Disk Cache on Power Edge Servers

Hello rkart77

Yes, the majority of the time disk caching will improve performance. Whether or not it is recommended will be dependent on the controller.

If you are using a controller that has battery backed cache like our PERC 5/6/H700 then I would recommend using the default setting. By default caching is enabled on the controller if the battery is charged. If the battery falls below a certain threshold of charge it will turn off caching.

If you are using a controller that does not have cache like the SAS 5/6/H200 then I would recommend disabling caching. By default it is enabled on the SAS 5/6. On the H200 caching is disabled by default. The controller itself does not have cache. The option to enable or disable cache on these controllers determines whether or not the cache on the HDD's are used.

Whether you are using cache on a controller or the cache in a hard drive the result is the same in the event of sudden loss of power. If there is not a battery present there is a high probability of loss of data. It is highly recommended that any server in a production environment have an Uninterruptible Power Supply. A UPS provides another layer of protection against data loss. Cache memory is volatile memory. That means that the memory loses any stored data when there is no power. Any data that is in cache during a power loss will be lost. The batteries on our controllers will keep the memory powered for at least 24 hours. A brand new battery is capable of holding a 72 hour charge. If the battery gets to the point where it can no longer hold a 24 hour charge or greater then the controller will mark it as failed, and give an error similar to ROMB <= 24 hours.

Thanks

Hi Daniel,

Thanks for your Reply !

We always buy battery backed RAID controllers (H700/6i etc ..)and chose write back policy.Of course UPS is defualt power supply to all the servers in production environment.What i noticed at OMSA console is another policy called Disk Cache Policy.I would like know what will happen to data in case of Disk Cache Policy is enabled and server is abruptly down due to some other reason?

Thanks

Karthik R

I mean what is the protecton for data stored in the HDDs cache in the event of abrupt shtdown

Do HDDs also use RAID controller cache to store data?

I mean what is the protecton for data stored in the HDDs cache in the event of abrupt shtdown

There is no protection. The HDD cache is not battery backed so any data cached on the HDD would be lost in the event of an abrupt shutdown. The HDD cache should be disabled by default. I would recommend leaving it disabled.

Thanks