zfs scrub enable by default

[Allan Jude]

On 2020-08-05 00:32, Ryan Moeller wrote:
> On Tue, Aug 4, 2020 at 11:36 PM PK1048.COM <info at pk1048.com> wrote:
>>
>> On Aug 4, 2020, at 8:18 PM, Grant Gray via freebsd-fs <freebsd-fs at freebsd.org> wrote:
>>
>>> 2. That scrubbing PREVENTS data loss. Scrubbing can only tell you about data loss AFTER it happens. Yes, it could alert you to a problem that prevents further data loss, but it's already too late. Scrubbing is not a substitute for RAID, backups and proactive SMART testing.
>>
>> Scrubbing does not directly prevent data loss. Scrubbing identifies data returned from the underlying device as corrupt or incorrect. If the zpool has sufficient redundancy, then no data is lost.
>>
>> But, normal ZFS operation also identifies incorrect date returned from the underlying device(s).
>>
>> While I have only managed a few hundred drives under ZFS, I cannot recall one case where a proactive scrub found bad data _before_ normal operations. Once a checksum error is reported via `zpool status` then I have found a scrub useful to determine the extent of the drive failure.
>>
>> I know there are others with thousands of drives managed under ZFS and their experience may differ.
>>
> 
>> In terms of whether the periodic scrub should be enabled by default, I am undecided, as I can see both sides of the argument. I would prefer to make it a user choice during installation as one of the ZFS options (such as native 4K drives, ashift=12 or mirrored swap). Make the default enabled, but by putting it in the installation options, those who know they need it disabled can make sure it is off from day one and not have a rude surprise the first time it runs (35 days after installation).
>>
> 
> I don't think it was mentioned on this list, but that seems to be the
> current direction this is heading. Providing an option in the
> installer, whether enabled by default or not, should address
> everyone's concerns.
> 

I don't think the installer is the place to define defaults though. So
I'd lean towards: on by default, but offer it as an option in the
installer, so they can choose to disable it.

-- 
Allan Jude

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