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Actually it's better to just do mirrors. Avoid RAIDZ at all costs if you care about performance and the ability to resilver in a reasonable amount of time.
by feld 9y ago
Actually it's better to just do mirrors. Avoid RAIDZ at all costs if you care about performance and the ability to resilver in a reasonable amount of time.
- gigatexal 9y agoSure I agree. But nested mirrors still suffer from the same issue of losing a drive and you lose everything.
- aeorgnoieang 9y ago> But nested mirrors still suffer from the same issue of losing a drive and you lose everything. Are you referring to mirroring a volume or dataset on a single disk? Why would you want to do that instead of mirroring among multiple drives?
- gigatexal 9y agohow would you set up a large pool? two sets of say 5 disks in a mirror raidz1 would still fail if a disk in one set failed and a disk in the other set failed. I guess you could do a stripe setup of 5 sets of 2 disks in mirrors. Still it seems wicked risky to me. I do agree though mirroring has been the best for speed but a lot of that changes with nicer SSDs especially NVMe ones.
- aeorgnoieang 9y agoI was curious about what a "nested" mirror is really. What exactly is nested? I'd setup a large pool with mirror vdevs, i.e. n sets of 2 disks per mirror. My half-remembered reasoning was that backups manage the risk you'll lose data. But replacing a disk in a mirror vdev is much easier, and faster, than doing so with RAIDZ. The risk of RAIDZ is that resilvering impacts multiple vdevs, is much more intensive than a simple mirror resilvering, and thus the probability that additional drives will fail is much higher. Here's a blog post that I definitely read the last time I was reading up on this: - [ZFS: You should use mirror vdevs, not RAIDZ. – JRS Systems: the blog](http://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-not-raidz/ http://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs-...)
- aeorgnoieang 9y agoA Reddit post about that blog post in my other reply: - [You should use mirror vdevs, not RAIDZ. : DataHoarder](https://www.reddit.com/r/DataHoarder/comments/2v0quc/you_should_use_mirror_vdevs_not_raidz/?st=j5vfwpao&sh=926670a0 https://www.reddit.com/r/DataHoarder/comments/2v0quc/you_sho...)
- gigatexal 9y agoI wonder if resilvering is still an issue with SSDs. But I cede your point, nested vdevs of two disks making mirrors makes sense. It doesn't sit well still, but makes sense
- gigatexal 9y agoAccording to OpenZFS's changelog for v 0.7 resilvering is smarter now: https://github.com/zfsonlinux/zfs/releases/tag/zfs-0.7.0 https://github.com/zfsonlinux/zfs/releases/tag/zfs-0.7.0