1) Is it correct that "3x2TB RAIDZ1 + 3x2TB RAIDZ1" is equivalent to 6x2TB RAIDZ2? So I will have 8TB available and 4TB for parity?
2) If 1) is correct, is there an advantage of building the 6x2TB RAIDZ2 from the start rather than expanding the 3x2TB RAIDZ1?
3) If 1) is correct, what happens if I expand the 3x2TB RAIDZ1 with 3x3TB RAIDZ1? Is this even possible? Will I have 8TB available store or 10TB?
- What Milhouse said
- Yes there is an advantage to starting with raidz2
- Yes it's possible, though not entirely recommended for multiple reasons. You would have 4TB raw with the first vdev and 6TB raw with the second vdev.
If you start out with
raidz2 you not only get the additional redunancy, but all data is striped across all the drives equally.
If you start out with 3x2TB
raidz1 and
mostly filled it up you would get uneven access patterns when you added a second 3x2TB
raidz1 vdev. The original
vdev, being mostly full & having all the current data, would service most of the reads while the second new
vdev, having almost all the free space, would service most of the writes. All of which has a negative impact on performance and resilvering. It's not the end of the world by any means, but you want to have your fully built configuration as early as possible.
But why is it only possible to expand a RAIDZ1 by a RAIDZ1 of the exact same configuration if the two are independent anyways? Shouldn't it then be possible to extend a 3x2TB RAIDZ1 with e.g. a 5x2TB RAIDZ1?
As
Milhouse said it's possible, but not recommended. You would again run into uneven access patterns. It's also just a plain bad idea to mix different redundancy levels, which you did not in your example.
Everyone has their own threshold as to when to move to double-parity,
raidz2, vs single-parity,
raidz1. I'm actually OK with running 2TB drives in a 3 disk
raidz1 configuration. I am completely uncomfortable running with a 5x2TB disk
raidz1 vdev. Some of the quite knowledgeable storage regulars on here draw the line with 1TB drives, discounting mirrors. You could not make me run any 3TB drive with single parity besides in a mirror configuration. The rebuild/resilver times are simply too long and the chance of additional drive failure too great.