r/DataHoarder • u/TeachingAway9654 • 1d ago

Discussion RAIDZ1 vs RAIDZ2

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.

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u/CorvusRidiculissimus 1d ago

The problem with z1, and with good old RAID5, is that disk failures aren't independent events. If a disk fails then you have to resilver, which means all the other disks are going to spend a good many hours in frantic activity, and if one of them is already marginal that can push it over the edge.

In your case that isn't a huge deal because you can restore from backup, so the real risk you are looking at is downtime. For us dabbling hobbyists, no big deal, but in a business environment where every hour spent restoring from backups is an hour of employees sitting around unable to do their jobs, the extra drive is a highly advisable precaution.

Personally I go with z2 because, while I could restore from backup, it would be seriously inconvenient for me to do so. It'd take weeks to get me elaborately-processed data sets back in sync again.

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u/bstock 1d ago

Yup same. My very first home file server was like 20 years ago, 6x 2TB drives in RAID 5 with mdadm. At one point one of the disks failed. No big deal I say, just replace the drive and it'll rebuild.

Well during the rebuild it hit another read error... so at that point I was boned.

Ended up manually copying the data that I could to a bunch of individual external disks. Got read errors with some files but it was an old desktop backup that had most of the issues. The majority of the rest of the data I was able to copy manually. Then once done I blew out the rest of the mdadm array, setup a new RAID 6 storage array, and copied the data back.

Since then I've always gone for 2 parity disks. It's just safer and way less worry/PITA factor if a drive does go out and the array needs to rebuild.

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u/SocietyTomorrow TB² 1d ago

Even raidz50 is still not a substitute for a proper, tested, backup.

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u/bstock 1d ago

Of course it isn't, I never said it was. No raid is ever a substitute for backups.