Re: ZFS on labelled partitions (was: Re: LSI SAS2008 mps driver preferred firmware version)

[Freddie Cash]

On Tue, Nov 17, 2015 at 12:08 AM, Patrick M. Hausen  wrote:

> Hi, all,
>
> > Am 16.11.2015 um 22:19 schrieb Freddie Cash :
> >
> > ​You label the disks as they are added to the system the first time.
> That
> > way, you always know where each disk is located, and you only deal with
> the
> > labels.
>
> we do the same for obvious reasons. But I always wonder about the possible
> downsides, because ZFS documentation explicitly states:
>
> ZFS operates on raw devices, so it is possible to create a storage
> pool comprised of logical
> volumes, either software or hardware. This configuration is not
> recommended, as ZFS works
> best when it uses raw physical devices. Using logical volumes
> might sacrifice performance,
> reliability, or both, and should be avoided.
>
> (from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)
>
> Can anyone shed some lght on why not using raw devices might sacrifice
> performance or reliability? Or is this just outdated folklore?
>

​On Solaris, using raw devices allows ZFS to enable the caches on the disks
themselves, while using any kind of partitioning on the disk forces the
caches to be disabled.

This is not an issue on FreeBSD due to the way GEOM works.  Caches on disks
are enabled regardless of how the disk is accessed (raw, dd-partitioned,
MBR-partitioned, GPT-partitioned, gnop, geli, whatever).

This is a common misconception and FAQ with ZFS on FreeBSD and one reason
to not take any Sun/Oracle documentation at face value, as it doesn't
always apply to FreeBSD.

There were several posts from pjd@ about this back in the 7.x days when ZFS
was first imported to FreeBSD.

-- 
Freddie Cash
fjwc...@gmail.com
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ZFS on labelled partitions (was: Re: LSI SAS2008 mps driver preferred firmware version)

[Patrick M. Hausen]

Hi, all,

> Am 16.11.2015 um 22:19 schrieb Freddie Cash :
> 
> ​You label the disks as they are added to the system the first time.  That
> way, you always know where each disk is located, and you only deal with the
> labels.

we do the same for obvious reasons. But I always wonder about the possible
downsides, because ZFS documentation explicitly states:

ZFS operates on raw devices, so it is possible to create a storage pool 
comprised of logical
volumes, either software or hardware. This configuration is not 
recommended, as ZFS works
best when it uses raw physical devices. Using logical volumes might 
sacrifice performance,
reliability, or both, and should be avoided.

(from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)

Can anyone shed some lght on why not using raw devices might sacrifice
performance or reliability? Or is this just outdated folklore?

Thanks,
Patrick
--
punkt.de GmbH * Kaiserallee 13a * 76133 Karlsruhe
Tel. 0721 9109 0 * Fax 0721 9109 100
i...@punkt.de   http://www.punkt.de
Gf: Jürgen Egeling  AG Mannheim 108285



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Re: ZFS on labelled partitions (was: Re: LSI SAS2008 mps driver preferred firmware version)

[krad]

From what i remember its a  control thing. If you have another layer below
zfs, be it software based or hardware based, zfs cant be sure what is going
on, therefore cant guarantee anything. This is quite a big thing when it
comes to data integrity which is a big reason to use zfs. I remember having
to be very careful with some external caching arrays and making sure that
they flushed correctly as often they ignore the scsi flush commands. This
is one reason why I would always use the IT based firmware rather then the
RAID one, as its less likely to lead to issues.

On 17 November 2015 at 08:08, Patrick M. Hausen  wrote:

> Hi, all,
>
> > Am 16.11.2015 um 22:19 schrieb Freddie Cash :
> >
> > ​You label the disks as they are added to the system the first time.
> That
> > way, you always know where each disk is located, and you only deal with
> the
> > labels.
>
> we do the same for obvious reasons. But I always wonder about the possible
> downsides, because ZFS documentation explicitly states:
>
> ZFS operates on raw devices, so it is possible to create a storage
> pool comprised of logical
> volumes, either software or hardware. This configuration is not
> recommended, as ZFS works
> best when it uses raw physical devices. Using logical volumes
> might sacrifice performance,
> reliability, or both, and should be avoided.
>
> (from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)
>
> Can anyone shed some lght on why not using raw devices might sacrifice
> performance or reliability? Or is this just outdated folklore?
>
> Thanks,
> Patrick
> --
> punkt.de GmbH * Kaiserallee 13a * 76133 Karlsruhe
> Tel. 0721 9109 0 * Fax 0721 9109 100
> i...@punkt.de   http://www.punkt.de
> Gf: Jürgen Egeling  AG Mannheim 108285
>
>
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Re: ZFS on labelled partitions

[Miroslav Lachman]

Patrick M. Hausen wrote on 11/17/2015 09:08:

Hi, all,


Am 16.11.2015 um 22:19 schrieb Freddie Cash :

​You label the disks as they are added to the system the first time.  That
way, you always know where each disk is located, and you only deal with the
labels.


we do the same for obvious reasons. But I always wonder about the possible
downsides, because ZFS documentation explicitly states:

ZFS operates on raw devices, so it is possible to create a storage pool 
comprised of logical
volumes, either software or hardware. This configuration is not 
recommended, as ZFS works
best when it uses raw physical devices. Using logical volumes might 
sacrifice performance,
reliability, or both, and should be avoided.

(from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)

Can anyone shed some lght on why not using raw devices might sacrifice
performance or reliability? Or is this just outdated folklore?


It was on Solaris but not on FreeBSD. If you were using partitions on 
Solaris the drive cache was disabled (or something like that, I am not 
100% sure)


Miroslav Lachman

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Re: ZFS on labelled partitions

[krad]

It was a control thing again, if you were using a partition another
application could be using the drive on another partition, therefore zfs
couldn't guarantee exclusive use of the disk so had to be more careful in
the way it operated the drive. I think this meant I went into write through
mode like you say.

On 17 November 2015 at 08:22, Miroslav Lachman <000.f...@quip.cz> wrote:

> Patrick M. Hausen wrote on 11/17/2015 09:08:
>
>> Hi, all,
>>
>> Am 16.11.2015 um 22:19 schrieb Freddie Cash :
>>>
>>> ​You label the disks as they are added to the system the first time.
>>> That
>>> way, you always know where each disk is located, and you only deal with
>>> the
>>> labels.
>>>
>>
>> we do the same for obvious reasons. But I always wonder about the possible
>> downsides, because ZFS documentation explicitly states:
>>
>> ZFS operates on raw devices, so it is possible to create a
>> storage pool comprised of logical
>> volumes, either software or hardware. This configuration is not
>> recommended, as ZFS works
>> best when it uses raw physical devices. Using logical volumes
>> might sacrifice performance,
>> reliability, or both, and should be avoided.
>>
>> (from http://docs.oracle.com/cd/E19253-01/819-5461/gbcik/index.html)
>>
>> Can anyone shed some lght on why not using raw devices might sacrifice
>> performance or reliability? Or is this just outdated folklore?
>>
>
> It was on Solaris but not on FreeBSD. If you were using partitions on
> Solaris the drive cache was disabled (or something like that, I am not 100%
> sure)
>
> Miroslav Lachman
>
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