Tiered Storage Space writing to slow tier (Server2019)

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Cutha

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Sep 24, 2016
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I setup a tiered storage space with 45x 10TB drives and 3x NVMe 1.6TB SS's. The writes were slow and looking into it a bit it seems the writes all land on the slow tier instead of the fast tier.

I have tried with ReFS and NTFS and the results are the same.

Any suggestions?
 

Cutha

Member
Sep 24, 2016
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I figured out my problem. The Performance tier (SSD) is smaller than the free space threshold that triggers the writes to land directly on the HDD's. I'm reasonably sure it's configured as a % of the total volume. When I created the volume with a capacity tier of 600GB instead of 300TB the writes go the fast tier.

I found this registry key which may have an impact on this but it's a decimal so the highest I can go is 99% which would be about 3TB in my case.

HKLM:\SYSTEM\CurrentControlSet\Policies
Type: DWORD
Name: DataDestageSsdFillRatioThreshold

Does anyone know how to modify the threshold of when SS decides to bypass the fast tier (SSD) and land writes directly to the capacity tier (HDD) instead?
 

Cutha

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Sep 24, 2016
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As far as I can figure, Storage Spaces will not use the fast tier (SSD) if the fast tier is below a particular percentage (~3.2%) of the total size of the virtual drive.

This is a PITA because 1TB of a fast tier with a large dual parity capacity tier still speeds up large writes nicely.

The ReFS performance counters show that what SS considers for fast tier size and fill % is adjusted based on the capacity tier, at least to some degree. As I shrunk the capacity tier the fast tier % full more closely matched the size of the fast tier.

I have scoured the interwebs and found many people with issues getting tiering working and I now think most of the problems are from the fast tier being below some threshold value that is not configurable.

I adjusted the registry key above, rebooted and re-created a VHD with the same results. The key does work as far as adjusting when the fast tier destages but it does not have any impact on this problem with where writes land.

Does anybody know if there is another registry key that can be added to control this threshold?

SSD TierHDD TierData on VHDReFS reported Fast Tier Data Fill %
1TB30TB3.76GB2%
1TB31TB3.76GB8%
1TB32TB3.76GBN/A*
1TB10TB60.0GB8%
1TB10TB400.0GB51%
*All writes go directly to slow tier even when drive has zero data on it.

I don't think NTFS worked even when I shrunk the capacity tier.
 

burdell

New Member
Jun 30, 2020
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Did you find any solution to this problem? I was considering building an SSD tier for storage spaces but I may skip it now if there is a minimum size it has to be in order to function.
 

Cutha

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Sep 24, 2016
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Did you find any solution to this problem? I was considering building an SSD tier for storage spaces but I may skip it now if there is a minimum size it has to be in order to function.
Sadly no. I should have tried a Multi-Resilient Volume with the HDD's making up both tiers. I have done it before and it worked ok.
 

Net-Runner

Member
Feb 25, 2016
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I've had similar problem on NTFS and couldn't make it writing to hot tier. The only way it worked, when I reformatted the volume with ReFS.