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I have a very basic storage setup in my workstation PC. At the moment 1 SSD + 1 HDD set up in a tiered Storage Space. I'm on Windows 10 Pro so I've been doing this all via Powershell manually.

I was expecting Windows to take a bit of time to recognise the "hot data" and move it to the SSD. But at the moment it seems to be very conservative. At the moment an optimisation run gives me this:

PS C:\WINDOWS\system32> defrag D: /g /h /#
>>
Microsoft Drive Optimizer
Copyright (c) Microsoft Corp.

Invoking tier optimization on Tiered Storage (D:)...


The operation completed successfully.

Post Defragmentation Report:

        Volume Information:
                Volume size                 = 4.05 TB
                Free space                  = 2.99 TB

        Storage Tier Optimization Report:

                % I/Os Serviced from Perf Tier  Perf Tier Size Required
                100%                            9.70 GB *
                95%                             8.94 GB
                90%                             8.18 GB
                85%                             7.42 GB
                80%                             6.65 GB
                75%                             5.89 GB
                70%                             5.13 GB
                65%                             4.37 GB
                60%                             3.61 GB
                55%                             2.85 GB
                50%                             2.09 GB
                45%                             1.68 GB
                40%                             1.30 GB
                35%                             942.96 MB
                30%                             577.80 MB
                25%                             376.39 MB
                20%                             249.18 MB
                15%                             163.62 MB
                10%                             88.75 MB
                5%                              30.78 MB

        * Current size of the Performance tier: 459.98 GB
          Percent of total I/Os serviced from the Performance tier: 7%

        Size of files pinned to the Performance tier: 12.00 KB
        Percent of total I/Os: 0%

        Size of files pinned to the Capacity tier: 0 bytes
        Percent of total I/Os: 0%

What I don't understand is the difference between "% I/Os Serviced from Perf Tier" and "Percent of total I/Os serviced from the Performance tier". The later is really low so it seems strange that Windows would move more to the SSD to increase this number.

I have been running the defrag quite often to see if this number changes. Perhaps the frequency of optimisation can also have an effect? I haven't been able to find any information about this.

How come Windows isn't moving more to the SSD tier? Why is the "Percent of total I/Os serviced from the Performance tier" different and so much lower than "% I/Os Serviced from Perf Tier"?

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"% I/Os Serviced from Perf Tier" – the capacity of PerfTier which is required to benefit from autotiering based on your past workload. For example, you need only 9.7GB Perf Tier

"Percent of total I/Os serviced from the Performance tier" – I think it is self-explaining. Only 7% was served by PerfTier.

More explanation here - https://docs.microsoft.com/en-us/previous-versions/windows/it-pro/windows-server-2012-r2-and-2012/dn789160(v=ws.11)

You can benefit from Autotiering only for writes (depending on OS and file system) and reads of frequently accessed data.

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  • But the "% I/Os Serviced from Perf Tier" are a percentage also. So why don't the two percentages align? Is it that the former is a prediction based on past usage and the data that is being moved whereas the latter is an actual historical calculation? In which case that would imply my workload is changing significantly which I wouldn't expect. Primocache manages to get to 50% hit rates pretty quickly (within a day). I'm not using Primocache anymore BTW. – Sam Pattuzzi May 18 at 8:10
  • "% I/Os Serviced from Perf Tier" shows required capacity for percentage SSD tier utilization. So, if you would have only 2.85GB of SSD tier, than only 55% of 100% possible I/O are “boosted”. The prediction is based on past usage (cannot tell about what timeframe) According to this metrics, adding more as 10GB SSD tier capacity shouldn’t result in any performance improvement. "Percent of total I/Os serviced from the Performance tier” shows actual percentage of IOPS, which benefits from autotiering. In your case only 7% have been predicted. The details, how it works in provided link. – batistuta09 May 18 at 11:27
  • That doesn't make sense to me. If it knows that only 7% would be serviced from the PerfTier then why wouldn't it use more of the SSD space available (I have a 500GB drive). So 100% is far from the 9.70GB in use so wouldn't seem to align with your explanation. Furthermore, if I manually pin files then the "Percent of total I/Os serviced from the Performance tier" increases. Why isn't Storage Spaces make more use of this space? – Sam Pattuzzi May 19 at 20:58

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