On 2015-12-02 08:45, Duncan wrote:
> Austin S Hemmelgarn posted on Wed, 02 Dec 2015 07:25:13 -0500 as
> excerpted:
>
>> On 2015-12-02 05:01, Duncan wrote:
>
> [on unverified errors returned by scrub]
>>>
>>> Unverified errors are, I believe[1], errors where a metadata block
>>> holding checksums itself has an error, so the blocks its checksums in
>>> turn covered are not checksum-verified.
>>>
>>> What that means in practice is that once the first metadata block error
>>> has been corrected in a first scrub run, a second scrub run can now
>>> check the blocks that were recorded as unverified errors in the first
>>> run, potentially finding and hopefully fixing additional errors[.]
>
>>> ---
>>> [1] I'm not a dev and am not absolutely sure of the technical accuracy
>>> of this description, but from an admin's viewpoint it seems to be
>>> correct at least in practice, based on the fact that further scrubs as
>>> long as there were unverified errors often did find additional errors,
>>> while once the unverified count dropped to zero and the last read
>>> errors were corrected, further scrubs turned up no further errors.
>>>
>> AFAICT from reading the code, that is a correct assessment.  It would be
>> kind of nice though if there was some way to tell scrub to recheck up to
>> X many times if there are unverified errors...
>
> Yes.  For me as explained it wasn't that big a deal as another scrub was
> another minute or less, but definitely on terabyte-scale filesystems on
> spinning rust, where scrubs take hours, having scrub be able to
> automatically track just the corrected errors along with their
> unverifieds, and rescan just those, should only take a matter of a few
> minutes more, while a full rescan of /everything/ would take the same
> number of hours yet again... and again if there's a third scan required,
> etc.
>
> I'd say just make it automatic on corrected metadata errors as I can't
> think of a reason people wouldn't want it, given the time it would save
> over rerunning a full scrub over and over again, but making it an option
> would be fine with me too.
>
I was thinking an option to do a full re-scrub, but having an automatic 
reparse of the metadata in a fixed metadata block would be a lot more 
efficient that what I was thinking :)