Vdev in a Vdev?

[RRRRRRR123456789]

Hi all.

I had a 4 x 4tb WD Red set up.
However, one drive now needs to be RMA-ed. I only have 2 x 2tb drives on standby.... Is it possible to set it to a 4tb vdev and use as a drive in the Raidz i currently have? Thanks!

Edit. Running 9.3

 

[nojohnny101]

I'm afraid I can't understand what you want to do.

As per forum rules, please post your drill hardware setup along with FreeNAS version.

Hope you didn't setup up an Raidz1 or you have sufficient backups.

 

[Dice]

Is it possible to set it to a 4tb vdev and use as a drive in the Raidz i currently have? Thanks!

No.
That would cause you to add a striped vdev into your existing pool. That would be detrimental to redundancy as you suddenly will be susceptible to a drive failure of any of those 2 drives, would cause the entire pool to fail.
Do not go this route, unless you have enough space to <recreate> your pool at a later stage, and have <ALL> of the data backed up.

 

[Dice]

For reference: Check my signature, there is a link to cyberjocks newbie guide. It will explicitly explain what you can and cannot do for pool expansions without violating redundancy.

 

[RRRRRRR123456789]

I'm afraid I can't understand what you want to do.

As per forum rules, please post your drill hardware setup along with FreeNAS version.

Hope you didn't setup up an Raidz1 or you have sufficient backups.

Could you explain why not raid 1?

I was under the impression I could lose one drive.

 

[Arwen]

No, you can not use 2 x 2TB disks as a replacement for 1 x 4TB disk.

There are tricks experts, (including myself), can perform. However, none are for
the faint of heart or people who have to ask how. (Meaning if you have to ask, it's
too complicated to walk someone through remotely.)

As for the RAID-Z1 issue, if you loose another disk, you loose your pool. Further,
if you have a bad block on the existing disks during your disk re-silver, you will
likely loose some data.

 

[RRRRRRR123456789]

No.
That would cause you to add a striped vdev into your existing pool. That would be detrimental to redundancy as you suddenly will be susceptible to a drive failure of any of those 2 drives, would cause the entire pool to fail.
Do not go this route, unless you have enough space to <recreate> your pool at a later stage, and have <ALL> of the data backed up.

I thought that if I add a virtual 4tb (made of 2 x 2tb) to my raid5 set up, I can either lose 1 actual 4tb or both 2tb and still be ok.

See below for what I mean

4tb 4tb 4tb (2x2tb)

In a 4tb raid5 set up

 

[RRRRRRR123456789]

No, you can not use 2 x 2TB disks as a replacement for 1 x 4TB disk.

There are tricks experts, (including myself), can perform. However, none are for
the faint of heart or people who have to ask how. (Meaning if you have to ask, it's
too complicated to walk someone through remotely.)

As for the RAID-Z1 issue, if you loose another disk, you loose your pool. Further,
if you have a bad block on the existing disks during your disk re-silver, you will
likely loose some data.

Oh okay. Mmm I guess I'll have to buy a 4tb drive now then. Thanks!

 

[danb35]

I thought that if I add a virtual 4tb (made of 2 x 2tb) to my raid5 set up, I can either lose 1 actual 4tb or either 2tb and still be ok.

With the bolded change, this statement is correct. Yes, it's theoretically possible to do what you're suggesting--ZFS will let you create a bunch of crazy configurations. Fortunately, FreeNAS prevents you from most of them, though you can still do it at the command line if you're persistent and know how. But as @Arwen says, if you have to ask how, you probably shouldn't be doing it. And, to emphasize my change, loss of either of the 2 TB disks would result in failure of that "virtual 4tb"

And just for clarity, if FreeNAS is managing your RAID (as it should be), you don't have RAID 5; you have RAIDZ1. Though superficially similar, there are a number of significant differences.

 

[Vito Reiter]

While EXTREMELY UNRECOMMENDED and having the potential to lose ALL your data, you could put those 2 disks in a MoBo or other hardware Raid0 let FreeNAS see it as a 4TB drive and replace as usual. It's a trick and really dangerous but to some logic at least there would be something holding the data in the meantime and if any 1 drive in the Raid0 failed you'd just lose the equivalent of losing 1 drive in your RaidZ1. Seriously, unrecommended.

