[zfs-discuss] Replacing multiple drives at once - performance

Chris Siebenmann cks at cs.toronto.edu
Tue Sep 1 10:59:29 EDT 2015

> My question is about the performance. Since the resilvering takes a
> couple of weeks to complete (replacing one drive), would replacing
> two drives at once be slower or faster than replacing them one after
> another?

 Replacing multiple drives at once will generally resilver at
essentially the same speed as replacing a single drive. Resilvers are
generally limited by how fast your disks can seek (generally your
existing disks), not by how much data you can read and write at once.

(To simplify a lot, resilvers work by walking down the ZFS tree of
objects, identifying objects that need to be written to the new disks,
and then writing them there. Walking down the tree requires random IO
all over your existing disks and that's generally the limiting part.  If
you have only a little space used, resilvers and scrubs are much faster
than normal software RAID bulk copying; if you have a lot of space used,
they can be significantly slower on spinning rust HDs. Pervasive SSDs
will make this a lot better.)

	- cks

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