Am 29.06.2011 15:53, schrieb Frediano Ziglio: > 2011/6/29 Anthony Liguori <anth...@codemonkey.ws>: >> On 06/29/2011 07:16 AM, Kevin Wolf wrote: >>> >>> Am 29.06.2011 14:06, schrieb Anthony Liguori: >>>> >>>> On 06/29/2011 06:59 AM, Kevin Wolf wrote: >>>>> >>>>> Hi, >>>>> >>>>> I think we have touched this topic before during some IRC discussions or >>>>> somewhere deep in a mailing list thread, but I think it hasn't been >>>>> discussed on the list. >>>>> >>>>> Our default cache mode of cache=writethrough is extremely conservative >>>>> and provides absolute safety at the cost of performance, >>>> >>>> But for the most part, we track bare metal fairly well in terms of block >>>> performance, no? >>>> >>>> Or are you really referring to qcow2 as a specific example? In the >>>> past, we used a different default caching mode for qcow2. I think that >>>> could be done again if there was a compelling reason. >>> >>> No, people are also complaining about bad performance with raw. Which >>> isn't really surprising when you do a flush after each single write >>> request. O_SYNC is really much more than is needed in the average case. >> >> Which file system on the host? >> >> At any rate, I'm a big fan of making wce tunable in the guest and then I >> think setting wce=1 is quite reasonable to do by default. >> >> Regards, >> >> Anthony Liguori >> >>> >>> Kevin >> >> >> > > Personally I think default lead to poor performance but currently > people know that even critical application are handled correctly. > Assuming a client connect to a database server on a qemu guest when > server reply "transaction ok" you know that even a host crash lead to > a successful transaction. At least the fact that this assumption won't > be true has to be stated.
If the guest is acting correctly, then this assumption is definitely true with cache=none/writeback. What needs to happen is that the guest issues a disk flush before sending your "transaction ok" message. If it doesn't do that it can fail even on real hardware. cache=writethrough makes things safe that aren't even safe on real hardware, that's why I consider it extremely conservative. Kevin