 

[danb35]

While EXTREMELY UNRECOMMENDED and having the potential to lose ALL your data, you could put those 2 disks in a MoBo or other hardware Raid0 let FreeNAS see it as a 4TB drive and replace as usual.

Yuck. I'd favor the ZFS solution; at least that way the system can track disk health and such. But I guess your suggestion doesn't involve the CLI-fu, so that's a point in its favor. But neither is a "good" idea.

 

[Stux]

Far as I know, no way to vdev in a vdev.

What's the ZFS solution? Make a pool then make a vdev from a file on that pool?

 

[danb35]

What's the ZFS solution?

To be honest, I don't know--I'd thought there would be a way (and a less kludge-y way than making a pool, then a file on that pool, then adding that file into the other pool), and I have very vague memories of having seen it done, but nothing clear enough to say how it was done. Perhaps the Princess can clarify further, though her Elvish-ZFS might work differently...

 

[Stux]

On Linux I think you'd use mdadm, is there a FreeBSD equivalent?

 

[Arwen]

I guess there are 3 highly un-recommended ways to create a 4TB LUN from 2 x 2TB disks;

1. Hardware / semi-hardware RAID-0
2. New ZFS pool striping the 2 x 2TB disks into 1, 4TB pool. Create a sparse file or ZVol in it. May have to tweek the settings to get enogh space.
3. Use the FreeBSD software RAID tools to create a stripe of the 2 x 2TB disks.

All quite clumsy and prone to single disk failure taking out the LUN exposed to the destination Zpool.

That said, I have done tricks like that temporarily to migrate data around. I once migrated an entire server's file systems from one set of disks to another set, live without rebooting.

Actually number 3 is not too bad, though I don't know the syntax. If I recall correctly, Corral was using mirrored swap using the FreeBSD RAID tools so that a loss of a disk, (and it's swap), did not cause server crashes.

 

[RRRRRRR123456789]

With the bolded change, this statement is correct. Yes, it's theoretically possible to do what you're suggesting--ZFS will let you create a bunch of crazy configurations. Fortunately, FreeNAS prevents you from most of them, though you can still do it at the command line if you're persistent and know how. But as @Arwen says, if you have to ask how, you probably shouldn't be doing it. And, to emphasize my change, loss of either of the 2 TB disks would result in failure of that "virtual 4tb"

And just for clarity, if FreeNAS is managing your RAID (as it should be), you don't have RAID 5; you have RAIDZ1. Though superficially similar, there are a number of significant differences.

Apologies but could you explain why it's either 2tb and not both 2tb?

Note : I am not doing this but asking for educational purposes

Would it still be a raidz? So the remaining 3 4tb will still have sufficient information to rebuild.

 

[danb35]

Apologies but could you explain why it's either 2tb and not both 2tb?

The two 2 TB disks are going to have to be in some kind of striped configuration (i.e., RAID 0 or equivalent). In RAID0, failure of any one disk kills the array. That's true in ZFS stripes as well as in traditional RAID 0. But yes, your pool would still be RAIDZ.

 

[Vito Reiter]

Apologies but could you explain why it's either 2tb and not both 2tb?

Note : I am not doing this but asking for educational purposes

Would it still be a raidz? So the remaining 3 4tb will still have sufficient information to rebuild.

Here is the best explanation on how you would go about this, personally I recommend Next-Day shipping on a new 4TB but your data is your responsibility and you already know the risks here. The only way to 'Replace' that missing drive using your 2x2TB is to make FreeNAS see those 2x2TB as a single 4TB drive using a stripe or RAID0, either through hardware RAID or software RAID through CLI not attached to your volume. I'm no Van Gough but here is my rough sketch of what the set-up would be

If any one of the 2TB drives fails, both of them will, however your main array will only see it as a single disk loss so it's a little safer than a normal RAID0? That is the best way to explain the setup, I still wouldn't do it. I like to call it RaidZ1.5, good luck and seriously just next-day a 4TB.