[zfs-discuss] Is a scrub a data read-only or read-write operation?

Durval Menezes durval.menezes at gmail.com
Sun Jan 26 16:09:57 EST 2014


Hi Michael, 

On Sun, Jan 26, 2014 at 11:30 AM, Michael Kjörling <michael at kjorling.se> 
wrote:

> On 25 Jan 2014 22:55 -0200, from durval.menezes at gmail.com (Durval 
> Menezes):
> > On Jan 25, 2014 10:43 PM, "Cédric Lemarchand" <
> cedric.lemarchand at ixblue.com>
> > wrote:
> >> But yes, there are some writes too, I don't know what and why exactly.
> >
> > Wild guess: scrub progress/checkpoint counter?
>
> Yes; I think I covered those in my original question. :-)
>

Well, not explicitly, and I was responding to Cedric.

I suppose
>
>> the real question is what, if anything, _other than that sort of pool
>
>> metadata_ gets updated or rewritten during a scrub. Particularly, what
>
>> if anything _relating to the user-stored data_ gets updated or
> rewritten during a scrub?
>

Of course one would have to look at the source to be sure, but I would be 
very surprised if scrub writes anything but *its own* metadata (not even 
file metadata) to disk... what would be the point? 
 

> Obviously if an error is detected then depending on the level of
>
>> redundancy available that fact will be recorded and/or the data
> healed.
>
>> However, for the purposes of my question, let's play the naivity game
>
>> and for a moment assume a _perfect_ storage device, defined as one
>
>> that when asked to read a specific location never presents anything
>
>> but exactly what was last written to that location. Yes, that includes
>
>> the entire path from RAM to physical storage media and back, including
> cabling, drive firmware, etc.
>

Again, what would be the point of scrub writing anything but its own 
"metadata" (ie, aforementioned progress/checkpoint counter) to disk, except 
when it detects problems during the process? 

Cheers, 
-- 
   Durval.

 


> --
>
>> Michael Kjörling • http://michael.kjorling.semichael at kjorling.se
>
>> OpenPGP B501AC6429EF4514 http://michael.kjorling.se/public-keys/pgp
>
>>                 “People who think they know everything really annoy
>
>>                 those of us who know we don’t.” (Bjarne Stroustrup)
>
>>
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