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Disk Response Time

IOMeter is sending a stream of requests to read and write 512-byte data blocks with a request queue depth of 1 for 10 minutes. The disk subsystem processes over 60 thousand requests, so the resulting response time doesn’t depend on the amount of cache memory.

The four-disk arrays are almost equal to the single drive in terms of read response time, but the eight-disk arrays are considerably worse. The controller either has to spend more time to “recall” what address is stored on what disk or just lacks performance to process so many requests. Anyway, the small lag is obvious.

It’s logical at writing: the larger the total cache of the array, the smaller the response time. The dependence is almost directly proportional. And of course, it doesn’t work with the checksum-based arrays because they can’t just put data into the disks’ cache. These arrays suffered the most from the controller’s disabled cache and have a write response of 20 milliseconds and more. Typically, the write response of such arrays is comparable their read response, but not four to five times worse.

Take note that the four-disk RAID5 and RAID6 are similar in this test whereas the eight-disk RAID6 is considerably slower. The low performance of the controller’s processor shows up again.

 
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