SSD Read Cache

[osamamansoor]

Hi Experts,

I have recently procured HP MSA 2062 and Created Two POOL A and POOL B.

I have 12 SAS Drives and 2 SSD Drives.

I used 2 SAS Drives as GLOBAL SPARE and 4 SAS Drives as POOL B and 6 SAS Drives as POOL A.

Now I want to use the Read Cache feature for SSD therefore I assigned 1 Drive in a POOL B as a READ CACHE and wanted to assign 1 more remaining SSD drives as reading Cache but Disk Group Button is disabled.

Can I assign in a similar manner to assign 1 SSD on one POOL and 1 SSD on the second POOL or I have to assign both in a single POOL.

[SUBHAJIT KHANBARMAN_1]

@osamamansoor 

You can create 1 SSD read cache per Pool

Please try below command from Command line,

Add a read-cache diskgroup to pool A. The resulting group will be named rcA01

# add disk-group type read-cache disks <drive_position> pool a

Hope this helps!
Regards
Subhajit

I am an HPE employee

If you feel this was helpful please click the KUDOS! thumb below!

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[sudhirsingh]

Hi @osamamansoor 

For your case, create read cache disk group from CLI., when a single SSD drive left in MSA add disk option will be grayed out.

Create the read-cache disk group with the following CLI command: 

add disk-group type read-cache disks <disk number> pool <letter>

You may find following information useful:

1. To increase existing Rad-cache disk group size you need to delete and recreate the Read-cache.
2. Only a single read-cache diskgroup may exist within a pool. 
3. Each read cache disk group consists of one or two SSDs with a maximum usable capacity of 4TB.
4. The read cache feature uses one SSD read cache disk group per pool as a read cache for “hot” pages only,
5.    When a read-cache group consists of one SSD, it automatically uses NRAID. When a read-cache group consists of two     SSDs, it automatically uses RAID0.
6. A virtual pool can not contain both read cache and a Performance tier.
7. Best practice: Choose read cache only when a workload is known to have a very low percentage of random writes,
8. SSD read cache accelerates random reads, and does not accelerate random writes or sequential I/O. If random writes are frequent, use performance tiering.

Hope this helps!

Regards,

Sudhir

[osamamansoor]

What do you mean by Performance tier?

Can we create TWO SSD for Performance Tier?

[sudhirsingh]

Hi,

Disk tiers are comprised of aggregating 1 or more Disk Groups of similar physical disks. The MSA 2060 supports 3 distinct tiers:

A performance tier with SSDs
A standard SAS tier with Enterprise SAS HDDs
An archive tier utilizing Midline SAS HDDs

Regards,

Sudhir

[SUBHAJIT KHANBARMAN_1]

@osamamansoor 

Yes you can create Virtual Disk group (VDG) with two SSD drives as Performance tier. Please note SSD read cache and Performance tier can't exists together in same Pool.

Another important thing to remember, SSD read cache only help with random read operation but Performance tier helps with both read/write operation.

I would suggest to go through Quickspec and Best Practice technical paper to gain more knowledge on this product,

https://h20195.www2.hpe.com/v2/getdocument.aspx?docname=a00094630enw#

https://psnow.ext.hpe.com/doc/a00105260enw?jumpid=in_lit-psnow-red

Hope this helps!
Regards
Subhajit

I am an HPE employee

If you feel this was helpful please click the KUDOS! thumb below!

*************************************************************************

[ArunKKR]

Hi Osama,

I believe that Sudhir and Subhajit has shared all the required information.

Please let me know whether you have any further queries.

[Cali]

I would recommend you to only use only one pool anyway.
This is MSA2060 Best Practice and in your configuration, give more Performance.
There is also a much better gross/net ratio.
Cali

[osamamansoor]

Thank you so much for your quick responses and apologies for not a quick reply.

@Cali As i have a total two SSD so i use one with each pool as i have two pools or did you mean to say that two SSD with one POOL ?

Can someone suggest how can i observe the performance of READ Cache is there any chart or statistics to observe i put the load on my POOL?

[osamamansoor]

What HP recommends for SSD with READ CACHE RAID N or RAID 0 ?

[Cali · Accepted solution]

Since it is a Read Cache, there is no need for Redundancy.

When using as read cache, SSDs should account for 10-20% of the fastest HDD tier, so ideally the Standard tier.

Each read-cache disk group consists of one or two SSDs with a maximum usable capacity of 4TB.
When a read-cache group consists of one SSD, it automatically uses NRAID.
When a read-cache group consists of two SSDs, it automatically uses RAID0.

Cali

[JonPaul]

I think Cali's recommendation got missed before.  The recommended configuration would include ALL drives in a single Pool. With the number of drives you have in your configuration, the drives will be the performance limiter.  Therefore, a single SAS disk group of RAID 6, 10 drive + 2 spares,  or a 12 Drive MSA-DP+ disk group and a RAID 1 SSD tier would likely be your simpliest configuration with the best performance + you would have the ability to expand the MSA-DP+ disk group incrementally if you needed to in the future.