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Comment by keeperofdakeys | Hacker News Reader

A mirror resilver is a relatively linear and sequential rewrite, so its very fast. Raidz resilvers require lots of random reads and writes across all the drives, and requires waiting for all drives to read data before it can be written to the replaced drive - "herding cats" sounds appropriate here.

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That makes sense. Usually the received wisdom I hear is "RAIDZ is slower due to the parity calculations", which has always seemed dubious to me on modern CPUs, given we can do much more complex compression and encryption at way faster then disk speed (for HDDs, at least)

Usually what makes RAID-Z slower is not really the parity calculations, it's the fact that each RAID-Z vdev only has the read IOPS of the slowest disk in it.

So for example, a pool with 2 RAID-Z vdevs each containing 5 disks effectively has only 2x the IOPS of a single disk, while a pool with 5 mirror vdevs (i.e. RAID-10) has 10x the IOPS of a single disk.

It's not a big problem if you mostly do sequential I/O but it's a huge difference if/when you do small random reads (e.g. traversing a non-cached directory tree).

In the context of resilvers, currently RAID-Z pools can only be done by traversing the block tree which, due to fragmentation of CoW filesystems, usually leads to a lot of random reads/writes, while a RAID-10 pool can basically resilver a pool while doing almost fully sequential I/O which can be much, much faster.

Do random reads and writes out more stress on the drive mechanism? I’m wondering if this increase in resliver time adds risk of data loss, which is probably the more important factor than clock time. (But I’m open to learning if I am wrong)

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