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I don't quite understand the use case here. If I'm setting up RAID it's because I want the system to stay up. That's the only purpose for it. If a device goes

I don't quite understand the use case here. If I'm setting up RAID it's because I want the system to stay up. That's the only purpose for it. If a device goes missing for "unknown reasons", then the machine should still work, and I'll figure out what happened when monitoring pokes me and says RAID is degraded.

viraptor 4y ago
The use case is: Enough drives failed that your raid is degraded. Any more data you write is not replicated and it may be due to software/hardware issue that will kill more drives soon. It's up to you to choose at that point - is availability more important for you (add degraded to fstab), or data consistency (deal with the array first).
_kdave 4y ago
Yeah but monitoring is not something that comes with the filesystem. If you have to set up the system to be a HA and configure monitoring, email notifications whatever, making sure the filesystem is created with redundant profiles, then I'm expecting that also adding the 'degraded' to the fstab is part of the configuration.
p1necone 4y ago
> That's the only purpose for it. That's not the only purpose for it. There's three reasons I can think of that you might set up a RAID array: * You want better uptime. (your use case) * You want to protect from data loss. (my assumption was that this is the most common use case, but I could be wrong. This also helps with uptime because there's nothing worse for uptime than having to restore lost data from a cold backup) * You want better performance, data integrity be damned. (RAID 0) Booting a RAID array with a failed disk is a bad idea if you care a lot about not losing data, because now you're one less disk failure away.
- jonhohle 4y ago
  RAID 0 should be called AID, since it’s not really RAID.
  - vetinari 4y ago
    The '0' says it exactly: the amount of data you are left with, once one of the drives fails.
- wtallis 4y ago
  Another way of thinking about it: should uptime with bad data or services making false guarantees about data durability actually count as uptime?
- justsomehnguy 4y ago
  > Booting a RAID array with a failed disk is a bad idea Booting a RAID array with a failed disk is absolutely fine idea. How else I get access to the tools to identify the bad drive and resilver RAID on a replacement, be it in the same bay or not?
  - wtallis 4y ago
    Booting from a degraded array is only a fine idea in some circumstances, not all. That's why the kernel should not default to automatically doing so; but a distro or sysadmin that has better knowledge of the broader situation (eg. presence of hot spares or a working monitoring/alert system) can reasonably change that default when the risks of booting from a degraded array have been mitigated.
    - justsomehnguy 4y ago
      Ie you are treating RAID as a backup.
      - happymellon 4y ago
        No, you are treating RAID as a protection against longer outage of restoring from backups.
      - wtallis 4y ago
        Backups cannot be perfectly real-time unless they are very nearly RAID. Any time you are generating/collecting important data, you will unavoidably have some amount of that important data in the state of not yet backed up. It's reasonable to want to preserve all the data you currently have—some of which probably hasn't been backed up yet—and not accept new data to be written with the durability guarantees the array was originally configured for silently violated. Since the kernel has no way of knowing which volumes may contain important data that didn't get the chance to be backed up, it should try its best to maintain the original durability standards the filesystem was configured until some mechanism outside the kernel authorizes the relaxation of those standards.
RealStickman_ 4y ago
The system stays up just fine. You just can't reboot it without fixing/ignoring the problem. I think that's fair.

viraptor 4y ago

The use case is: Enough drives failed that your raid is degraded. Any more data you write is not replicated and it may be due to software/hardware issue that will kill more drives soon. It's up to you to choose at that point - is availability more important for you (add degraded to fstab), or data consistency (deal with the array first).

_kdave 4y ago

Yeah but monitoring is not something that comes with the filesystem. If you have to set up the system to be a HA and configure monitoring, email notifications whatever, making sure the filesystem is created with redundant profiles, then I'm expecting that also adding the 'degraded' to the fstab is part of the configuration.

p1necone 4y ago

> That's the only purpose for it. That's not the only purpose for it. There's three reasons I can think of that you might set up a RAID array: * You want better uptime. (your use case) * You want to protect from data loss. (my assumption was that this is the most common use case, but I could be wrong. This also helps with uptime because there's nothing worse for uptime than having to restore lost data from a cold backup) * You want better performance, data integrity be damned. (RAID 0) Booting a RAID array with a failed disk is a bad idea if you care a lot about not losing data, because now you're one less disk failure away.

jonhohle 4y ago
RAID 0 should be called AID, since it’s not really RAID.
- vetinari 4y ago
  The '0' says it exactly: the amount of data you are left with, once one of the drives fails.
wtallis 4y ago
Another way of thinking about it: should uptime with bad data or services making false guarantees about data durability actually count as uptime?
justsomehnguy 4y ago
> Booting a RAID array with a failed disk is a bad idea Booting a RAID array with a failed disk is absolutely fine idea. How else I get access to the tools to identify the bad drive and resilver RAID on a replacement, be it in the same bay or not?
- wtallis 4y ago
  Booting from a degraded array is only a fine idea in some circumstances, not all. That's why the kernel should not default to automatically doing so; but a distro or sysadmin that has better knowledge of the broader situation (eg. presence of hot spares or a working monitoring/alert system) can reasonably change that default when the risks of booting from a degraded array have been mitigated.
  - justsomehnguy 4y ago
    Ie you are treating RAID as a backup.
    - happymellon 4y ago
      No, you are treating RAID as a protection against longer outage of restoring from backups.
    - wtallis 4y ago
      Backups cannot be perfectly real-time unless they are very nearly RAID. Any time you are generating/collecting important data, you will unavoidably have some amount of that important data in the state of not yet backed up. It's reasonable to want to preserve all the data you currently have—some of which probably hasn't been backed up yet—and not accept new data to be written with the durability guarantees the array was originally configured for silently violated. Since the kernel has no way of knowing which volumes may contain important data that didn't get the chance to be backed up, it should try its best to maintain the original durability standards the filesystem was configured until some mechanism outside the kernel authorizes the relaxation of those standards.

RealStickman_ 4y ago

The system stays up just fine. You just can't reboot it without fixing/ignoring the problem. I think that's fair.