Re: Doesn't "writes" do what resync does ?

[Stan Hoeppner <stan@hardwarefreak.com>]

On 12/30/2013 2:36 AM, Pieter De Wit wrote:
> On 30/12/2013 20:25, Stan Hoeppner wrote:
>> On 12/29/2013 5:16 PM, Pieter De Wit wrote:
>>> <snip>
>>> Should that resync not have had more completed ?
>> Your question is invalid.  What you meant to ask is
>>
>> "Why are pvdisplay and mdstat reporting what seems to be conflicting
>> state data?"
>>
>> Did you also ask on the lvm list why pvdisplay says most PEs are
>> consumed, yet mdstat says resync is only 17% complete?
>>
> Hi again Stan,
> 
> pvdisplay says most PEs are consumed because I moved that data to the
> device. My question, rephased then:
> 
> Shouldn't writes to a RAID device count as resyncs ?

The resync process is independent of normal IO.  It starts at the
beginning and soldiers on to the end doing a read of each sector pair
then comparing them (for RAID1).  So no, writes don't count as resync
operations.  md doesn't perform write/read/verify in normal operation,
only write.  Linux relies on hardware to report write errors, and
assumes the data hit the disk intact if no error.  There is no mechanism
to pass a new write as verified to the resync process.  If you think it
should you may want to shoot the idea past Neil.  Though I'm sure he's
already considered that and rejected it for various reasons.

And unless you restart the resync, it won't verify any sectors you wrote
up to its current position.  Any sectors you wrote after that point it
will be verifying.  You don't need to restart the resync or do another
one.  I'm simply explaining how the resync is independent of normal
write IO.

-- 
Stan