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koden

Controller cache and battery

14 posts in this topic

Hi

I'm working with servers.

maybe this is not the right place to ask about hardware but will try anyway.

I'm just wondering how controller cache and battery is working.

1. I asume that the cache module does that server is faster, because it can write to cache even if C drive is occupied by another job.

2. I also asume that cache can hold last data if server goes down, because of the battery, and restore this data when server starts again.

But is battery nessesary in ex. 1 or only in 2?

Can ex. 1 be possible without battery and only cache module?

or am I totally wrong about the cache and battery use?

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This sounds more like a hardware question. What is the controller you are using?

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Exactly what kind of battery are you talking about, some kind of external battery backup system or what?

Cheers and Regards

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Hi

thanks... as I wrote it's hardware and it was just a try :-)

I will try in a hardware forum somewhere.

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Well we have a hardware forum (I just move the topic there) so you can post what controller you have.

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it's many types of controllers.

But I can find an controller ex. tomorrow.

But think thats not so important for the question... i think.

I more like to know what the cache and battery are for and if it's not there.

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Well let's take the 3ware 9750 as an example. Batter backup p/n iBBU07 is an optional component which can be connected to the card. It says this about it:

Optional LSI iBBU07 battery backup unit.

Up to 72 hours of data retention in the event of sudden power loss or unclean shutdown
and

RAID controller caches can significantly increase performance when writing data. A typical example for such a cache would currently consist of 256, 512 or 1024 MB. If the power were to fail, the content of this cache would be lost, unless the content has been protected by a battery backup unit (BBU) or battery backup module (BBM). BBUs and BBMs have integrated batteries, which can generally power the content of the cache for up to 72 hours. If the server is re-started during that period, the data in the cache can be recovered.

Note: The battery status should be checked at periodic intervals, since capacity will reduce over the life of the battery. When the battery becomes too weak (generally after one to three years), it should be replaced (just like a notebook battery). If the battery status is not checked, there is the risk after several years that the battery will only be able to retain the content of the cache for very short period, which would risk data loss if the power failure were to continue for a longer period. The Battery Backup Unit (BBU/BBM) Maintenance for RAID Controllers article will provide detailed information about this issue.

Note: RAID controllers, which do not use a BBU to protect the cache (but instead copy the content of the cache to flash memory in the event of a power failure), do not require special cache protection maintenance. The new Adaptec Series 5Z Controller systems using the so-called “Zero-maintenance Cache Protection “(ZMCP)” feature would provide this capability.

From here:

http://www.thomas-krenn.com/en/wiki/RAID_Controller_and_Hard_Disk_Cache_Settings

Edited by Tripredacus
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Thanks a lot. :-)

I'm not english speaken, but if I understand it right, then it is as I assumed.

A cache can be used without battery for performance.

The battery is only there to hold data up to 72 hours if server goes down.

So when I have servers with battery that need replacement can I ignore this error if data loss at a power failure is acceptable

Off course I need to be aware of that old batteries can leak fluid.

The last part I did not understand:

"Note: RAID controllers, which do not use a BBU to protect the cache (but instead copy the content of the cache to flash memory in the event of a power failure), do not require special cache protection maintenance"

1. Is there another way to cache than using cache module?

2. To protect data I assume a battery of some kind is always needed

Why I ask al this is because I have several servers without cache and battery. I have mounted cache and battery in one of them and performance then got much better.

But to do that in all servers it would cost some money and time. And another problem...downtime :-)

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The cache on the raid controller is primarily used (needed) for performance. The cache is storing the data before being written to the drives. The battery is needed in case of power outage to keep the data stored in the cache until power is restored.

Some controllers need a bbu to enable some functionalities like write cache. So in some cases, a working bbu is required to allow maximum performance even if there is no need of it (an ups should be used to avoid power outage and then the bbu shouldn't be needed for its primary role).

Edited by allen2
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ok.. so it depends on controller if it needs bbu or not just to write to cache.

I just assumed that cache module was getting power from server and not battery.

battery was only for restoring data from cache.

But yoy say that some cache module needs battery just to work, even that it does not use power from battery.

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All controllers work a little bit differently from others. That's why I asked for models. You look up the technical docs for those controllers to learn what their capabilities and requirements are.

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But yoy say that some cache module needs battery just to work, even that it does not use power from battery.

On the HP ProLiant SmartArray devices I have used, the cache is disabled when the battery loses charge or registers a fault.

This is done to prevent the loss of data in the event of a power outage. It's easier for HP to support a disk performance issue that they can just tell you to replace the battery for, than it is to recover a corrupt database.

When the battery fails, or has no charge, event ID 1206 from Storage Agents will be logged to tell you about the battery failure. Event ID 1204 will also be logged to tell you that the cache accelerator module has been disabled.

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But yoy say that some cache module needs battery just to work, even that it does not use power from battery.

On the HP ProLiant SmartArray devices I have used, the cache is disabled when the battery loses charge or registers a fault.

This is done to prevent the loss of data in the event of a power outage. It's easier for HP to support a disk performance issue that they can just tell you to replace the battery for, than it is to recover a corrupt database.

When the battery fails, or has no charge, event ID 1206 from Storage Agents will be logged to tell you about the battery failure. Event ID 1204 will also be logged to tell you that the cache accelerator module has been disabled.

ok so when no battery cache disable and can't be used

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