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Date: Tue, 26 Aug 2014 09:03:18 +0200
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Subject: Re: [Qemu-devel] Using cache=writeback safely on qemu 1.4.0 and
	later
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To: Andrew Martin <amartin@xes-inc.com>, Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu-devel <qemu-devel@nongnu.org>

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