SmartArray P400 questions

[paulfoel]

Got a few questions about this and configuring via ACU.

1) Physical Drive Write Cache. Can you explain what this is? Is it the actual physical drive write cache and nothing to do with the array accelerator?

Also, am I right in saying the accelerator battery has no effect on this. i.e. if you lose power you may have problems?

2) Ratio settings. 25/75 Read/Write seems to be the default. Do you have any guidelines on correct use of this ratio?

In real world, what sort of improvements, if any, would you expect to see, for exampe, for a RAID1 configured windows system disk?

3) If battery is fault, am I right in saying that write cache only gets disabled (in effect, switches to 100/0 Read)? Or does it turn off acceleration for read also?

4) Have various levels of firmware and ACU installed on many servers. Some have the option to "Enable Write Cache when Battery/Capacitor Not Present or Not Completely Charged" but some have this option missing. Can you confirm whether a minimum level of firmware is required for this functionality?

[Nestorov]

Hi, since you posed quite a few questions, each of them demanding a rather detailed explanation, i'm gonna go ahead and copy your message and go through each question separately. I've tried to be as detailed and accurate as i can and to provide links to additional sources when possible.

1) Physical Drive Write Cache. Can you explain what this is? Is it the actual physical drive write cache and nothing to do with the array accelerator?
 
Also, am I right in saying the accelerator battery has no effect on this. i.e. if you lose power you may have problems?

--> Answer: if you open the Array Configuration Utility User Guide you can find the following description for the physical drive write cache option:

Enabling or disabling the drive cache
On controllers and drives that support physical drive write cache, you can use this command to enable or
disable the write cache for all drives on the controller.
CAUTION: Because physical drive write cache is not battery-backed, you could lose data if a power failure
occurs during a write process. To minimize this possibility, use a backup power supply.

http://www.hp.com/ctg/Manual/c00709035.pdf

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Basically some (not all!) Hard Disks have their own small write cache and if the controller recognizes it, you have the option to enable it or disable it. As the Quote from the User Guide states this cache is not battery backed and hence you are correct in assuming that data written to it will be lost in case of an outage.

That is the reason why we recommend installing an accelerator and a battery as the accelerator module has larger capacity, generally works faster and can be battery backed. Also, if you have a cache module and a battery (or capacitor for the newer generations of smart Array Controllers) it is advised to disable the physical drive write cache option.
 
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2) Ratio settings. 25/75 Read/Write seems to be the default. Do you have any guidelines on correct use of this ratio?
 
In real world, what sort of improvements, if any, would you expect to see, for example, for a RAID1 configured windows system disk?

--> Answer: Yes, the 25 read/75 write setting is the default one. Of course depending on what applications you're running and what data is being stored on the volume you might wanna change that. If you have a logical drive that functions as a data base that rarely has new elements added to it then why not change the setting to 75 read and 25 write? It is a consideration that users have to make depending on their specific environment.

A bit more info on this can be found in the same user guide:
http://www.hp.com/ctg/Manual/c00709035.pdf 

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Changing the controller cache ratio
The controller cache ratio setting determines the amount of memory allocated to read and write
operations. Different types of applications have different optimum settings. You can change the ratio only
if the controller has a battery-backed cache (because only battery-backed cache can be used for write
cache) and if there are logical drives configured on the controller.

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3) If battery is fault, am I right in saying that write cache only gets disabled (in effect, switches to 100/0 Read)? Or does it turn off acceleration for read also?

--> Answer: yes, you are correct; the controller would say its accelerator is temporarily disabled due to the battery being faulty, but only the write cache would be disabled; reading would still be accelerated.
* if you want more detail on why the default setting for write cache is larger you can also check out the HP Smart Array Technology technical paper:
http://www8.hp.com/h20195/v2/getpdf.aspx/4AA5-4124ENW.pdf?ver=1.0

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the pasage about the cache starts on p.3; bear in mind that the p400 are a bit outdated already as they were introduced with the G5 servers and we're now at gen9 and p440 so not all information on this technical paper is really relevant to the p400; most if it is for pX2X controllers and newer. But the sections about what read and write cache mean and why the default settings for write cache are always bigger are valid also for the p400

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4) Have various levels of firmware and ACU installed on many servers. Some have the option to "Enable Write Cache when Battery/Capacitor Not Present or Not Completely Charged" but some have this option missing. Can you confirm whether a minimum level of firmware is required for this functionality?

--> Answer: yes, not all controllers allow for that option, and newer firmware allows for it if the controller can work with an accelerator module, but without a battery you again accept the risk of a data loss (same as if you choose to use the physical drive write cache)

* you can again turn to the Smart Array Technology technical paper:
http://www8.hp.com/h20195/v2/getpdf.aspx/4AA5-4124ENW.pdf?ver=1.0

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and consult the section about the cache/accelerator technology, starting on p.3 where the following is written:
HP Smart Array battery-backed or flash-backed cache backup mechanisms protect the cache data against a server crash and power loss. The controller disables caching when battery-backed or flash-backed cache is an option but not installed. You can override this behavior but doing so opens a window for possible data loss.

Also in the newer versions of the ACU's user guide and the Smart Storage Administrator User guide you can find the following info:
NoBatteryWriteCache
With this option, the controller can enable write cache when no battery is present or when the battery fails.
Values are Enable or Disable. The default is Disable.

So if you do not see the option as available for you controller, make sure it is running the latest firmware, ACU 8.70 or newer and then try enabling it if you wish to do so. As stated previously having a battery or capacitor module is always better as it is less risky.

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Additional remark: for any newer controllers, meaning anything equal to or newer then pX1X i'd recommend you use the Smart Storage Administrator:
www.hp.com/go/hpssa

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as it is the successor to the HP Array Configuration Utility and is the program that will continue to be updated and developed as it was introduced with newer generation Smart Array Controllers.

[paulfoel]

Thanks for the reply. VERY helpful.