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Re: [zfs-discuss] Size discrepancy (beyond expected amount?)

Subject: Re: [zfs-discuss] Size discrepancy beyond expected amount?
From: Harry Putnam
Date: Fri, 20 Mar 2009 10:42:19 -0500
Bob Friesenhahn <bfriesen@xxxxxxxxxxxxxxxxxxx> writes:

>>> With five disks, raidz1 becomes useful.
>>
>> The three 500gb I have now are all one brand and model number and IDE ata.
>> If I were to expand to 5, those 2 would need to be sata or else I'd
>> also need to add a PCI IDE controller.
>>
>> With that in mind would it be problematic to make up the 5 by adding 2
>> sata 500gb to the mix?
>
> Adding a few more drives so that you have at least five drives would
> get you to the point where raidz1 is useful and you will have more
> space available.

So is 5 with 1 hotswap (total 6) a sensible arrangement? And would
that leave me with something like 2tb (minus manufacturer exaggeration)
and one disk would be swallowed for parity data.

Also, I'm not getting why a 4 disk raidz arrangement would not be
practicle. I see now why 3 isn't smart, with no spc advantage over 2
disk mirror.  But wouldn't a 4 disk raidz1 of 500gb drives give about
1500 gb for space where as a mirror setup with 4 500gb would only give
1000 gb. (again minus manufacturer bragging).

Or is the needed hotswap for the raidz1 the deal breaker?  Meaning it
would really take 5 disks to get 1500gb of space in a raidz1 of all
500gb disks.

>  . . . . . .  At the moment there is no way to expand the size of
> a raidz vdev other than by replacing all of the disks with larger
> disks.  You can't add additional drives to a vdev.  So you need to
> install the drives from the very beginning.

So far, I'm still at the point where I can just destroy whatever I've
created and start over, since no data has been written to the zpool so
far.

But what about mixing the 3 IDE ata I have currently with 2 sata (all
500gb)
(I have no more IDE controllers, available  but do have 2 sata ports
available).

Is mixing IDE and SATA likely to be a problem?

And another thing... if all controllers are full and used in a raidz
is it still possible to add a hotswap?  Would I pull out one of the
raidz drives to do it or what?

I understand the hotswap disk would not have to remain in the machine
but still, all available controllers would be used for the 5 disk
raidz so something would have to come out to be able to add the
hotswap disk.

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