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Write Flush Policy Tests
The new write flush policy tests were developed in response to a pattern of problem reports regarding application responsiveness. In these on-going reports, people experience their UI intermittently hesitiating, glitching or even hanging. From the customer perspective, not only is this very frustrating, it is also unclear what the root cause is and how to fix it.
The goals for these new tests are to identify storage device behaviors that can cause real-world responsiveness problems, and provide tests that help identify problematic components, leading to improved components and improved customer experiences.
After investigation, the root cause of this set of problem reports was found to be storage devices with latency issues (long IOs). Changing the device or updating the device’s firmware led to a significantly improved user experience.
By playing back user traces associated with this category of problem report across a range of drives, Microsoft was able to identify IO patterns associated with the latency issue. The WinSAT write flush policy tests were then built as a small set of mixed read/write tests which could run quickly and identify devices with those latency issues.
Both the WinSAT write flush policy tests and the user problem traces have been re-run across a range of hardware. There is a good correlation – the set of disks flagged by the write flush policy tests corresponds well to the set flagged by the problem traces. However, these tests are still being refined – during the Release Candidate (RC) milestone, Microsoft will run the tests across an even wider range of hardware, and will update them to maximize the accuracy of the tests.
Although Vista contained an early version of the write flush policy test, it was not included in WinSAT/WinEI scoring as it was preliminary, not yet well-defined enough to crisply identify the problem. For Win7, Microsoft has a much better understanding of the underlying issues, and so have been able to build more accurate tests. Therefore, the Windows 7 write flush policy tests are now included in WinSAT/WinEI scoring.
It’s worth noting that these latency issues are not limited to old or slow devices – even devices with higher throughputs and RPMs can have long IOs that lead to transitory hangs.
Assessing Enhanced Storage Systems
WinSAT can measure the performance the NVRAM portion of Enhanced Storage Systems (ESS). The challenge is how to map the size and performance of the NVRAM to a WinEI score. Microsoft do not yet have good data as to how size and throughput factor into the user experience.
In Windows 7, the plan is that hybrid drives will be scored using only their mechanical performance characteristics.
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