----- Original Message ----- From: tkaczma@gryf.net Cc: toasters@mathworks.com Sent: Thursday, February 24, 2000 2:52 AM Subject: Re: NVRAM memory
On Fri, 18 Feb 2000, Bruce Sterling Woodcock wrote:
Basically, it will effect your write performance. Less NVRAM means it can cache fewer writes, thus having to write to disk more often.
This is not necessarily true. It will affect the speed of some bursty writes where there might not be enough space in the NVRAM to log the complete write stream. If the writes are continuous no loss of performance should be noticed, i.e. the bottleneck will be the speed of writing to disk. In fact, since the number of transactions in the NVRAM will be smaller the banks will clear faster allowing more data to be logged quicker.
Yes necessarily true. The amount of time you have to wait depends on how much you can cache; the disk may be slower, but if I have more NVRAM, then the time it takes for me to fill up, the less time I have to wait until the disk write completes (if I have to wait at all). Also, once you start filling up, your writes won't be "continuous" because the client will start backing off when the filer stops responding.
As I understand from NetApp documentation, and please
correct me if I'm wrong, at no point is the NVRAM used as a cache during normal operation. It serves only as a log that is written to, but not read unless a reboot occurs. Although the amount of NVRAM does affect the performance of the write cache, especially during frequent rewrites of the same block, I'm not sure how many applications/NFS stacks actually put out such a mix of ops.
This is true but irrelevant. The NVRAM itself is not always directly utilized, but the size of the NVRAM dictates the size of the DRAM write cache, so the result is the same. Look, if you don't believe me, feel free to take out half the NVRAM in your filer, write a 100MB file, and see if it takes more or less time.
If you don't really notice the difference on your filer, chances are it is not often very heavily loaded and doesn't experience a lot of heavy write traffic.
Or it is heavily loaded all the time, but the same blocks aren't very frequently overwritten, i.e. the write cache serves very little.
Your conclusion is spurious; rewriting the same block isn't the issue.
Bruce