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Use cache on RAID Controller?


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I'm currently using my 3Ware 9500S-12 with unraid 5b12a which is working fine so far (apart from a missing/faulty SMART function and the according errors in the log...)

I've exported all disks in single disk mode of course and included in unraid. The parity and cache disk is not on the 3Ware but on the internal SATA controller as suggested in the tut.


What I'm unsure about is that the controller has a 256MB Cache Module which is enabled per default. Is it wise to use it? Or is it possibly confusing unraid and should be disabled?



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If you have the BBU (Battery backup unit) then the cache on the controller is in effect for reads and writes and is protected. (if enabled)

If you do not have the BBU, the controller only cache's reads.


The cache does not confuse unRAID. It aids in providing faster reads and allowing the writes to complete without waiting for the drive to rotate and acknowledge the write has succeeded.


I don't know where it says to keep the parity off the 3ware. But I use parity on a raid controller with cache and it helps allot.


Granted parity and cache are the only drives on my raid controller, but it provides a nice boost of performance.



You can access SMART information via smartctl with the -d3ware option.


In my situation I chose to use simpler SATA controller's for the data drives and an advanced controller for the PARITY / CACHE.


This let's unRAID show the details of the data drives via gui and I use my own tools to monitor parity and cache.

Also the ARECA ARC-1200 has an alarm on it should a drive fail. Maybe the 3ware does too, but I'm not sure.


In any case, It would be highly recommended to use a UPS on the machine's electrical side along with some USB or SERIAL cable to shut the system down if power has failed. Then you do not have to worry about data being left in the controller cache.




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Thanks for your reply. Was most helpfull.  :)


I've been brave and just disabled the cache for tryouts and truth to be said the results were dreadfull. Before the writhe performance was at ~20 to 25MByte/s sustained write, after disabling the cache it crashed to 7-8MByte/s. So I think it's safe to say it's a good thing to keep.  ;D

I do have a BBU but with 3ware controllers geeky as they always were you could use the cache in write mode even without a BBU although you get a nice warning window.


The reason the parity drive is on the normal SATA is simply that the 3Ware being a native PCI-X Device sits in a normal PCI Slot in my config which is limiting the troughput to the other controllers. The 12 2TB Drives on the 3ware contain more or less just "old" data and I add new drives only to the faster OK. lets say the 'better attached' controller in the system.


The USV is a good idea I've been thinking about it some time now but never actually invested the money. Any suggestions on a cheapish model with USB would be highly appreciated.





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