How to create spare for existing pool in 8.2?

[danzg]

I'm trying to add a 6th disk as a spare for a 5-disk raidz1 pool.

I go to "Volume Manager" ... if I click ZFS, I see the "spare" option, but when I click on the "member disk", that option disappears.

And am I "extending" a volume?

Confused ... do I need to do this from CLI?

 

[cyberjock]

You do know that spare disks are not working completely correctly.. right?

 

[danzg]

From what I read, I got that a hot spare MIGHT automatically take over for a failed disk, or it might require intervention.
I figured it couldn't hurt.

But now I'm stuck .... please see my post here

 

[cyberjock]

I thought there was an issue somewhere about a spare automatically taking over for a failed disk could fail causing problems for the zpool... I can't find the page I saw that at though :( I could be totally out to lunch. I don't build servers that need spares, and I always keep a spare in its box until it's needed. If a disk fails I want to know so I can take appropriate actions as I see fit :P I prefer the manual way, especially since I know there's some issues with spares.

 

[danzg]

My problem is, the server is in New York; I'm in LA! So I have to rely on non-techies over there if I need to lay on hands.

 

[cyberjock]

Eew.. /vomit.

You said you don't want to reboot because of how far away you are... why not? I'm a little confused.

 

[danzg]

No, I meant that someone needs to be physically there to reboot; the machine needs a manual selection of the USB drive at power up through BIOS.

 

[danzg]

I'm back online now, and zpool status shows the spare as AVAILABLE.

But now you have me worried ... should I detach it? Is it safe to have a spare? I don't mind if it fails to auto-replace, but I do very much mind if it could corrupt our data!

 

[cyberjock]

No, I meant that someone needs to be physically there to reboot; the machine needs a manual selection of the USB drive at power up through BIOS.

Ouch. Why not set it permanently through the BIOS?

I'm back online now, and zpool status shows the spare as AVAILABLE.

But now you have me worried ... should I detach it? Is it safe to have a spare? I don't mind if it fails to auto-replace, but I do very much mind if it could corrupt our data!

That's something someone else would have to comment on. I just thought "SPARE=BAD" so I've been avoiding them until I hear otherwise.

 

[oxyris]

From http://doc.freenas.org/index.php/Volumes

Spare: will create a hot spare that is only used when another disk fails. Hot spares speed up healing in the face of hardware failures and are critical for high mean time to data loss (MTTDL) environments. One or two spares for a 40-disk pool is a commonly used configuration. Use this option with caution as there is a known bug in the current FreeBSD implementation. This will be fixed by zfsd which will be implemented once it is committed to FreeBSD.

I have a different question regarding using a spare drive. What is the disadvantage of using RAIDZ1 + 1 spare drive vs. using RAIDZ2 apart from being vulnerable during the rebuild of a failed drive? Also, wouldn't RAIDZ1 have better performance than RAIDZ2?

 

[cyberjock]

WIth RAIDZ2 you have double the failure protection of RAIDZ1.

Theoretically, if you have a disk fail you still have redundancy on RAIDZ2. With RAIDZ1 you have zero redundancy. During the scrub to bring the spare into the zpool if another disk fails and you lose everything. Any bad sectors can cause corrupted files.

But, if you use RAIDZ1 and a spare, the spare won't have the "wear and tear" from being used if it were in a RAIDZ2.

So it's a choice between the two possible scenarios. I take the stance that RAIDZ2 is better only because if you're having disks drop from old age in a RAIDZ1 you'll lose everything and with a RAIDZ2 you still have to lose 2 more disks before you lose data.

Personally, I never do RAIDZ1+1 spare. If I"m going to build a server and money is tight I build it as a RAIDZ2 and then a month or two later when I can afford it I buy the spare. That way you end up with RAIDZ2 + 1 spare without having to pay for the extra spare right now. If you go with RAIDZ1 + spare you can't make a RAIDZ2 later without recreating the zpool.