ZFS: Parallel I/O to vdevs that appear to be separate physical disks but really are partitions

[Adam McDougall]

On 10/23/2010 4:58 AM, Pawel Jakub Dawidek wrote:
> On Fri, Oct 22, 2010 at 03:52:36PM -0700, Eugene M. Kim wrote:
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>> Greetings,
>>
>> I run a FreeBSD guest in VMware ESXi with a 10GB zpool.  Lately the
>> originally provisioned 10GB proved insufficient, and I would like to
>> provision another 10GB virtual disk and add it to the zpool as a
>> top-level vdev.
>>
>> The original and new virtual disks are from the same physical pool (a
>> RAID-5 array), but appears to be separate physical disks to the
>> FreeBSD guest (da0 and da1).  I am afraid that the ZFS would schedule
>> I/O to the two virtual disks in parallel thinking that the pool
>> performance will improve, while the performance would actually suffer
>> due to seeking back and forth between two regions of one physical pool.
>>
>> 1. Will ZFS schedule parallel access to different top-level vdevs?
> Yes, ZFS stripes requests over all top level vdevs.
>
>> 2. If so, how can I suggest ZFS that certain vdevs should be treated
>> not as separate physical disks but as partitions on the same physical
>> disk?
> Nope, that's not possible.
>
> Instead of adding next 10GB disk maybe it is possible to grow the one
> you have? ZFS should be able to grow automatically.
>
What about creating a new 20gb "disk" then using zpool replace, export 
and import it to see the new size, and then retiring the old 10gb "disk"?