[dm-devel] Desynchronizing dm-raid1

[malahal at us.ibm.com]

Martin K. Petersen [mkp at mkp.net] wrote:
> >>>>> "Malahal" == malahal  <malahal at us.ibm.com> writes:
> 
> [I sent this last week but it never made it to the list]
> 
> Malahal> Very few drivers require it, so how about an interface to
> Malahal> lock the pages of an I/O available to drivers. Only needed
> Malahal> RAID drivers would lock the I/O while it is in progress and
> Malahal> they only pay the performance penalty.  mmap pages are a bit
> Malahal> tricky. They need to go into read-only mode when an I/O is in
> Malahal> progress. I know this would likely be rejected too!!!
> 
> I have exactly the same problem with the data integrity stuff I'm
> working on.
> 
> Essentially a checksum gets generated when a bio is submitted, and
> both the I/O controller and the disk drive verify the checksum.
> 
> With ext2 in particular I often experience that the page (usually
> containing directory metadata) has been modified before the controller
> does the DMA.  And the I/O will then be rejected by the controller or
> drive because the checksum doesn't match the data.

Your problem is very similar to an iSCSI problem sumitted here:
http://now.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=137902

Fortunately, you can detect the problem and the I/O can be retried if
possible. In the RAID case, it goes undetected until you hit the
eventual corruption!