In a device that has raid 5, and a "raid cache" of 4GB, does this help performance? I have 32MB cache hard drives but I was wanting to know if the 4GB cache will make up for the difference in performance between a 128MB hard drive cache and a 32mb cache for the hard drive. Hardware raid. The front end of the device is a 2008 R2 server, Dell R610.
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1Actually some driver vendors disable the disk drive cache when the array cache is in use. You should be able to verify this in device manager. Sometimes you can even set the drive cache policy but the driver will reset it. See ewwhite answer for more info.– Greg AskewDec 18, 2012 at 0:00
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3 Answers
With a battery-backed write cache on your RAID controller, you can safely enable write-back mode for your array. Without this, you would need to write-through and suffer the RAID 5 write penalty. HDD cache doesn't really factor in here.
If you have the budget for it, BBWC is a no-brainer on all but the most idle of systems.
Yes. Especially in the case of a battery or flash-backed array cache.
The purpose of a hardware controller's (write) cache is to acknowledge writes at low-latency (before coalescing and flushing to disk). Cache on the controller is an order of magnitude faster than that of rotating media, so this will help accelerate your applications.
Typically, individual drive cache should be disabled or run on a system with consistent external battery (UPS) protection. Your physical drive cache is probably a non-factor here. 32MB versus 128MB probably won't matter. There are specific edge-cases where it may.
What will you be doing on this system?
See:
What is the memory module on a RAID card needed for?
Incredibly low disk performance on HP ProLiant DL385 G7
Non-volatile cache RAID controllers: what kind of protection is there against NVCACHE failure?
Yes.
HOW MUCH is another question and depends on the patterns.
