple regions, you'll want a distributed file system on top of your ZFS to manage that. Something like Lustre or Ceph. It would still be very challenging, though.
- magicalhippo 6y ago
  > Can I use 100 drives in a RAID-Z AFAIK yes you could, but you wouldn't want to put them in a single RAID-Z(1,2 or 3) VDEV, but rather multiple VDEVs. This is because I/O operations scale with the number of VDEVs rather than number of disks[1]. So there's a tradeoff between space efficiency and IOPS. But AFAIK you absolutely could put 100 disks in a single RAID-Z VDEV. On the mailing lists there's frequently people posting with that or more disks in a single pool (split across multiple VDEVs). [1]: https://www.delphix.com/blog/delphix-engineering/zfs-raidz-stripe-width-or-how-i-learned-stop-worrying-and-love-raidz https://www.delphix.com/blog/delphix-engineering/zfs-raidz-s...
  - tutfbhuf 6y ago
    Let's say I build such a system and in 2 years from now, I want to add another 20 disks, is that possible too?
    - magicalhippo 6y ago
      That's the easy thing with ZFS, just add the disks as new VDEVs. So just to clarify. A ZFS pool stores data in VDEVs. A single VDEV can be: - a single disk (no redundancy, but same error checking) - N disks in a N-way mirror - N disks in a RAID-Z (single for disk parity, min 3 disks) - RAID-Z2 with 2 disk parity - RAID-Z3 with 3 disk parity (much slower than other two RAID-Z's due to complex math) You can always _add_ VDEVs to a pool, and ZFS will start using it. It will try to be clever about it, so for example if existing VDEVs are quite full, it will redirect most of the writes to the new VDEV. What you cannot do (yet) is _remove_ a VDEV. It's also possible to replace _all_ the disks in a single VDEV to larger disks in turn (important!), and once the final disk is replaced the VDEV will suddenly appear to have the added capacity.
      - rewtraw 6y ago
        It is actually now possible to remove a vdev. It's a new feature, not sure if it works for all RAID types, but I just removed a mirror from my pool without any issue. It migrated data off that vdev and into the rest of the pool just like a resilver.

tutfbhuf 6y ago

I mean practically speaking. Lets say I actually were to buy 100 x 10TB drives. I know that I can span a ceph cluster across those drives (e.g. CERN has created some 30PB cluster in the past). Can I use 100 drives in a RAID-Z? I'm asking because the main advantage I can see for a zettabyte file system (ZFS) is at larger scale, thus I'm interested in how to actually use it e.g. for a server setup of a company that needs to save data on many disks.

gh02t 6y ago
ZFS can use that many drives, but it's not a distributed filesystem so it's intended to use within a single server. For 100 disks (or some arbitrarily large number) I'd assume you want something distributed, in which case ZFS could serve as the underlying FS on each node with e.g. Gluster or Ceph. ZFS as the backing store for Gluster is fairly common, for example.
- deleted 6y ago
  [deleted]
- secabeen 6y ago
  I have two systems with 140 drives in a single ZFS pool. (14 VDEVS of 10 drives each). They work great, and we haven't had any issues with them.
  - tutfbhuf 6y ago
    What type of connection do these system have (e.g. 10GbE) and how much latency? Would it be possible to span a ZFS Pool across multiple regions (us, europe, ...)?
    - secabeen 6y ago
      The two systems are in different buildings on campus. Each system (with 140) drives is a single unit, with 60-drive JBODs running on SAS-external connections. If you want to span a pool across multiple regions, you'll want a distributed file system on top of your ZFS to manage that. Something like Lustre or Ceph. It would still be very challenging, though.
magicalhippo 6y ago
> Can I use 100 drives in a RAID-Z AFAIK yes you could, but you wouldn't want to put them in a single RAID-Z(1,2 or 3) VDEV, but rather multiple VDEVs. This is because I/O operations scale with the number of VDEVs rather than number of disks[1]. So there's a tradeoff between space efficiency and IOPS. But AFAIK you absolutely could put 100 disks in a single RAID-Z VDEV. On the mailing lists there's frequently people posting with that or more disks in a single pool (split across multiple VDEVs). [1]: https://www.delphix.com/blog/delphix-engineering/zfs-raidz-stripe-width-or-how-i-learned-stop-worrying-and-love-raidz https://www.delphix.com/blog/delphix-engineering/zfs-raidz-s...
- tutfbhuf 6y ago
  Let's say I build such a system and in 2 years from now, I want to add another 20 disks, is that possible too?
  - magicalhippo 6y ago
    That's the easy thing with ZFS, just add the disks as new VDEVs. So just to clarify. A ZFS pool stores data in VDEVs. A single VDEV can be: - a single disk (no redundancy, but same error checking) - N disks in a N-way mirror - N disks in a RAID-Z (single for disk parity, min 3 disks) - RAID-Z2 with 2 disk parity - RAID-Z3 with 3 disk parity (much slower than other two RAID-Z's due to complex math) You can always _add_ VDEVs to a pool, and ZFS will start using it. It will try to be clever about it, so for example if existing VDEVs are quite full, it will redirect most of the writes to the new VDEV. What you cannot do (yet) is _remove_ a VDEV. It's also possible to replace _all_ the disks in a single VDEV to larger disks in turn (important!), and once the final disk is replaced the VDEV will suddenly appear to have the added capacity.
    - rewtraw 6y ago
      It is actually now possible to remove a vdev. It's a new feature, not sure if it works for all RAID types, but I just removed a mirror from my pool without any issue. It migrated data off that vdev and into the rest of the pool just like a resilver.