HPE Smart Array P816i-a SR Smart-Cache not able to change Ratio

[Wayne]

Hi

We use HPE Smart Array P816i-a SR on Gen10 DL 380 server and added a 400GB SSD disk for SmartCache. The problem is, everytime we create the smart cache, we are not able to change the Cache-Ration from 0/100 to 50/50 what we prefer. In the current Smart Staorage Administrator 3.47.6.0 there is no option to change the Ratio. In the details we get the current settings of the Ration and it always shows 0% Read / 100% Write, but where the hell can we change the Ratio?

Thanks

[KM26]

Hi Wayne,

With regard the query raised, we found the following information to assist you.

The following features are not available unless SmartCache is DISABLED:
• Expand Array
• Advanced Capacity Expansion
• Move Logical Drive
• Change Array Drive Type
• Mirror Splitting and Recombining (offline only)
• Split Mirror Backup and Rollback (online and offline)
• Heal Array
• Extend Logical Drive
• Migrate RAID/Strip Size
• Change Cache Ratio
• Align Logical Drives

You can find more about SmartCache and its features in the Smart storage administrators user guide below. 
HPE Smart Storage Administrator
User Guide
https://support.hpe.com/hpsc/doc/public/display?docId=c03909334

[Wayne]

Thank you, read this notification hundred times but never realized the cache ratio part.

It's just no logical for me, if we wanna use a 400GB SSD disk as cache, why we are not allowed to set the ratio of the cache and it's just always using the full amount for writing? For our backup repository which we use to send the files to the tape once a month we would prefer to have 80% read and 20% write.

So there is no way to control the usage of SmartCache, it's just always 100% write?

[KM26]

Hi Wayne,

The SmartCache is functioning as per design.

Please refer to the extracts given below to help you understand this further.

HPE SmartCache uses one or more SSDs as dedicated caching devices for other volumes—increasing storage performance by copying the most frequently accessed data to low latency SSD(s) for quicker access that is completely transparent to host applications.

Originally, HPE SmartCache only supported write-through cache capabilities. This allowed it to deliver improved performance for read operations to Smart Array volumes, but not for write operations. Write-intensive application such as virtualization did not see significant performance gains with SmartCache.

For Gen9, HPE SmartCache has added write-back cache capabilities as well. Write-back caching allows Smart Array volumes to first store writes in the low latency SSD cache, and then write the data from the cache to the Smart Array volume at a later point in time. With write-back caching, HPE SmartCache can now improve the performance of applications with heavy write workloads.

Please refer to page 9 in the article given below for further details regardimg SmartCache.

https://h20195.www2.hpe.com/v2/GetDocument.aspx?docname=4AA5-4526ENW&doctype=Business%20white%20paper&doclang=EN_US&searchquery=&cc=us&lc=en

[Wayne]

Thnx KM26

So if I understand correct, it means we would get better results if we disable SmartCache for this volume and just use the 4GB controller cache and set the Ratio to 100% read 0% write? Because we have the much better read performance that way compared with the SmartCache enabled 400GB SSD disk, doesn't matter if write-back or pass-through mode.

[KM26]

Hi Wayne,

The Smartcache would have better read or write performance based on the type of SSDs used. 

The document shared previosuly explains the functionality of SmartCache.

It may or may not provide better performance than controller cache.

This can only be determined by testing scenarios.