HPE MSA 2050 - Read Cache Full (No Free Space)

[LunarOperation]

Good day Expert,

I have an HPE MSA 2050 configured like this:

Disk Group A1 - Volume A has 15 Disks (SAS) in RAID 6 with a Total Space of 31 TB (Capacity Tier)

Disk Group A2 - Volume A has 2 Disks (SSD) in RAID 0 for Read Cache with Total Space of 1596 GB

We have created different volumes on Disk Group A1 that host most of our VMs on vSphere and the IOPS is simply crazy and super slow.

When looking at the Performance, I do see that the SSD Read Cache always has a Free Space of 0 MB, all space consumed, rarely I can see like 60 MB free.

I should mention that the 2 x SSD only have 800 GB each and I am thinking that the SSD Read Cache is small for the current workload which may be causing this VM slowness in our environment. Is my assumption correct?

[support_s]

System recommended content:

1. HPE MSA 1050 2050 2052 1060 2060 and 2062 Storage - Hosts May Not Have Access to MSA Volumes When Updating to VMware ESXi 7.0 Update 2, has been released

2. HPE MSA 1050/2050 CLI Reference Guide

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[Sahana_S]

Hi @LunarOperation ,

If you have configured a read cache, this is only related to read performance. If you try to write in this array, you may not see any significant improvement. Read cache does not add to the overall capacity of the pool to which it has been added.

You can add or remove it from the pool without any adverse effect on the volumes and their data for the pool, other than to impact the read-access performance.

You can refer to the below document which explains about performance: page no 136

https://support.hpe.com/hpesc/public/docDisplay?docId=a00017707en_us

[LunarOperation]

@ Sahana_S

Thank you for the update. But I am concerned about the fact that the Read Cache may be smaller for the overtall Capacity Tier?

What is a good SSD Read Cache size for a Capacity Tier of 31 TB? Is 1.5 TB good enough as a Read Cache for such big capacity tier?

This is where I am kind of confused and doubting at the moment.

What is a good practice about sizing for the SSD Read Cache Tier?

[JonPaul]

@LunarOperation 
You can find some information on capacity sizing in the MSA Gen5 Virtual Storage Technical Guide:  https://www.hpe.com/psnow/doc/a00087404enw
The READ CACHE being completely full is a good thing and is also good when it comes to having an SSD performance tier.  It means that as much data as possible is in the fastest tier possible.
One performance component to look at is the I/O Workload graph.  This will give a 'general' view of how 'wide' your daily data set, the data you actively connect to each day.  
If you are looking to improve performance then removing the READ-CACHE and adding a SSD performance tier would help.  Note that a license is required for the performance tier and with the current lifecycle stage of the MSA 2050 it would be good to determine if you need the performance tier (Advanced Data Services license) sooner than later.