Enable Write Cache on a CX without SPS Battery

CX300 = Not Possible but just do add for future reference/searches, EMC do have DC Powered CLARiiON CX4 arrays which do not use a standby power supply.

Ankit

I've never worked with the pure DC CLARiiONs Ankit, but this seems curious to me. Do you know why the DC only versions wouldn't need SPSes? In my mind the SPS would still be required. Without the SPS if the array loses power coming into it there is no way to dump the cache and shut down "gracefully". The SPS provides the temporary power required to do that as well as the sense and signal to shutdown if the power into the SPS fails?

Maybe I'm missing something on the architecture of these arrays...?

Good Question! Rather than I convey my understanding I reckon attached Whitepaper (13 Page Only) will give you clear and better picture about Concept, Design and Configuration.

Thanks Ankit. I read through this doc, but it doesn't answer the question of how one of these arrays would deal with a power outage. I don't see how they could handle a power outage and still destage the write cache on this array.

Allen Ward wrote: I've never worked with the pure DC CLARiiONs Ankit, but this seems curious to me. Do you know why the DC only versions wouldn't need SPSes? In my mind the SPS would still be required. Without the SPS if the array loses power coming into it there is no way to dump the cache and shut down "gracefully". The SPS provides the temporary power required to do that as well as the sense and signal to shutdown if the power into the SPS fails? Maybe I'm missing something on the architecture of these arrays...?

Since, I do not have access to powerlink. I took a lil help from an EMC friend in Michigan He correctly found the document about CX4 DC Installation which had the information required:

Systems with DC power are intended for use in environments with redundant and highly available power sources (for example, "Central Office" grade power within the telecommunications industry), and DC power provided by the site must meet this requirement. The sudden loss of all incoming DC power to a storage system may cause unexpected abnormal behavior of the storage system and loss of write-cache data.

This should answer your query. I have not asked him for the document (email me) yet but this is the document which you may download and refer to :

EMC CLARiiON DC-Powered CX4 Series Enclosures

Installation and Operation Guide

P/N 300-007-920

Ankit

You are forgetting redundancy.

Ideally such environment/Data Centers have redundant power source (DC )so that if one side only goes down, they are still OK. If the whole room with DC Storage goes down then probably they will have more pressing issues and concerns than DC Powered Storage going down!

Thanks Ankit, I guess that answers the question. They just put the responsibility on the customer to make sure power is uninterrupted. I guess that makes sense. Definitely not something for our smaller data centers, and no real reason for us in the bigger data centers.

Allen,

Here is some internal marketing material (VNX FAQ) regarding the DC powered systems:

[...]

Given that DC environments have very reliable power, the likelihood of a power failure to both circuits is negligible. Therefore, DC models do NOT use Standby Power Supplies. DC models have different PSNT part numbers than their AC powered peers.

[...]

You are correct, when a DC powered system is purchased, the responsibility is on the client to make sure it is maintained and they accept this and understand the consequences.  When pushed further and asked for clarification, corporate has suggested the following:

[...]

DC powered environments like for Telco equipment typically run off batteries and have more reliable power

[...]

Thanks guys. I'm not actually interested in deploying in my data centers.. I just wanted to better understand why the seemingly critical function of the SPS wasn't included on this version of the array. I'm still a bit confused over why they wouldn't include it anyway, but since I'll likely never have one of these on my floor it is more curiosity than anything :-)

Should anyone need: there is a way that seems to allow write cache enable without SPS. However it's RISKY. Use it at your own risk and if you REALLY know what you are doing and what you'll be exposed to.

Connected to Flare Windows:

flarecons d f a
setcache -nosps 1

flarecons d f b
setcache -nosps 1