Re: mismatch_cnt questions - how about raid10?

[Peter Rabbitson <rabbit@rabbit.us>]

Neil Brown wrote:
> On Tuesday March 6, rabbit@rabbit.us wrote:
>> Neil Brown wrote:
>>> When we write to a raid1, the data is DMAed from memory out to each
>>> device independently, so if the memory changes between the two (or
>>> more) DMA operations, you will get inconsistency between the devices.
>> Does this apply to raid 10 devices too? And in case of LVM if swap is on 
>> top of a LV which is a part of a VG which has a single PV as the raid 
>> array - will this happen as well? Or will the LVM layer take the data 
>> once and distribute exact copies of it to the PVs (in this case just the 
>> raid) effectively giving the raid array invariable data?
> 
> Yes, it applies to raid10 too.
> 
> I don't know the details of the inner workings of LVM, but I doubt it
> will make a difference.  Copying the data in memory is just too costly
> to do if it can be avoided.  With LVM and raid1/10 it can be avoided
> with no significant cost.
> With raid4/5/6, not copying into the cache can cause data corruption.
> So we always copy.
> 

I see. So basically for those of us who want to run swap on raid 1 or 
10, and at the same time want to rely on mismatch_cnt for early problem 
detection, the only option is to create a separate md device just for 
the swap. Is this about right?