Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and later

[Paolo Bonzini]

Il 25/08/2014 20:13, Andrew Martin ha scritto:
> Even if the python daemon or apache2 did not fsync the modified files, isn't 
> there some action that the OS takes periodically to flush dirty pages to 
> disk? 
> This seems to be implied in the SuSE documentation:
> https://www.suse.com/documentation/sles11/book_kvm/data/sect1_1_chapter_book_kvm.html
> "the normal page cache management will handle commitment to the storage 
> device."

... using fsync :)

> In the case of the files uploaded by apache2, they were added to the server 
> days 
> before  the power outage, so it seems like there would have been ample time 
> for 
> those changes to have been flushed.

Remember that cache=writeback has a huge cache, whose size is several
gigabytes.

At some point there was a QEMU bug that caused the guest not to do
fsyncs.  It was fixed by commit ef5bc96268.

Paolo