[gentoo-user] [OT] Using an odd number of drives in ZFS RaidZ

[Frank Steinmetzger]

Hello fellows

This is not really a Gentoo question, but at least my NAS (which this mail
is about) is running Gentoo. :)

There are some people amongst this esteemed group that know their stuff
about storage and servers and things, so I thought I might try my luck here.
I’ve already looked on the Webs, but my question is a wee bit specific and I
wasn’t able to find the exact answer (yet). And I’m a bit hesitant to ask
this newbie-ish question in a ZFS expert forum. ;-)

Prologue:
Due to how records are distributed across blocks in a parity-based ZFS vdev,
it is recommended to use 2^n data disks. Technically, it is perfectly fine
to deviate from it, but for performance reasons (mostly space efficiency) it
is not the recommended way. That’s because the (default) maximum record size
of 128 k itself is a power of 2 and thus can be distributed evenly on all
drives. At least that’s my understanding. Is that correct?

So here’s the question:
If I had three data drives, (c|w)ould I get around that problem by setting a
record size that is divisible by 3, like 96 k, or even 3 M?



Here’s the background of my question:
Said NAS is based on a Mini-ITX case which has only four drive slots (which
is the most common configuration for a case of this formfactor). I started
with two 6 TB drives, running in a mirror configuration. One year later
space was running out and I filled the remaining slots. To maximise
reliability, I went with RaidZ2.

I reached 80 % usage (which is the recommended maximum for ZFS) and am
now evaluating my options for the coming years.
1) Reduce use of space by re-encoding. My payload is mainly movies, among
   which are 3 TB of DVDs which can be shrunk by at least ⅔ by re-encoding.
   → this takes time and computing effort, but is a long-term goal anyway.
2) Replace all drives with bigger ones. There are three counter arguments:
   • 1000 € for four 10 TB drives (the biggest size available w/o helium)
   • they are only available with 7200 rpm (more power, noise and heat)
   • I am left with four perfectly fine 6 TB drives
3) Go for 4+2 RaidZ2. This requires a bigger case (with new PSU due to
   different form factor) and a SATA expansion card b/c the Mobo only has
   six connectors (I need at least one more for the system drive), costing
   250 € plus drives.
4) Convert to RaidZ1. Gain space of one drive at the cost of resilience. I
   can live with the latter; the server only runs occasionally and not for
   very long at a time. *** This option brings me to my question above,
   because it is easy to achieve and costs no €€€.

-- 
Grüße | Greetings | Qapla’
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In this sentance are definately three error’s!