[pve-devel] Default cache mode for VM hard drives

Eric Blevins ericlb100 at gmail.com
Fri May 29 20:37:53 CEST 2015


Suggestion for another test case.

Create a linux VM that has tons of swap and just enough RAM to boot.
Run a single threaded program that allocates and modifies more RAM
than the VM has ensuring many of the modifications are to swap.
This should result in out of sync blocks in the swap area.

Test case inspired by Linbit:
http://lists.linbit.com/pipermail/drbd-user/2008-May/009282.html

We already know that the swap code has similar behavior. In case
a page gets touched while it is under write out IO, the swap allows
the modification to the page although it is under IO by the block
layer. Therefore the swap code does not know which version actually
got written to disk, but it does not care, since it knows that it
has the up to date version in core memory.





On Fri, May 29, 2015 at 9:25 AM, Dietmar Maurer <dietmar at proxmox.com> wrote:
>> >> 1) On any guest OS (Linux or Windows) where swap is used actively
>
> So maybe the bug can be triggered using mmap? Unfortunately, I have no real idea
> how mmap work internally, but I can imagine that it produces a similar pattern?
> Any mmap expert here?
>
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