zfs discuss - Oct 2008 - Building a 2nd pool, can I do it in stages?

Hi, my present RaidZ pool is now almost full so I''ve recently bought a
Adaptec 3805 to start building a 2nd one, but since these are so expensive I
don''t have enough money left over to buy 8x1TB disks, but I can still
buy 3, and I have 5x 0.5TB disks lying around.

Is it possible to build a RaidZ with 3x 1TB disks and 5x 0.5TB disks, and then
swap out the 0.5 TB disks as time goes by? Is there a documentation/wiki on
doing this?


(I''m using NexentaCore and not OpenSolaris, but ZFS is still ZFS
isn''t it?)
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On Tue, 21 Oct 2008, H?vard Kr?ger wrote:
> Is it possible to build a RaidZ with 3x 1TB disks and 5x 0.5TB 
> disks, and then swap out the 0.5 TB disks as time goes by? Is there 
> a documentation/wiki on doing this?
Yes, you can build a raidz vdev with all of these drives but only 
0.5TB will be used from your 1TB drives.  Once you replace *all* of 
the 0.5TB drives with 1TB drives, then the full space of the 1TB 
drives will be used.

Depending on how likely it is that you will replace all of these old 
drives, you might consider using the new drives to add a second vdev 
to the pool so that the disk space on all the existing drives may be 
fully used and you obtain better mutiuser performance.

Bob
=====================================Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

Bob Friesenhahn schrieb:
> On Tue, 21 Oct 2008, H?vard Kr?ger wrote:
> 
>> Is it possible to build a RaidZ with 3x 1TB disks and 5x 0.5TB disks,
>> and then swap out the 0.5 TB disks as time goes by? Is there a
>> documentation/wiki on doing this?
> 
> Yes, you can build a raidz vdev with all of these drives but only 0.5TB
> will be used from your 1TB drives.  Once you replace *all* of the 0.5TB
> drives with 1TB drives, then the full space of the 1TB drives will be used.
> 
> Depending on how likely it is that you will replace all of these old
> drives, you might consider using the new drives to add a second vdev to
> the pool so that the disk space on all the existing drives may be fully
> used and you obtain better mutiuser performance.
> 
> Bob
But in this case one should be aware that if one adds another vdev, it
is currently impossible to get rid of it afterwards. I.e. the pool will
always have to RaidZ vdefs, and the new vdev which would consist in this
scenario of 3 1T disks couldn''t be grown by adding another disk. So one
would be forced to add another raid-z vdev.

IMO, I''d go for replacing the 0.5TB disks one by one and stick to a
single vdev.

- Thomas

On Wed, 22 Oct 2008, Thomas Maier-Komor wrote:
>
> But in this case one should be aware that if one adds another vdev, it
> is currently impossible to get rid of it afterwards. I.e. the pool will
> always have to RaidZ vdefs, and the new vdev which would consist in this
> scenario of 3 1T disks couldn''t be grown by adding another disk.
So one
> would be forced to add another raid-z vdev.
Yes, this is a good point.  Given sufficient drive slots and power, 
this quandary once again illustrates the value of simple mirroring, 
which wastes half the disk space but is quite flexible and fast.

If the user does not get around to replacing those 0.5TB disks then he 
might as well have started using mirror pairs since there is almost as 
much disk space wasted in the mean time and the mirror pairs would 
offer more performance.

Bob
=====================================Bob Friesenhahn
bfriesen at simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/

Hello there,
 It?s not a wiki, but has many considerations about your question:
 http://www.opensolaris.org/jive/thread.jspa?threadID=78841&tstart=60

 Leal.
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