VM drive cache mode and performance

Hello everybody,

Could anybody try to explain why cache=directsync and cache=none (which is write-back with host cache disabled) give completely same results? Shouldn't it be better on writing with cache=none?

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Hardware:
- Dell PE 710
- PERC H700 Integrated
- RAID 0+1
- Current Cache Policy: WriteBack, ReadAdaptive, Direct, No Write Cache if Bad BBU
- BBU is in place

Proxmox:
- Proxmox VE 2.3 (is under some daily load during the tests)

Virtual machine:
- Windows 2008 R2 SP1
- 3 drives (virto HDD, 10GB raw image on ext3, i.e. on local storage)
-- drive E: cache=directsync
-- drive I: cache=none
-- drive F: cache=writethrough
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Regards,
Stanislav