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Okay so I am facing a strange situation with a server build I've completed. Prior to getting a Tyan S7012, sticking it in a case with some RAM, I didn't notice any strange disk behaviour. But now, delving into the world of host bus adapters and all those good things, there are things I'm noticing on disk transfer speeds.

When using a .7z file (of around 90GB odd), it goes at 150MB/s. Then decides to drop and go to 98MB/s. This is transferring from a WD 4TB RED drive to a WD 3TB RED drive. However, I've seen this issue happen with a multitude of drive combinations (WD Green, Hitachi Deskstar).

I'm not sure why this would happen - the HBA's are Dell PERC H310's in IT mode so no RAID functionality or anything like that to interfere with things on that front.

Here is a screenshot of a pattern created when a transfer involves a 2TB WD Green disk

And here is the screenshot when transferring from the 4TB RED to 3TB RED

So, in terms of what's actually going on - errrrm? Starts off all well and good in both cases - reaching 150MB/s on the REDs and 115MB/s on the Green's but after a certain period, it drops off to a rate that seems a little bit slow.

Any ideas?

FoolishlyWise
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At first, it's limited by the read speed only. The write speed doesn't matter because data can be cached in RAM. Eventually, the system hits the limit of dirty pages it will allow to reside in RAM and the read operation will be throttled until more dirty pages are flushed to disk. So, most likely, 150MB/s is your read device limit and 100MB/s is your write device limit.

David Schwartz
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  • This makes sense. Is there any way to turn off the cache in Windows Server? Selecting the properties tab on device manager and deselecting write cache enabled isn't doing the trick. – FoolishlyWise Jan 21 '17 at 00:49
  • @FoolishlyWise No, there isn't. It's not really a cache, it's the operating system's core paging mechanics (how block devices interact with RAM). The write cache that you can disable is the cache on the drive itself. – David Schwartz Jan 21 '17 at 03:46