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>Not according to any of the docs I've read in the past. Happy to be proven wrong if you have a citation though. https://raid.wiki.kernel.org/index.php/Growing

by zaarn 8y ago

>Not according to any of the docs I've read in the past. Happy to be proven wrong if you have a citation though. https://raid.wiki.kernel.org/index.php/Growing https://raid.wiki.kernel.org/index.php/Growing ext4 online resize works by invoking resize2fs. >I don't disagree with that but how many of those users are running LVM + mdadm? And how many of those people are just using unraid (or similar purpose built solution for home users)? How many users are just using an off-the-shelf NAS like QNAP et al? QNAP and Synology don't do magic. They deploy mdadm, LVM and ext4 or btrfs (though btrfs only in single drive mode, they use LVM and mdadm to span the volume for btrfs). >That hardly seems fair, don't you think? It's the real world experience I had with the software and I think it's totally fair to point out and complain about the shortcomings. >No raid would give you 3TB of extra storage when you add a 2TB HDD. That would be breaking the laws of physics because you're now asking your server to create 1TB from thin air. Reread what I wrote, I mentioned adding 2x2TB not 1x2TB. > in your situation you don't get redundancy so your data becomes at risk if you have a failure Still having redundancy is exactly the point. The resulting array in the example would span 6x1TB and 2x1TB in RAID5, giving me a single drive for parity in each span and resulting in an effective RAID5 for the entire RAID. I can loose any drive in that 4x1+2x2TB array without loosing data. And it does that online without having to reboot or even halt any ongoing file operations. The volume will simply increase in size once it has finished formatting the new drive and integrated it into the array. >So you can just add an additional HDD to a ZFS tank but you wouldn't get redundancy on that additional HDD (which is why nobody suggests you should do it). Again, what Syno offers (and what can be replicated on any Linux system) allows you to add a single drive to expand the storage without loosing parity and more drives to span any drive increases with parity. This allows adding a single drive at a time and at worst you loose any additional capacity without parity yet (in the 1TB/2TB example, that would be loosing 1TB of storage until the second drive is added)

laumars 8y ago
> ext4 online resize works by invoking resize2fs. That's resizing the filesystem not the pool (eg `mdadm --grow`). You're also missing the point about what I said. To run those commands you must first `umount` to pool. Hence it being an offline job. > QNAP and Synology don't do magic. They deploy mdadm, LVM and ext4 or btrfs (though btrfs only in single drive mode, they use LVM and mdadm to span the volume for btrfs). That's completely irrelevant to the point I was making. How many home users do you know who configure their own home raids using LVM and mdadm? > Reread what I wrote, I mentioned adding 2x2TB not 1x2TB. You actually said neither. You said, and I quote, "2TB" (ie without any accumulation prefix). > Still having redundancy is exactly the point. The resulting array in the example would span 6x1TB and 2x1TB in RAID5, giving me a single drive for parity in each span and resulting in an effective RAID5 for the entire RAID. I can loose any drive in that 4x1+2x2TB array without loosing data You can actually do that in ZFS. In fact what you've just described were is the exact solution that you called "unacceptable" in ZFS! > And it does that online without having to reboot or even halt any ongoing file operations. The volume will simply increase in size once it has finished formatting the new drive and integrated it into the array. Indeed. Lots of commercial tools do support that. I thought we were comparing ext4, LVM and mdadm to ZFS though? I'm guessing you just glossed other my post where I said you were flip-flopping all over the place? It's easy to pick any specific tool ("tool a")and compare it to a thousand other tools and argue that "tool a" is crap because it doesn't work as well as tool b, c and d in very specific edge cases but all you're doing is trying to win an argument via death from a thousand papercuts rather than forming a sane basis for analysis. To be honest I think it's probably best we give up on this discourse because your points are all over the place. This feels very much like you've got an axe to grind and no practical experience to back up your prejudices.
- zaarn 8y ago
  >To run those commands you must first `umount` to pool. Hence it being an offline job. No you don't need to unmount the pool under mdadm. You can do a live resize of the array. See the link I posted. >How many home users do you know who configure their own home raids using LVM and mdadm? Not many, hence me mentioning Syno and QNAP. >You said, and I quote, "2TB" (ie without any accumulation prefix). I said, quote, "if you add 2TB to a 4x1TB RAID5 array, you get 4TB of storage. If you add another 2TB you get 7TB of storage." >In fact what you've just described were is the exact solution that you called "unacceptable" in ZFS! Again, this is not what ZFS does, the mdadm solution stripes multiple RAID arrays over the same drive, which to my recollection is not a recommended configuration in ZFS by a long shot. >Lots of commercial tools do support that. I thought we were comparing ext4, LVM and mdadm to ZFS though? The commercial tools I mentioned previously use ext4, LVM and mdadm native functionality.
  - rkeene2 8y ago
    Not only that, Linux MD let's you resize a RAID5 into a RAID6 (online, of course)

