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

[antlists <antlists@youngman.org.uk>]

On 29/06/2021 14:56, Frank Steinmetzger wrote:
> 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 €€€.
> 
5) Dunno if this is possible but ... replace one 6TB by a 12TB (any 
reason you don't like Helium?) and raid-0 two of the remaining 6's 
together. Dunno anything about what the raidZ's are but I presume this 
would give you 12TB of mirrored storage. It would also only use 3 slots, 
so you could use the 4th for eg your videos, and back them up on 
external storage ie the drive you've just removed :-)

(The raid-0, I'd probably stripe rather than linear for performance.)

Cheers,
Wol