Re: fdisk(8) vs gpart(8), and gnop

----- Original Message ----- 
From: "Nathan Whitehorn" <nwhitehorn@freebsd.org>
To: "Steven Hartland" <killing@multiplay.co.uk>; <freebsd-hackers@freebsd.org>; <freebsd-fs@freebsd.org>
Sent: Sunday, June 01, 2014 5:07 PM
Subject: Re: fdisk(8) vs gpart(8), and gnop

> On 06/01/14 09:00, Steven Hartland wrote:
>>
>> ----- Original Message ----- From: "Nathan Whitehorn" 
>> <nwhitehorn@freebsd.org>
>> To: <freebsd-hackers@freebsd.org>; <freebsd-fs@freebsd.org>
>> Sent: Sunday, June 01, 2014 4:55 PM
>> Subject: Re: fdisk(8) vs gpart(8), and gnop
>>
>>
>>> On 06/01/14 08:52, Steven Hartland wrote:
>>>> ----- Original Message ----- From: "Mark Felder" <feld@freebsd.org>
>>>>
>>>>> On May 31, 2014, at 20:57, Freddie Cash <fjwcash@gmail.com> wrote:
>>>>>
>>>>>> There's a sysctl where you can set the minimum ashift for zfs. 
>>>>>> Then you
>>>>>> never need to use gnop.
>>>>>>
>>>>>> I believe it's part of 10.0?
>>>>>
>>>>> I've not seen this yet. What we need is to port the ability to set 
>>>>> ashift at pool creation time:
>>>>>
>>>>> $ zpool create -o ashift=12 tank mirror disk1 disk2 mirror disk3 disk4
>>>>>
>>>>> I believe the Linux zfs port has this functionality now, but we 
>>>>> still do not.
>>>>
>>>> We don't have that direct option yet but you can achieve the
>>>> same thing by setting: vfs.zfs.min_auto_ashift=12
>>>>
>>> Does anyone have any objections to me changing this default, right 
>>> now, today?
>>> -Nathan
>>
>> I think you will get some objections to that, as it can have quite an 
>> impact
>> on the performance for disks which are 512, due to the increased 
>> overhead of
>> transfering 4k when only 512 is really required. This has a more dramatic
>> impact on RAIDZx due too.
>>
>> Personally we run a custom kernel on our machines which has just this 
>> change
>> in it to ensure capability with future disks, so I can confirm it does 
>> indeed
>> have the desired effect :)
> 
> So the discussion here is related to what to do about the installer. The 
> current ZFS component unconditionally creates gnops all over the place 
> to set ashift to 4k. That's across the board worse: it has exactly the 
> performance impact of changing the default of this sysctl (whatever that 
> is), it can't easily be overridden (which the sysctl can), and it's a 
> horrible hack to boot. There are a few options:
> 
> 1. Change the default of vfs.zfs.min_auto_ashift
> 2. Have the same effect but in a vastly worse way by adjusting the 
> installer to create gnops
> 3. Have ZFS choose by itself and decide to do that permanently.
> 
> Our ATA code is good about reporting block sizes now, so (3) isn't a big 
> issue except for the mixed-pool case, which is a huge PITA.
> 
> We need to choose one of these. I favor (1).

I wasn't aware of that but it should do #3

min_auto_ashift is a bigger discussion.

    Regards
    Steve
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