On Fri, 18 Feb 2000, Bruce Sterling Woodcock wrote:
Just having extra CPU won't help. A better indication is how often you see the writes on the systat being done. If you just see them in bunches every 5-10 seconds, you'll be fine. (Is it still every 10 seconds?) If you see them more often than that, the less NVRAM will hurt you.
This is not neccesarily true. It will hurt you in certain, in my opinion exotic, scenarios.
Suppose I write a 7MB file to your F330. You had 8MB of NVRAM. I start writing. At 4MB you switch half of the NVRAM over and start flushing it to disk. I fill up the rest of your NVRAM and the filer returns back success and my client is ready to go do something else while the filer busily commits everything to disk.
Now, same situation, only you have 4MB. After 2 MB you switch, then I fill up the next 2MB, and now I'm stuck waiting for you to finish your disk activity. You do, and I write the next 2MB, and I have to wait again. Instead of being able to write to you at "memory" speeds (basically as fast as I can go and the network will allow), I am stuck waiting on the disks.
Suppose you write a 10MB file at a blazing speed. With 8MB NVRAM, you'll fill up both parts of the NVRAM and wait while 4MB of data is written to disk. With 4MB NVRAM, you'll happily fill up both sides, but then wait only for 2MB of data to be written to disk before you can write some more. That's half the waiting time. If the filer is smart, it will cache NFS packets without acknowledging them before they are put in the NVRAM. As you see it is a case of using larger spoons at a slower pace or smaller spoons at a faster pace. The large spoons take more time to empty.
The quantitative impact depends on how big the files are being written and how often they are written. If it's just users writing small files and the infrequent large one, at most they'll notice an extra second or two wait when saving something. If it's a database or news server getting written to every second, the whole thing will be slowed down considerably. Maybe 20-40%?
Again, please see above before you accept this theory. I'm not sure that it is all this clear cut. I would expect the amount of NVRAM to have a significantly smaller impact on continuous "contiguous" writes than what is advertised above. However, just like the previous comentator, I don't have real data to substantiate this.
Tom