gmirror, gpart and MBR vs GPT in the Handbook

[Warren Block]

On Tue, 3 Dec 2013, Julien Cigar wrote:

> On Mon, Dec 02, 2013 at 08:56:13AM -0700, Warren Block wrote:
>> On Mon, 2 Dec 2013, Julien Cigar wrote:
>>
>>> If you want to use GPT with gmirror you may want to mirror each
>>> partition instead of the whole disk.
>>>
>>> For example on my box I have the following: https://dpaste.de/Rb3S
>>
>> There are a couple of potential problems with that.  The big one is when
>> a disk fails and is replaced.  If you're not careful, the rebuild of all
>> those mirrored partitions will start at the same time.  Head contention
>> will bring that to a near-standstill.  It also puts a heavy load on the
>> drive that still works.  Hopefully it is not the same model and age as
>> the one that failed, or its "warranty timer" may also be close to
>> expiring.
>
> That's true, I turned off autosynchronization of stale components to
> avoid this kind of bad scenario ..
>
>>
>> A less-serious problem is that only the partitions are mirrored.  That
>> leaves out metadata like the partition tables and bootcode, but those
>> typically do not change very often and might not be a problem.  The
>> admin has to remember to manually install such things on a new
>> replacement disk, though.
>
> It's just a matter of gpart backup / gpart restore, right ?

That will get the partition tables but not the bootcode or PMBR.  A 
failed disk could then leave a mirror which has all the data but is 
unbootable.

At some point, we'll have to address the conflict between gmirror and 
GPT.  ZFS mirrors can replace gmirror in some situations, but not all.