[Xen-users] qemu disk cache mode

[Stefan Berder <stefan.berder@xxxxxxxxxxxxxxxxx>]

Hi all,
I can't find any good talk about this subject and would like some insights and advices on the cache side in xen.
I discovered that a domO power outage can lead to a severe filesystem corruption of the domUs. The domO is a dual disk dell server with a PERC controler in writethrough cache mode, the disk cache is disabled, the scheduler in the domO/domU is NOOP, the domO is holding an LVM in which LVs are created to be used as physical disks (phy:) in the domU. Using xm destroy to shut the domU is ok and the filesystem doesn't crash. Pulling the power plug from the domO leads to severe damage on the domU ext3 filesystem. It is easily reproductible and always leads to fs corruption (therefore database inconsistencies). Did some test with the same domU converted in PV, the corruption does not appear but the write performance is really low.

Going through various search results on google, I discovered that qemu is doing some writeback caching by default. It seems that xen is not supporting this option in the configuration files therefore I can't find any good way to disable this behavior when running a domU. I can't find a good explanation of the internal of a domU creation (maybe I'm bad at searching for this one) and is struggling with the python code to make a cache=[none|writeback|writethrough] option but I'm not even sure it is necessary/useful.

Is there a good way to avoid the domU to cache?

Stefan
-- 
Stefan Berder | Operations Department | ChinaNetCloud
X2 Space 1-601, 1238 Xietu Lu, Shanghai 200032, China | 上海市斜土路1238号1-601室
Stefan.Berder@xxxxxxxxxxxxxxxxx | Phone: +86(21) 6422-1946 | Mobile: (+86)15800622253


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