ZPool & VDev config questions.

[freenasrast]

Consider:

Option 1
--------
ZPOOL[VDEV0(10 x 3TB ZFS+RAIDZ2)] = 24GB of usable storage requiring 30GB RAM since it's 1GB of RAM for each TB [10 x 3TB = 30TB]


Option 2
--------
ZPOOL[VDEV0(6 x 3TB ZFS+RAIDZ2) + VDEV1(6 x 3TB ZFS+RAIDZ2)] = 24 GB of usuable storage requiring 36 GB RAM since its 1GB RAM for each TB [2 x (6 x 3TB) = 36TB]


Q1/
Why would option 2 need that extra 6GB to produce the same amount of usable storage as option 1. Am I doing the math incorrectly?

Q2/
Option 2 somehow feels like the best way to go, but I am no expert. Which of the two options is the preferred option.

Thanks!

 

[paleoN]

Why would option 2 need that extra 6GB to produce the same amount of usable storage as option 1. Am I doing the math incorrectly?

Try subtracting the parity first. There is some ambiguity about this, but that's the conclusion I reached when I had thought about it some time ago. I haven't encountered anything sufficient to change my mind so far. Of course depending on the use case you can get away with less at the possible cost of some performance.

Option 2 somehow feels like the best way to go, but I am no expert.

+1

 

[freenasrast]

Try subtracting the parity first. There is some ambiguity about this, but that's the conclusion I reached when I had thought about it some time ago.

It's what makes common sense to me too, only what gave me pause is the documentation I seem to happen upon on the topic always lumps in both the usable space and the parity when calculating the RAM needed.

 

[cyberjock]

Keep in mind those are thumbrules. I have a 30TB zpool and with 12GB of RAM it was horribly slow for only 1 user(me). I couldn't stream a movie and look for a file without the video stuttering, etc. Upgraded to 20GB of RAM and I'm smooth sailing. Depending on the number of users, the loading you use and the performance you are expecting you may need more than thumbrule, or less. Honestly, I'd start with 2x8GB sticks and see how things go. If you need more, buy another 8GB stick. If you still need more, buy yet another 8GB stick. Of course, if money is no object, then just go to 32GB right now ;).

 

[freenasrast]

Keep in mind those are thumbrules. I have a 30TB zpool and with 12GB of RAM it was horribly slow for only 1 user(me). I couldn't stream a movie and look for a file without the video stuttering, etc. Upgraded to 20GB of RAM and I'm smooth sailing. Depending on the number of users, the loading you use and the performance you are expecting you may need more than thumbrule, or less. Honestly, I'd start with 2x8GB sticks and see how things go. If you need more, buy another 8GB stick. If you still need more, buy yet another 8GB stick. Of course, if money is no object, then just go to 32GB right now ;).

The board supports p to 32GB so I'll put it in so I don't have to worry about it later.
What power supply are you using to drive this setup with all of those drives?