Are RAM memory requirements for ZFS a function of raw drive capacity or useable drive capacity?

[jsnas]

I was wondering if RAM memory requirements for ZFS are a function of raw drive capacity or useable drive capacity? And does the answer depend on RaidZ2 versus 3-disk Mirrored vdev’s?

For example, I’m planning on using 3-disk mirrored vdev’s. I’ll add drives in sets of 3 per vdev (so can survive up to 2 drive failures). For example:

vdev1:
3*6TB = 18TB raw storage, but only 6TB useable storage

Then I’ll add more vdev’s later as needed. So the total storage for this approach would be:

raw storage = #vdev * (3 * #TB)
useable storage = #vdev * #TB

Looking at this link (“How Much RAM is needed?”):
http://www.freenas.org/whats-new/20...design-part-i-purpose-and-best-practices.html

it mentions “8GB of RAM will get you through the 24TB range. Beyond that 16GB is a safer minimum, and once you get past 100TB of storage, 32GB is recommended.”

So is “24TB range” referring to raw or useable storage? And does a 3-disk Mirrored vdev solution like above require less/same/more RAM than RaidZ2 would for a given amount of useable storage?

 

[SweetAndLow]

Zfs likes lots of memory. Everyone has their guidelines and formulas but those are just guesses. You will be happy starting out with 16GB memory and add more when you add more drives or feel kind you want better performance.

With a 100TB pool 32GB would work but performance would probably not be very good. I would probably want 64-96GB of memory at that point.

 

[Bidule0hm]

In theory the 1 GB per TB rule uses the raw capacity and is not linked to the vdev type (RAID-Zx, mirror, ...) but as @SweetAndLow said it's not a hard rule so don't worry too much.

 

[Darren Myers]

also depends if youre running jails, such as Plex...that can be both CPU and RAM intensive so you would want to be sure you have enough for FreeNAS and any jails to use and play nicely

 

[jsnas]

Thanks everyone for the DDR-to-DriveCapacity ratio feedback. I'll start with 16GB DDR, and move up to 32GB later.

On the topic of #cores in the CPU, if I'm just using FreeNAS as a pure/simple file server (no jails such as Plex, etc.), does ZFS ever use more than one core/thread at a time? In other words, will I see any benefit whatsoever with an 8-core vs 4-core vs 2-core CPU (ignoring all other CPU factors such as frequency)? Do ZFS scrubs or replication take advantage of multiple cores/threads?

 

[SweetAndLow]

Everything in today's world takes advantage of multiple cores. In your case though processor with 2 cores should be fine.