Volume status showing 8 HDDs in two raidz2? I am confused.

[ringohung]

I'm a newbie. I have a very simple and dumb question. I have tried to search but still can't find anyone dumb enough to post similar question....

I have set up my first Freenas Nas. I have 8 x 8TB drives and wanted to set up them in one Volume in Raidz2. However, I am feeling that I may have done something wrong and set up them as two 4 x 4TB in Raidz2, because I see raidz2-1 and raidz2-0 on the Volume Status. Could any experienced user verify for me please? Thank you for much!

 

[PhiloEpisteme]

Yes, you do in fact have two RADIZ2 vdevs. This possibly happened by trying to "extend" the pool and adding the second set of 4 drives as a RADIZ2 vdev. Keep in mind that while a pool can be extended by adding vdevs currently a RADIZ1|2|3 vdev cannot be extended in this way. You'll need to create the vdev all at once using all of the drives at the same time if you want an 8-drive RADIZ2 vdev.

 

[garm]

You'll need to create the vdev all at once using all of the drives at the same time if you want an 8-drive RADIZ2 vdev.

Which will destroy any data present on the pool

 

[ringohung]

Yes, you do in fact have two RADIZ2 vdevs. This possibly happened by trying to "extend" the pool and adding the second set of 4 drives as a RADIZ2 vdev. Keep in mind that while a pool can be extended by adding vdevs currently a RADIZ1|2|3 vdev cannot be extended in this way. You'll need to create the vdev all at once using all of the drives at the same time if you want an 8-drive RADIZ2 vdev.

I see. I will have to backup all data and remake my volume. Thank you very much!

 

[ringohung]

Which will destroy any data present on the pool

Noted. Thank you!

 

[ringohung]

Yes, you do in fact have two RADIZ2 vdevs. This possibly happened by trying to "extend" the pool and adding the second set of 4 drives as a RADIZ2 vdev. Keep in mind that while a pool can be extended by adding vdevs currently a RADIZ1|2|3 vdev cannot be extended in this way. You'll need to create the vdev all at once using all of the drives at the same time if you want an 8-drive RADIZ2 vdev.

BTW, would a 8 x 8TB in raidz2 or two 4 x 8TB in raidz1 be better and more reliable? Thanks in advance.

 

[anmnz]

RAIDZ-1 is not recommended around here for disks over 1TB in size, as a general rule, because if a disk fails then its vdev will have no redundancy until replacement & resilvering is complete, and that takes a while for a large disk.

With 8 disks in RAIDZ-2 the pool can survive failure of any 2 disks. And if one disk fails you are still protected against data errors on the other disks.

With 2 4-disk RAIDZ-1 vdevs, if two disks in the same vdev fail then the pool is gone. And if one disk fails you can only detect, not correct, data errors on the 3 other disks in the same vdev.

From a reliability perspective the RAIDZ-2 configuration is unquestionably better.

 

[PhiloEpisteme]

BTW, would a 8 x 8TB in raidz2 or two 4 x 8TB in raidz1 be better and more reliable? Thanks in advance.

My guess is less reliable, depending on what you mean by reliable. For one, RAIDZ2 tolerates two drive failures without losing a vdev and thus your pool whereas RAIDZ1 tolerates only a single drive failure.

Also, with RAIDZ1 when you lose a single drive and replace it via resilvering you have no additional parity; if an unrecoverable read error occurs on one of your disks during resilvering you will not be able to recover that bit and this a file is likely to be corrupted. With RAIDZ2 if a single drive is lost you'll have parity data still and thus read errors can be corrected by ZFS. There is a rather famous article claiming that RAID5 (and thus RAIDZ1) is dead because of the unrecoverable read error rate being high enough such that on large enough disks (~2TB) the likelihood of at least one read error during resilvering is high enough to warrant not using the setup. This argument should be tempered with the fact that new drives have a better unrecoverable read error rate and if one occurs it will not corrupt every single file, only the one living at that place on your disk.

A final reason RAIDZ1 is less reliable is that if during resilvering you lose another drive with RAIDZ1 you're in trouble, you've lost your vdev and thus your pool. With RAIDZ2 you're alright because the vdev can still function with two simultaneous drive failures.

Where RAIDZ1 might be more reliable is if you're trying to ensure that the sectors on the disk line up correctly with the way ZFS stores and stripes data. With RAIDZ2 I believe it is 6 disks, with RAIDZ1 it is 5 disks, I believe. So if you only had 5 disks and wanted to pick the layout that uses the space most optimally you'd go RAIDZ1.

In your case because the drives are so large and resilevering would take a while so I would personally go with RAIDZ2 to avoid losing the vdev if another failed during that process.