[dm-devel] all cache blocks marked as dirty in writethrough mode, "Data loss may occur", constant write activity

[Joe Thornber]

On Mon, Jun 30, 2014 at 10:14:03AM +0100, Joe Thornber wrote:
> - There's a constant 4k background load to the metadata device.
> 
>   I'll look into this.  Sounds like the periodic commit is re writing
>   the superblock even if there's no change to the mappings.

I've added this test to the dm test suite:

https://github.com/jthornber/device-mapper-test-suite/blob/master/lib/dmtest/tests/cache/io_use_tests.rb

Using my latest development tree I can't see any spurious IO.

I also instrumented to see exactly when the superblock is written,
again nothing happens while the cache is idle.  So I'm puzzled about
what you are seeing.

- Joe