[dm-devel] Desynchronizing dm-raid1

[malahal at us.ibm.com]

Mikulas Patocka [mpatocka at redhat.com] wrote:
> Ideas how to fix it:
>
> 1. lock the buffers and unmap the pages while they are being written.
> --- upstream developers would likely reject it. No other driver than 
> dm-raid1 has problems with this and they wouldn't damp performance because 
> of one driver.

Very few drivers require it, so how about an interface to lock the pages
of an I/O available to drivers. Only needed RAID drivers would lock the
I/O while it is in progress and they only pay the performance penalty.
mmap pages are a bit tricky. They need to go into read-only mode when an
I/O is in progress. I know this would likely be rejected too!!!

> 4. make more region states.
> --- If the region is in RH_DIRTY state and all writes drain, the state is 
> changed to RH_MAYBE_DIRTY. (we don't know if the region is synchronized or 
> not). The disk dirty flag is kept.
> --- periodically (once in few minutes, so that it doesn't affect 
> performance much), the change all regions in RH_MAYBE_DIRTY state to 
> RH_CLEAN_CANDIDATE, then issue sync() on all filesystems. If, after the 
> sync(), the region is still in RH_CLEAN_CANDIDATE (i.e. it hasn't been 
> written during the sync()), it is moved to RH_CLEAN state and the on-disk 
> bit for the region is turned off.

Sounds good except that it uses sync()! Is there a way to sync only
pages related to a certain block device? How hard it is to implement
such an interface?

--Malahal.