[zfs-discuss] Is a scrub a data read-only or read-write operation?
durval.menezes at gmail.com
Sun Jan 26 16:09:57 EST 2014
On Sun, Jan 26, 2014 at 11:30 AM, Michael Kjörling <michael at kjorling.se>
> On 25 Jan 2014 22:55 -0200, from durval.menezes at gmail.com (Durval
> > 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.
>> 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
>> 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?
>> Michael Kjörling • http://michael.kjorling.se • michael 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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