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Adaptive Flash Cache problems on 7400 with 3.2.2 (MU6)

 
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rmay_bk
Frequent Advisor

Adaptive Flash Cache problems on 7400 with 3.2.2 (MU6)

Recently turned this feature on for the first time.  Now seeing two errors; the first is from Infosite: "High slab space usage detected"

The second I'm seeing in Events & Alerts: "Flash Cache I/O times are slow on node: 3"

Any suggestions?

 

2 REPLIES 2
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rmay_bk
Frequent Advisor

Re: Adaptive Flash Cache problems on 7400 with 3.2.2 (MU6)

As a follow-up, I've looked at some performance reports in SSMC.  Looking at "user time", node 3 has been busier than the others for days/weeks.  However, when we enabled Flash Cache yesterday around 10AM the "system time" metric for node 3 spiked above the other nodes as well.

Looking at Physical Drives Compare by Performance, I see that these SSDs became very busy overnight, with write latency spiking to 14ms during one stat sample.  All 16 SSD drives chart identically.

Adaptive Flash Cache Performance shows all four nodes charting pretty much identically.  The used percentage peaked at 400% roughly 20 hours after enablement but it's now drifting downward for some reason and is currently at 340%.

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Re: Adaptive Flash Cache problems on 7400 with 3.2.2 (MU6)

I understand you have mentioned Write latency 14ms which can be due to different reason.

AFC related to read operation only.

A write will continue to be serviced in exactly the same way as  it should be even with an AFC (Advanced Flash Cache) implementation as it is only read data that can be read from AFC. The AFC is used by writes only to invalidate data not for retrieval.

When a read request is received DRAM is still used as the primary cache and is checked first, next the AFC is checked and if the data is present on the SSD’s a cache hit is registered and the data does not need to be serviced from spinning disk.

It's very difficult to comment without knowing your setup.

HP have built in a simulation mode which doesn’t even require any SSD’s present in the system. Simulation mode allows you to look at your cache stats and see if AFC would be beneficial to your system.

Flushing data from the SSD’s that has been placed there by AFC occurs through a LRU (Least recently used) algorithm. When data arrives in the AFC it is admitted at normal temperature it will be promoted to hot when data is accessed frequently and marked cold as it eventually ages and will then be subject to eviction from flash cache. So to summarise what we are seeing here is essentially a tiered cache system, DRAM is used as primary cache, then destages to AFC which then further destages to spinning disk as data becomes cold. The take home benefit from all of this is a larger cache providing improved response times for random read workloads.

You can go through the below white paper to get more details,

https://h20195.www2.hpe.com/v2/getpdf.aspx/4AA5-5397ENW.pdf

 

Hope this helps!
Regards
Subhajit

I am an HPE employee

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