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On 5/16/2014 9:18 AM, Nicolas Haller wrote:
> On 16/05/2014 09:36, Karl Denninger wrote:
>>
>> On 5/16/2014 8:23 AM, Nicolas Haller wrote:
>>> Hello,
>>>
>>> I've got a new server and I installed FreeBSD 10 on it. I have a
>>> problem to create a new zfs pool. The command stalls on IO wait (D
>>> state / zio->io_cv).
>>>
>>> The device for the pool is a 1.7T partition (index 4) of a gmirror
>>> device.
>> Why?
>>
>> ZFS provides its own mirroring and it is superior, as it checksums eac=
h
>> block and thus does not rely on the drive returning an error to detect=
>> problems. It can also rewrite a bad block (assuming the problem is
>> transient) and scrub also relies on independent components.
>>
>> You're destroying the data integrity advantage that ZFS gives you by
>> using a gmirror under it. Stop doing that and see if your problem
>> disappears.
>>
>> (In other words it sounds like the problem is real but you shouldn't b=
e
>> doing that anyway, so it also shouldn't bite you.)
>>
>
> Yes you're right but I have two disks on this server which host my=20
> non-zfs root fs. The first 98G partition you show on "gpart show"=20
> output is my UFS root fs and I want it mirrored.
>
> As I'm not sure it's a good idea to mix gmirror and ZFS mirror on the=20
> same drives (What do you think about this?), I let gmirror handle the=20
> mirror thing.
There are ways to do that (e.g. gmirror a partition or slice; yes, that=20
works) but I wouldn't for several reasons (complexity and the risk of=20
making a mistake in the future that winds up screwing you.)
> So, yes, I drop the data integrity advantage, but I keep the=20
> flexibility given by pools/datasets, send/receive, compression...
> It's not so bad :-)
>
Ah, ok, I see what you're doing.... I opted to switch my root partition=20
to zfs and boot from a mirrored ZFS volume. If I'm going to run ZFS=20
*anyway* then I figure I may as well boot from it.
There is a potential risk in doing that however -- if something goes=20
wrong with the zfs code you're in deep kimchee. This is most-likely if=20
your emergency boot media is incompatible with a feature flag in the=20
on-disk data set. You have to be CAREFUL doing this to avoid getting=20
nailed; UFS boot is arguably "safer" in that regard, I suppose in that=20
if push comes to shove the individual disks in a gmirror can be mounted=20
since the metadata is confined to the tail end of the drive.
--=20
-- Karl
karl@denninger.net
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