[one-users] Setting the "SHAREABLE" attribute

[Stefan Kooman]

Quoting Gareth Bult (gareth at linux.co.uk):
> Mmm, there has been some discussion around this ..
> 
> Technically using cache=writeback "should" be safe for migration.
> (which is the method I use)
> 
> So .. I need "cache=writeback" and "sharable" for migration to happen.
> (the performance hit with cache=off is an unacceptable performance hit, at least for me ..)
> 
> From the QEMU lists, 2012;
> "In short, if you're using a recent kernel with ext3 or ext4, cache=writeback is absolutely safe. If you're using an older version of ext3, cache=writeback is still safe but ext3 itself isn't. cache=writeback can make the situation worse."

Hmm, the guys at big blue think otherwise [1]:

With caching set to writeback mode, both the host page cache and the
disk write cache are enabled for the guest. Because of this, the I/O
performance for applications running in the guest is good, but the data
is not protected in a power failure. As a result, this caching mode is
recommended only for temporary data where potential data loss is not a
concern.

Gr. Stefan

[1]:
http://pic.dhe.ibm.com/infocenter/lnxinfo/v3r0m0/index.jsp?topic=%2Fliaat%2Fliaatbpkvmasynchio.htm

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