On 2018-06-29 11:15, james harvey wrote:
On Thu, Jun 28, 2018 at 6:27 PM, Chris Murphy <li...@colorremedies.com> wrote:
And an open question I have about scrub is weather it only ever is
checking csums, meaning nodatacow files are never scrubbed, or if the
copies are at least compared to each other?
Scrub never looks at nodatacow files. It does not compare the copies
to each other.
Qu submitted a patch to make check compare the copies:
https://patchwork.kernel.org/patch/10434509/
This hasn't been added to btrfs-progs git yet.
IMO, I think the offline check should look at nodatacow copies like
this, but I still think this also needs to be added to scrub. In the
patch thread, I discuss my reasons why. In brief: online scanning;
this goes along with user's expectation of scrub ensuring mirrored
data integrity; and recommendations to setup scrub on periodic basis
to me means it's the place to put it.
That said, it can't sanely fix things if there is a mismatch. At least,
not unless BTRFS gets proper generational tracking to handle temporarily
missing devices. As of right now, sanely fixing things requires
significant manual intervention, as you have to bypass the device read
selection algorithm to be able to look at the state of the individual
copies so that you can pick one to use and forcibly rewrite the whole
file by hand.
A while back, Anand Jain posted some patches that would let you select a
particular device to direct all reads to via a mount option, but I don't
think they ever got merged. That would have made manual recovery in
cases like this exponentially easier (mount read-only with one device
selected, copy the file out somewhere, remount read-only with the other
device, drop caches, copy the file out again, compare and reconcile the
two copies, then remount the volume writable and write out the repaired
file).
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