4tb HDs for Raidz2

[messerchmidt]

Hello

If I go raid-z2, how drives do i lose to parity? 1 or 2?

I was going to order 5 or 6 x HGST 4tb drives via this deal -> http://forums.redflagdeals.com/newegg-ca-4tb-hgst-deskstar-nas-harddrive-179-99-a-1783965/ (yes I am in Canada)

I have a xeon 1230v3 with 32gb ecc that i made in late 2014 using some 1tb drives in raidz1 (carry over from previous build)

I have 6 sata ports on the board and 4 more via a pcie raid card (freenas detects it)

was going to order the IBM card: to confirm, this is the card I need?

http://www.ebay.com/itm/IBM-ServeRa...-SAS9220-8i-/151770387354?hash=item235638539a

 

[DrKK]

In RAID-Z2, you will be "losing" two drives' worth of storage. RAID-Z is one, RAID-Z2 is two, RAID-Z3 is three ;)

If you had 6x4TB on a RAID-Z2 vdev, you would expect, what, like 14.5TB or so of pool at the end of the day.

That card looks right to me. Maybe one of the other guys can confirm...@cyberjock? @Ericloewe ?

 

[cyberjock]

Math seems right to me.. something like 14.5TB give or take a little bit.

 

[Ericloewe]

Card is supposedly the right model. Possible "Fell out of the back of a truck in the shadiest alley in Shenzhen" issues aside, it'll do fine.

 

[messerchmidt]

you cannot add drives to a zfs pool later right? so i cannot add more 4tb drives at a later time, or?

 

[Ericloewe]

you cannot add drives to a zfs pool later right? so i cannot add more 4tb drives at a later time, or?

Sure you can. You can add as many vdevs as you like, but if any single vdev fails, you'll lose the pool, so you'll want to avoid stupid ideas like single HDDs as vdevs.

 

[messerchmidt]

so if i buy 5 or 6 drives now, i can add more 4tb drives in the future and increase the size of the array/pool?

 

[anodos]

Card is supposedly the right model. Possible "Fell out of the back of a truck in the shadiest alley in Shenzhen" issues aside, it'll do fine.

In that vein, the card doesn't come with a bracket. You'll probably want one of those.

 

[Ericloewe]

so if i buy 5 or 6 drives now, i can add more 4tb drives in the future and increase the size of the array/pool?

Right, you can do something like add another 5 or 6 drives to the pool. It's more flexible than that, but I feel adding more of the same type of vdev is generally the best idea.

 

[TXAG26]

I think what the OP is getting at is if he starts out with 6x 4TB HDD's and wants to add 2x more HDD's to "expand" the same pool and keep RaidZ2, then NO, you can't do that without first moving all of your data out of the pool, destroying the 6x HDD RaidZ2, and finally rebuilding it with the new 8x HDD RaidZ2 pool. You would then need to copy all of your data back to the 8x HDD pool.

What Ericloewe describes about vdev's is also correct, but you really need to add the same number of drives at the same RaidZ2 level if you're at all concerned with data security and performance.

 

[Ericloewe]

What Ericloewe describes about vdev's is also correct, but you really need to add the same number of drives at the same RaidZ2 level if you're at all concerned with data security and performance.

It's a bit more flexible than that. What keeps me from adding a four disk RAIDz2 vdev to the already existing six disk RAIDZ2 vdev is mostly OCD.

 

[diedrichg]

What's lost in adding a second vdev is storage due to parity and overhead. So even though moving your data off the server for a few hours is scary, I see lost storage space and the potential for pool loss an unacceptable negative. Not to mention, not only do I have to worry about the two-drive parity in my first vdev, but now I have to worry about a second vdev two-drive parity. That's 4 drives to keep on the shelf when I could have just kept 2 on the shelf with a larger rebuilt pool.

 

[Ericloewe]

What's lost in adding a second vdev is storage due to parity and overhead. So even though moving your data off the server for a few hours is scary, I see lost storage space and the potential for pool loss an unacceptable negative. Not to mention, not only do I have to worry about the two-drive parity in my first vdev, but now I have to worry about a second vdev two-drive parity. That's 4 drives to keep on the shelf when I could have just kept 2 on the shelf with a larger rebuilt pool.

I disagree. Having four drives ready for a small server is overkill unless extreme high availability is desired.
Yes, there is a roughly 2x chance of a drive failure, but there is also more redundancy in the pool. If I had two drives failing in quick succession before I had a new spare ready, I'd definitely shut down the server and investigate (small server is implied, of course).

There are clear disadvantages (cost of additional drives and support structure, including power and heat extraction), but I disagree that two sane vdevs are less reliable in practical terms than a single vdev of equal capacity.