by zaarn 8y ago

laumars 8y ago

> ext4 online resize works by invoking resize2fs. That's resizing the filesystem not the pool (eg `mdadm --grow`). You're also missing the point about what I said. To run those commands you must first `umount` to pool. Hence it being an offline job. > QNAP and Synology don't do magic. They deploy mdadm, LVM and ext4 or btrfs (though btrfs only in single drive mode, they use LVM and mdadm to span the volume for btrfs). That's completely irrelevant to the point I was making. How many home users do you know who configure their own home raids using LVM and mdadm? > Reread what I wrote, I mentioned adding 2x2TB not 1x2TB. You actually said neither. You said, and I quote, "2TB" (ie without any accumulation prefix). > Still having redundancy is exactly the point. The resulting array in the example would span 6x1TB and 2x1TB in RAID5, giving me a single drive for parity in each span and resulting in an effective RAID5 for the entire RAID. I can loose any drive in that 4x1+2x2TB array without loosing data You can actually do that in ZFS. In fact what you've just described were is the exact solution that you called "unacceptable" in ZFS! > And it does that online without having to reboot or even halt any ongoing file operations. The volume will simply increase in size once it has finished formatting the new drive and integrated it into the array. Indeed. Lots of commercial tools do support that. I thought we were comparing ext4, LVM and mdadm to ZFS though? I'm guessing you just glossed other my post where I said you were flip-flopping all over the place? It's easy to pick any specific tool ("tool a")and compare it to a thousand other tools and argue that "tool a" is crap because it doesn't work as well as tool b, c and d in very specific edge cases but all you're doing is trying to win an argument via death from a thousand papercuts rather than forming a sane basis for analysis. To be honest I think it's probably best we give up on this discourse because your points are all over the place. This feels very much like you've got an axe to grind and no practical experience to back up your prejudices.

zaarn 8y ago
>To run those commands you must first `umount` to pool. Hence it being an offline job. No you don't need to unmount the pool under mdadm. You can do a live resize of the array. See the link I posted. >How many home users do you know who configure their own home raids using LVM and mdadm? Not many, hence me mentioning Syno and QNAP. >You said, and I quote, "2TB" (ie without any accumulation prefix). I said, quote, "if you add 2TB to a 4x1TB RAID5 array, you get 4TB of storage. If you add another 2TB you get 7TB of storage." >In fact what you've just described were is the exact solution that you called "unacceptable" in ZFS! Again, this is not what ZFS does, the mdadm solution stripes multiple RAID arrays over the same drive, which to my recollection is not a recommended configuration in ZFS by a long shot. >Lots of commercial tools do support that. I thought we were comparing ext4, LVM and mdadm to ZFS though? The commercial tools I mentioned previously use ext4, LVM and mdadm native functionality.
- rkeene2 8y ago
  Not only that, Linux MD let's you resize a RAID5 into a RAID6 (online, of course)