Arisadmin,

Hi, I’m Joe with Dell EqualLogic. Although we support having members with mixed RAID policies in the same pool, in your case this is not advisable due to the two different drive types on your two members, i.e., your PS6000E is a SATA and your PS6000XV is a SAS. Mixing different drive types in the same pool, will most likely degrade performance in the group.

If the arrays were of the same drive type, i.e., both SATA (or both SAS), then combining the two (RAID 10 and RAID 6), would not be a problem, however the actual benefits, in your case, may not be as great as expected.

In order for load balancing to work “efficiently”, the array will analyze the disk I/O for several weeks (2-3) and determine if the patterns are sequential (RAID’s 10/5/6) or random (RAID 10), and would migrate those volumes to the corresponding member. However in a two member group this is often less efficient, since the array will try to balance across both member, and you may end up with 80% of the volume on one member and 20% on the other member instead of a 50/50 split.

We also support manually assigning a raid level to the volume, but this would in effect, eliminate the load balance that you are trying to achieve, since it is only a two member group.

So in summary, we don’t recommend combining different Drive types in the same pool.

You can go to http://www.delltechcenter.com/page/Guides and review the following documents for more information:
Deploying Pools and Tiered Storage in a PS Series SAN
PS Series Storage Arrays Choosing a member RAID Policy

Regards,
Joe

Hi!
We have a similar situation. We have one PS6000 (SAS RAID-50) and one new PS4000 (SATA).

We are using VMware ESX (only) together with EqualLogic and created 5 volumes (4x1TB and 1x500GB). On these volumes we store the vmdk-files.
I was planning to set the RAID-level for the PS4000 to RAID-5 and add it to the same storage pool as the PS6000. But after reading your answer, I'm confused...

I thought that EqualLogic load balancing was done at block-level? In our case, will it actually move the whole (1TB) volume to a single member (SAS or SATA) based on the performance benchmark? In that case, we still need to have control of what data we actually put in the different volumes to get the best performance.

Regards,
Anders Jansson

jansson81,

I’ll try to clear up any confusion, for the best practices when configuring a storage pool:

1. Do not mix arrays with different drive speeds within a single pool unless they are running a unique RAID policy

2. Do not mix arrays with different drive technologies (SATA, SAS, SSD) within a single pool unless they are running a unique RAID policy.

3. Do not mix arrays with different controller speeds (1GbE, 10GbE) within a single pool unless they are each running unique RAID policies.

To override the automated performance decisions for a specific volume, indicate a ―preferred
RAID type for that volume (can be done in the GUI or CLI). If that RAID type exists within the pool, the volume will reside on those arrays that match the preferred RAID policy for the volume.

For your example, you have two members with different drive technologies (Item #2), so you can create each member with a different RAID preference (as you indicated Array1 w/Raid50, Array2 w/RAID5), and then specify the preferred RAID policy for the volume(s), without any issues.

Regards,
Joe

Thanks for your fast reply Joe!

Can you please also confirm that a volume only exists on one member in a storage pool at a time? Or is it possible that diffrent data from same volume exist on diffrent members?

Quote from my previous post:
"In our case, will it actually move the whole (1TB) volume to a single member (SAS or SATA) based on the performance benchmark?"

Best Regards
Anders Jansson

To answer your question, it depends on a number of factors.

With volume RAID preferences set to Auto, the volume will span both members in the pool. If using the RAID preference, it isn’t a guarantee that it will adhere to the request and only place the volume on a given member; for instance if the volume is too large to fit on the member with the desired RAID level (you might end up with an 80/20 balance).

Note that when mixing SAS/SATA in the same pool the performance load balancer doesn’t take drive RPM into account in its calculations. A common issue is that if you combine fast SAS with slower SATA (often worst with the SATA setup as RAID 5) the SAS array won’t provide its potential performance, and the load balance may place more data on the larger SATA member (typically SATA Arrays have more space).

Regards,
Joe

Question, JoeSatDell:

Hate to resurrect an old topic, but I have a question.

The initial poster has a PS6000XV (SAS, RAID10) and PS6000E (SATA, RAID6). In your initial reply, you stated "Although we support having members with mixed RAID policies in the same pool, in your case this is not advisable due to the two different drive types on your two members, i.e., your PS6000E is a SATA and your PS6000XV is a SAS. Mixing different drive types in the same pool, will most likely degrade performance in the group."

Later, you listed the following criteria:
1. Do not mix arrays with different drive speeds within a single pool unless they are running a unique RAID policy
2. Do not mix arrays with different drive technologies (SATA, SAS, SSD) within a single pool unless they are running a unique RAID policy.
3. Do not mix arrays with different controller speeds (1GbE, 10GbE) within a single pool unless they are each running unique RAID policies.

Questions:
1. Arrays have different drive speeds and unique RAID policy. This criteria would be met in above scenario.
2. Arrays have different drive technologies (SATA, SAS) and unique RAID policy. This criteria would be met in above scenario.
3. N/A

If the PS6000E & PS6000XV each have a unique RAID policy, why the initial reply?

Thanks in advance :)

EQLfan,

No problem. The initial reply was based on the following:

If both members were added to the same pool, load balancing would try to balance the volumes across both members based on useable space. In this case, we can have both arrays in the same pool, since they are different drive technologies, using unique RAID policies. However, depending on the needs, such as, if SAS performance is not an issue, than using load balancing with both members in the same pool and setting the volume RAID preferences is an option (meaning when you combine drive technologies a percentage of the faster SAS disk performance might be sacrificed – depending on load, I/O’s etc. ). If SAS performance is important, than keeping both members in separate pools is the most likely option.

Joe

I can confirm that it is possible to mix SAS and SATA in the same pool with diffrent RAID-levels. I can also confirm that virtual machines running in the mixed pool (VMware) stopped working when we tried it (extremly slow and vMotion stopped workinig due to time-out). EqualLogic doesn't recommend mixing SAS and SATA in the same pool (but there might be cases when the customers wants to do this anyway).

Important info: If you want/need storage tiering (load balancing) and make use of it, you need at least three EqualLogic boxes in the same storage pool.

Best Regards

Now that firmware 5.1.1 is out with sub-volume tiering, do these same rules apply? I would rather mix my SAS and SATA drives into a single pool and let the software figure out what data blocks belong where. Otherwise I am just statically assigning volumes to one pool or another, and I may not make optimal placements, and I certainly won't be moving volumes as conditions change.

Moving discussion to this newer post created by khsieh:
http://www.delltechcenter.com/thread/4755028/Storage+tiering+with+SAS%2FSATA+drives+with+new+EqualLogic+5.1+firmware

Joe