questions about combining vdevs into a single zpool

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eweber1969

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Apr 26, 2012
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Hi All,

I had a few questions about combining 2 vdev's (using raidz1) into a single zpool. My questions are based on the following configuration...

vdev1 = 2TB + 2TB + 2TB (available space = 4TB)
vdev2 = 1TB + 1TB + 1TB (available space = 2TB)

1) please confirm that the total available space would be 6TB, and that the smaller vdev does not limit the total capacity of the zpool.

2) do files get striped across both vdev's, or does freenas decide that it will write a file to either one vdev or the other?

3) I would hope that the answer to question #2 is that freenas stripes a file across one or the other, but not both. I'm asking in regards to drive failure. I would assume that I could lose (1) one drive from vdev1 OR (2) one drive from vdev2 OR (3) one drive from vdev1 and one drive from vdev2 and that everything would still be recoverable.

4) What if I lost 2 drives from vdev1? I understand that vdev1 would be hosed, but would it affect the files on vdev2?

5) Since I don't have 6 drives laying around, I would first have to create vdev1, then copy all my existing files to freenas. Then I would add vdev2 (with the now empty hdd's). In my mind, vdev1 would be full and vdev2 would be empty. This feels unbalanced. Does this mean that only future writes to the zpool would begin utilizing vdev2 (i guess it depends on the answer to question #2)? Or does freenas fill up vdev1 before it starts using vdev2.

Thank you very much for your help!

Eric
 

ProtoSD

MVP
Joined
Jul 1, 2011
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3,348
Hi Eric,

1) The smaller vdev won't limit the size of your pool, but you should have vdevs of equal size or you will have redundancy issues.

2) Files get striped across both if they're created at the same time, otherwise it's somewhat random.

3) You're correct about losing a single disk from one or both vdevs

4) If you lose a vdev from any pool you're screwed

In my mind, vdev1 would be full and vdev2 would be empty. This feels unbalanced. Does this mean that only future writes to the zpool would begin utilizing vdev2

Yes, this is how it works

You could also create a pool of mirrors, that way you could add to the pool 2 disks at a time, but the same principle applies, if you lose 1 mirror in a pool you lose all of them.

Or does freenas fill up vdev1 before it starts using vdev2.

When you ask this question you should ask how ZFS or FreeBSD does it. FreeNAS is built on those and doesn't change the rules for it's own reasons.

-- Proto
 

eweber1969

Cadet
Joined
Apr 26, 2012
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Thanks Proto! Appreciate your time and expertise answering my questions.

Today I have 3 hdd's: 1TB + 1TB (in raid0), and 2TB (backup). I was thinking about buying 3 more drives (1TB, 2TB, 2TB) and creating the zpool I described earlier.

Based on your comment, "you should have vdevs of equal size or you will have redundancy issues," as well as the current price of hdd's, maybe I should stick with just 1 vdev (2TB x 3), and then either (1) add another equal sized vdev later or (2) maintain just 1 vdev and upgrade the drives from 2tb to 3tb to 4tb as technology allows (affords).

hmmmm....Don't really have a use for the 1TB drives after the nas gets built....i also bought a mobo with 6 sata ports.

Thanks again for your help.

Eric
 
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