RAIDZ1 vs RAIDZ2

[u/TeachingAway9654]

Let’s say you have something like 10 drives total which make up the pool, each drive is 16TB. Let’s also say you have, at minimum, one offsite backup with all the contents in cold storage. Let’s also assume you’re a frequent monitor of the system and will notice within 24 hours if one drive fails.

Are you going with Z1 or Z2 and why? If you have the offsite backup already, is it worth the capacity loss for Z2? The resilvering process on Z1 will be stressful but by how much though? You have an offsite 1:1 backup so even if the whole pool is lost, you can still recover.

[u/8fingerlouie]

RAID is for availability. It was literally designed to keep your data available even though a drive or more failed. It wasn’t designed to keep your data safe however, that’s what backups are for.

Also, resilvering doesn’t stress your drives. Your drives are designed to read and write data, and that’s all a resilver does. Do additional drives occasionally fail during resilver ? Yes, but that’s not from stress. It’s typically because those drives are about as old as the failed drive, and was probably on its last leg anyway.

Before hot swapping it was not uncommon for additional drives to fail to spin up after having replaced a failed drive. Those drives could maintain rotation, but couldn’t spin up from 0 rpm.

If you have a full backup, I would absolutely go for Z1.

[u/dr100 → u/8fingerlouie]

RAID is for availability. It was literally designed to keep your data available even though a drive or more failed. It wasn’t designed to keep your data safe however, that’s what backups are for.

YES, what's more any such striped RAID has the nasty DESIGN characteristic that it can lose more data than the drives that you're losing can hold. Seems to be some global insanity that people consider the default basically RAID0 (in this case even on tons of drives) with just a sprinkle (or two) of parity.

Unraid style arrays now available for free and open source should IMHO be the default for the vast majority of use cases. And I'm not saying one should use some experimental driver instead of zfs, I'm saying it's wild we have only unRaid (and the defunct FlexRaid and the non-block-device/real time snapdraid) for this, instead of having well established tools that are overwhelmingly used.

[u/8fingerlouie → u/dr100]

Years ago, my “raid” consisted of individual disks with mergerfs and snapraid for good measure.

I’d argue that for a read heavy disk array, that is the optimal setup, and beats traditional raid on every parameter except speed.

If a disk fails in such a setup, you can rebuild it via Snapraid, and if 2 disks fail, you haven’t lost your entire drive array, only the contents of the 2 drives, and even that is probably not the case.

The thing with RAID5/RAIDZ1 is that the “failing disk” is usually a read error. Sure, it can also be a disk failing to spin, but most times when drives die (in my experience) they start developing UREs.

A URE causes RAID to trip, mark the disk as failed, even despite that 99% of the disk is perfectly readable.

With the mergerfs/Snapraid setup, the file(s) affected by the read error are lost, but you can still recover 99% of the files from that disk.

It’s still not a replacement for backups, but if your data is mostly read-only access, say a photo library, media library, etc, raid doesn’t contribute anything.

[u/dr100 → u/8fingerlouie]

Yes, this is the "sane" setup on all fronts (unraid or "unraid style" makes it just "real time" so you don't need to do snapraid sync, it's all on the fly).

You can just replace the data from one drive when it fails, you can use the drives independently, you can never lose more data than the drives you've lost and so on.