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Query about Cache in RAID systems...

S

Silvermirage

Limp Gawd
Joined
Dec 14, 2005
Messages
228
I would like more info on what the onboard cache on RAID cards do and how it works. I am building a 2TB RAID system and I need to know what the differences are between 64MB, 128 MB and 1GB of cache on the RAID card are. I also want to know whether there is any need to get 16MB cache HDD's versus 8MB cache HDD's if I'm running RAID 5 or 6. What are the advantages/disadvantages? Will there be a boost performance? Will the performance boost be noticable?
 
Onboard cache serves as well a cache for data while the raid processor is full. This is especially common with RAID 5 (dunno bout 6), where parity has to be calculated. If the parity calculator gets bogged down the cache allows it to store the data that is waiting, to allow a more efficient flow. The HD cache is the same principle, data that is going to be written/read off the drive is cached while the drive is waiting for either the data transfer/disk write to complete. As for performance boost, I don't know at what size the law of diminishing returns comes in. I guess it would have to deal with what you are going to do with it. From what I hear though, if you have a cache it is a good idea to have a battery backup on the raid card, because if the power goes out before the cache clears the Array can fail. I could be wrong on this and if I am then anyone else can feel free to chastize me. :p I'm often wrong, its been said I know just enough to be dangerous.
 
So it is pretty much solid state storage until the card can calculate where to store the data.

Now I'm curious...is that cache used solely for buffering or is it also used as a solid state storage device as well?
 
Silvermirage said:
So it is pretty much solid state storage until the card can calculate where to store the data.

Now I'm curious...is that cache used solely for buffering or is it also used as a solid state storage device as well?
Click to expand...

If you think about it, if the data in the cache is not also "RAIDed" then there would not be much point in RAID would there?

So the cache will be divided into cachelines equal to the stripe size and the RAID algorithm run against it too.

So it is not somewhere to put data untill the RAID processor can get around to it, because then that data would not be RAIDed.. Rather it is a place to put data that has been RAIDed untill the drives can get around to saving it. Drives are still the slowest part of the system if you have a fast enough RAID processor on your RAID card.

This is touched on briefly here: http://www.intel.com/technology/magazine/computing/RAID-6-0505.htm

With RAID 6 being basically twice the amount of work of RAID 5, you really need a fast RAID processor to even consider using RAID 6, unless storage speed is not much of an issue.

==>Lazn
 
dirtydr said:
A brief read on write back vs write through caching.
Click to expand...

Oh, true, on the RAID card, it can sometimes be set to use some of it for write caching, and sometimes will only do read caching.. but either way the data in the cache will already be "RAIDed" as I said above.

==>Lazn
 
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