this post was submitted on 07 Mar 2024
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I really want to run ceph because it fits a number of criteria I have: gradually adding storage, mismatched disks, fault tolerance, erasure encoding, encryption, support out-of-the-box from other software (like Incus).

But then I look at the hardware suggestions, and they seem like an up-front investment and ongoing cost to keep at least three machines evenly matched on RAM and physical storage. I also want more of a single-box NAS.

Would it be idiotic to put a ceph setup all on one machine? I could run three mons on it with separate physical device backing each so I don't lose everything from a disk failure with those. I'm not too concerned about speed or network partitioning, this would be lukewarm storage for me.

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[–] [email protected] 5 points 8 months ago (9 children)

I mean, yeah, I'd prefer ZFS but, unless I am missing something, it is a massive pain to add disks to an existing pool. You have to buy a new set of disks and create a new pool to transition from RAID z1 to z2. That's basically the only reason it fails the criteria I have. I think I'd also prefer erasure encoding instead of z2, but it seems like regular scrub operations could keep it reliable.

BTRFS sounds like it has too many footguns for me, and its raid5/6 equivalents are "not for production at this time."

[–] [email protected] -1 points 8 months ago* (last edited 8 months ago) (7 children)

Adding new disks to an existing ZFS pool is as easy as figuring out what new redundancy scheme you want, then adding them with that scheme to the pool. E.g. you have an existing pool with a RAIDz1 vdev with 3 4TB disks. You found some cheap recertified disks and want to expand with more redundancy to mitigate the risk. You buy 4 16TB disks, create a RAIDz2 vdev and add that to the existing pool. The pool grows in storage by whatever is the space available from the new vdev. Critically pools are JBODs of vdevs. You can add any number or type of vdevs to a pool. The redundancy is done at the vdev level. Thus you can have a pool with a mix of any RAIDzN and/or mirrors. You don't create a new pool and transition to it. You add another vdev with whatever redundancy topology you want to the existing pool and keep writing data to it. You don't even have to offline it. If you add a second RAIDz1 to an existing RAIDz1, you'd get similar redundancy to moving from RAIDz1 to RAIDz2.

Finally if you have some even stranger hardware lying around, you can combine it in appropriately sized volumes via LVM and give that to ZFS, as someone already suggested. I used to have a mirror with one real 8TB disk and one 8TB LVM volume consisting of 1TB, 3TB and 4TB disk. Worked like a charm.

[–] [email protected] 3 points 8 months ago (4 children)

You end up wasting a ton of space though because each vdev has its own parity drives.

[–] [email protected] 1 points 8 months ago* (last edited 8 months ago) (2 children)

What you lose in space, you gain in redundancy. As long as you're not looking for the absolute least redundant setup, it's not a bad tradeoff. Typically running a large stripe array with a single redundancy disk isn't a great idea. And if you're running mirrors anyway, you don't lose any additional space to redundancy.

[–] [email protected] 3 points 8 months ago

Yep I feel this way.

No point in pricing a single HDD because I'm shooting for parity on every vdev I spin up.

[–] [email protected] 2 points 8 months ago

Fair enough, it does add a good chunk of power usage though as HDDs are pretty power heavy at 5-7W or so.

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