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0 pointsdeadbunny2y ago0 comments

ZFS using mirrors is extremely easy to expand. Need more space and you have small drives? Replace the drives in a mirror one by one with bigger ones. Need more space and already have huge drives? Just add another vdev mirror. And the added benefit of not living in fear of drive failure while resilvering as it is much faster with mirrors than raidX.

Sure the density isn't great as you're essentially running at 50% or raw storage but - touches wood - my home zpool has been running strong for about a decade doing the above from 6x 6tb drives (3x 6tb mirrors) to 16x 10-20tb drives (8x mirrors, differing sized drives but matched per mirror like a 10tb x2 mirror, a 16tb x2 mirror etc).

Edit: Just realised someone else as already mentioned a pool or mirrors. Consider this another +1.

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matheusmoreira2y ago

> Replace the drives in a mirror one by one with bigger ones.

That's exactly what I meant by "just as bad as RAID". Expanding an existing array is analogous to every single drive in the array failing and getting replaced with higher capacity drives.

When a drive fails, the array is in a degraded state. Additional drive failures put the entire system in danger of data loss. The rebuilding process generates enormous I/O loads on all the disks. Not only does it take an insane amount of time, according to my calculations the probability of read errors happening during the error recovery process is about 3%. Such expansion operations have a real chance of destroying the entire array.

deadbunnyOP2y ago

That's not the case it mirrored vdevs. There is no degredatuon of the array with a failed drive in a mirrored vdev, it continues humming along perfectly fine.

Also resilvers are not as intensive when rebuilding a mirror as you are just copying from one disk in the vdev to the other, not all X other drives and recalculating parity at the same time. This means less reads across the entire array and much much quicker resilver times, thus less window for drive failure.

But don't just take my word for it. This is a blog post that go much into much more detail https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs...

matheusmoreira2y ago

I see. That addresses my concerns, and it's starting to make a lot of sense. I'm gonna study this in depth, starting with the post you linked. Thank you.

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matheusmoreira2y ago

> Replace the drives in a mirror one by one with bigger ones.

That's exactly what I meant by "just as bad as RAID". Expanding an existing array is analogous to every single drive in the array failing and getting replaced with higher capacity drives.

deadbunnyOP2y ago

That's not the case it mirrored vdevs. There is no degredatuon of the array with a failed drive in a mirrored vdev, it continues humming along perfectly fine.

But don't just take my word for it. This is a blog post that go much into much more detail https://jrs-s.net/2015/02/06/zfs-you-should-use-mirror-vdevs...

matheusmoreira2y ago

I see. That addresses my concerns, and it's starting to make a lot of sense. I'm gonna study this in depth, starting with the post you linked. Thank you.

j / k navigate · click thread line to collapse