ZFS pools of consisting of several mirrors

[Rolf Nielsen]

Dan Nelson wrote:
> In the last episode (Dec 01), Rolf Nielsen said:
>> In experimenting a bit with ZFS, I, among other things, tried something
>> like this
>>
>> zpool create -R /test test mirror file[01]0 mirror file[01]1 mirror 
>> file[01]2 mirror file[01]3 mirror file[01]4 mirror file[01]5
>>
>> This, according to zpool status, gives me a (file backed) pool consisting
>> of six mirrors, each mirror consisting of two files.  Now for my question. 
>> Exactly how is the pool built?  Is it...
>>
>> 1. A RAID0 of the six mirrors?
>>
>> 2. A mirror of two RAID0 arrays, each array consisting of the six files 
>> file0[0-5] and file1[0-5] respectively?
>>
>> 3 and 4. Like 1 and 2 above, but with JBOD instead of RAID0?
>>
>> 5. Some other way I haven't thought about?
>>
>> I guess it's 1 or 3, as the zpool status output shows me six mirrors, 
>> but which is it? And, provided my guess is correct, is there a way to 
>> implement 2 or 4 without involving geom_stripe or geom_concat?
> 
> It's 1/3/5.  Each mirror is independant, and writes are balanced across the
> mirrors based on space usage.  If you add another mirror to grow the pool,
> it will get most of the writes until the usages balance out.
> 
> You usually don't want to build an array with options 2 or 4, since a single
> drive failure will degrade the entire mirror half.  Consider if you have 
> 
> concat00 -> file01 file02 file03 file04 file05
> concat01 -> file11 file12 file13 file14 file15
> mirror0 -> concat0 concat1
> 
> If file01 fails, concat00 fails, causing mirror0 to become degraded.  When
> you replace file01, mirror0 will have to resynch all of concat00 from
> concat01 since it doesn't know about the subdevices.  If you don't replace
> file01, and then file15 fails, you have lost your entire volume (unless you
> do some hackery to swap file05 and file15 to create a functioning concat01).
> 

Good point. Thanks for the reply.