Back when I was even less experienced in self-hosting I setup my media/backup server using a RAIDZ1 array and 3 x 8TB disks. It’s been running well for a while and I haven’t had any problems and no disk errors.
But today I read a post about ‘pool design rules’ stating that RAIDZ1 configurations should not have drives over 1TB because the chances of errors occurring during re-silvering are high. I wish I had known this sooner.
What can I do about this? I send ZFS snapshots to 2 single large (18TB) hardrives for cold backups, so I have the capacity to do a migration to a new pool layout. But which layout? The same article I referenced above says to not use RAIDZ2 or RAIDZ3 with any less than 6 drives…I don’t want to buy 3 more drives. Do I buy an additional 8TB drive (for a total of 4 x 8TB) and stripe across two sets of mirrors? Does that make any sense?
Thank you!
Honestly, if you’re doing regular backups and your ZFS system isn’t being used for business you’re probably fine. Yes, you are at increased risk of a second disk failure during resilver but even if that happens you’re just forced to use your backups, not complete destruction of the data.
You can also mitigate the risk of disk failure during resilver somewhat by ensuring that your disks are of different ages. The increased risk comes somewhat from the fact that if you have all the same brand of disks that are all the same age and/or from the same batch/factory they’re likely to die from age around the same time, so when one disk fails others might be soon to follow, especially during the relatively intense process of resilvering.
Otherwise, with the number of disks you have you’re likely better off just going with mirrors rather than RAIDZ at all. You’ll see increased performance, especially on write, and you’re not losing any space with a 3-way mirror versus a 3-disk RAIDZ2 array anyway.
The ZFS pool design guidelines are very conservative, which is a good thing because data loss can be catastrophic, but those guidelines were developed with pools that are much larger than yours and for data in mind that is fundamentally irreplaceable, such as user generated data for a business versus a personal media server.
Also, in general backups are more important than redundancy, so it’s good you’re doing that already. RAID is about maintaining uptime, data security is all about backups. Personally, I’d focus first on a solid 3-2-1 backup plan rather than worrying too much about trying to mitigate your current array suffering catastrophic failure.
I think it’s worth pointing out that this article is 11 years old, so that 1TB rule-of-thumb probably probably needs to be adjusted for modern disks.
If you have 2 full backups (18TB drives being more than sufficient) of the array, especially if one of those is offsite, then I’d say you’re really not at a high enough risk of losing data during a rebuild to justify proactively rebuilding the array until you have at least 2 or more disks to add.
Let’s do the math:
The error-reate of modern hard disks is usually on the order of one undetectable error per 1E15 bits read, see for example the data sheet for the Seagate Exos 7E10. An 8 TB disk contains 6.4E13 (usable) bits, so when reading the whole disk you have roughly a 1 in 16 chance of an unrecoverable read error. Which is ok with zfs if all disks are working. The error-correction will detect and correct it. But during a resilver it can be a big problem.
If the actual error rate were anywhere near that high, modern enterprise hard drives wouldn’t be usable as a storage medium at all.
A 65% filled array of 10x20TB drives would average at least 1 bit failure on every single scrub (which is full read of all data present in the array), but that doesn’t actually happen with any real degree of regularity.
Then why do you think manufacturers still list these failure rates (to be sure, it is marked as a limit, not an actual rate)? I’m not being sarcastic or facetious, but genuinely curious. Do you know for certain that it doesn’t happen regularly? During a scrub, these are the kinds of errors that are quietly corrected (althouhg the scrub log would list them), as they are during normal operation (also logged).
My theory is that they are being cautious and/or perhaps don’t have any high-confidence data that is more recent.
Those failure rates are nonsense based on theoretical limits. In practice: a few weeks ago I resilvered a 4 disk z1 array with 8tb disks at 85% capacity in less than 24 hours. I scrub the pool every month and it didn’t seem any more taxing than a scrub.
It’s fine. RAID is not a backup. I’ve been running simple mirrors for many years and never lost data because I have multiple backups. Focus on offsite and resilient backups, not how many drives can fail in your primary storage device.
Personally I’d just upgrade to RAIDZ2, and add as many disks to that as reasonably practical. To be honest, I fail to see any downsides to using four disks for this other than the storage inefficiency.
