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A question of RAID levels

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I have a SAN. Said SAN has RAID5 volumes. To create disk volumes on a Solaris host, I use RAIDZ1 to strap these together. For those not quite up to speed: RAIDZ1 is basically RAID5 for ZFS.

 

What happens when a disk in the controller fails?

 

zebra: you aren't allowed to buzz in yet.

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Ohhhher, I get what you're saying now.

 

Also I'm jealous as shit that you're running a SAN. I also may be wrong, I'm trying to remember a conversation I had with a Snr SAN Engineer about a year ago, but the underlying layer should have a stupid amount of redundancey.

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If the disk fails you lose the disk and any redundancy.

But the data and the array will still be there.

 

 

Unless you're fucking with us....

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If the disk fails you lose the disk and any redundancy.

But the data and the array will still be there.

 

 

Unless you're fucking with us....

He's fucking with you. There is more to it than meets the eye. Think about it like this. It's a RAIDZ1 atop a RAID5. What happens when something goes wrong.

 

z

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When a disk in an raid5 array of your raidz volume fails then your Raidz volume functions in degraded mode. The entire raid5 setup with the dud disk will drop to degraded mode but wont be used anyway since raidz has decided not to use the dead 'disk' and is running in degraded mode. You can now replace the physical disk and wait a stupid amount of time for the raid 5 array to rebuild itself at which point zfs heal will kick in and resilver the 'disk' with any updates since the time the disk failed.

 

Well thats what I think.

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It'd die in the arse I would think, the RAIDZ1 wouldn't know what was going on.

 

But why you'd put RAID on RAID, unless you are doing some weird porno thing, is beyond me.

Wouldn't you just give Solaris the disks and let ZFS do it's magic on it?

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So you wouldn't use RAID10? That's RAID on RAID.

 

But yes... the point is, both levels of RAID would be simultaneously trying to rebuild. And that... sounds horrible.

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So you wouldn't use RAID10? That's RAID on RAID.

I would, but that is not a RAID5 on top of a RAID5, which was what I was getting at.

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I'm using 'disk' to describe a raid5 array and disk to describe a physical disk.

 

I would think that zfs would drop the 'disk' that is connected to the raid5 array when it drops to degraded mode from a disk fail. ZFS will drop disks when it does not think they are behaving correctly. I thought (but now not so sure) that zfs would drop this 'disk' until it has been fixed by a raid5 rebuild (however a degraded mode 'disk' may not give zfs reason to drop it). Then a ZFS heal would resilver the 'disk'.

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forgive me if i'm wrong but don't the 3+ disks in a raid 5 array all get seen as a single drive? to have a raid 5 array ontop of a raid 5 array wouldn't you need minimum 9 drives? Like how with a raid 1+0 you need 4 drives?

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yeah. for real this would be with 9 drives minimum. but since you are going to pack raidz at the top you want to build it with as many raid 5 arrays as possible so you don't loose too much capacity. If we went with a theoretical unit of 9 drives of 1tb size then you would have 3*1tb in raid 5 = 2tb and 3* 2tb drives in raidz =4tb. not exactly a good use of space. Raidz3 would be a better use in this instance, despite still being overkill on 9 drives

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