Amount of cache needed?

[Mott Given]

Is there any industry standard for the ratio of cache to disk that you should have? I'm trying to evaluate a vendor claim that we need to get a certain amount of cache.
Also, what's a ballpark figure on how much 8 GB of cache costs (and is your figure list price or the typical discount)?

[Eugeny Brychkov]

Which disk? Which disk array? Which configuration (hardware and software)? Could you please be more specific?
Eugeny

[Vincent Fleming]

Eugeny's has a good point.

Typically, the more drives you have in an array, the less you need cache.

For example, with the XP we use a much higher ratio of cache-to-storage when you have only a few drives, but once you start getting a lot of drives (50-100), then you need not add more cache with more drives. Basically we front-load the cache because you haven't enough drives for good performance.

Pricing also is dependent upon the particular architecture, and time of year (memory keps getting cheaper...) For the XP line, list price for 2GB of cache is about $33,000. Street prices vary, depending on your reseller or hp-direct discount level.

Also, prices can change daily in this business.

What kind of disk array are you looking at, and how big is it?

-Vince

[Mott Given]

Thanks for the replies. Its EMC
disks and terabytes of data (sorry - I don't know the exact number of drives and the exact amount of storage). Some of it is the brand new EMC Symmetrix 8xx storage.

Mott Given

[Ron Singler]

We were recently quoted a DMX 2000 with 5.1 TB usable disk at 8 GB of cache.

You can always run under cached based on your I/O requirements. Are the application you will be running write intensive? We the app need to do a lot of reads? Just some stuff to ask yourself when thinking about cache.