3ware 9750 horrible multi user

[sdadept]

when I have several virtual machines hitting the hard drives at once, almost the entire machines IO access grinds to a halt. When I have these same vm's running on another server that is just straight 2tb hard drive (no hardware raid card) then it works just fine.

is there a setting I need to put in somewhere that makes this work with multi user? I know on regular drives its a matter of making sure NCQ is turned on. What do I need to do to make similar work with this hardware raid card?

 

[drescherjm]

Do you have the BBU installed? If not the raid5/6 write speed will default to be very slow because without a BBU the write cache is disabled. You probably can override the default and enable the cache without a BBU but that is dangerous unless you have a UPS.

Edit: Are you using raid 5 or 6?

 

[sdadept]

I don't have the BBU. I do however have a massive ups for this server. Should I turn that on?

it's raid 5 with 6 1tb wd drives

 

[dalef]

Sorry for bumping this thread, but I thought I had something useful to add to it.

With the write caching turned off, I was getting write speeds on the order of 4MB/s to an 8 disk RAID6 array (Seagate Barracuda XTs). With it enabled, speeds jumped up 600+MB/s.
(I've got a 9750-4i connected to a 36 port SAS expander running on Linux.)

So what are the risks for turning write caching on when you don't have the BBU? Data loss is the risk. How big of a risk this is is really something that varies from person to person. How much do you value your data? What are the chances of a power failure (you mentioned you had a UPS, but that won't help when you have a PSU or UPS failure)?
If this is just for scratch storage or something that isn't that important, I'd say go for it. If you need the data and don't back it up that often, then probably best to shell out for the BBU (and consider the remote mounting bracket, the card gets warm when it's running hard).