[c-nsp] Stackwise 3750 and buffers

[Peter Rathlev]

Hi all,

We have 6 stacked WS-3750G-24T switches, and have a small suspicion that
there are some performance problems. We've looked into some different
things, and looking at the buffers I found some questions I couldn't
find an answer to.

A show buffers on the master gives:

HOR-R03-ISC-01#sh buff
Buffer elements:
     478 in free list (500 max allowed)
     3667187052 hits, 0 misses, 0 created

Public buffer pools:
Small buffers, 104 bytes (total 50, permanent 50, peak 110 @ 7w0d):
     49 in free list (20 min, 150 max allowed)
     2014928381 hits, 20 misses, 60 trims, 60 created
     0 failures (0 no memory)
Middle buffers, 600 bytes (total 61, permanent 25, peak 106 @ 7w0d):
     59 in free list (10 min, 150 max allowed)
     55275850 hits, 277 misses, 795 trims, 831 created
     0 failures (0 no memory)
Big buffers, 1536 bytes (total 150, permanent 50, peak 686 @ 7w0d):
     150 in free list (5 min, 150 max allowed)
     407745931 hits, 1231637 misses, 3694857 trims, 3694957 created
     0 failures (0 no memory)
VeryBig buffers, 4520 bytes (total 25, permanent 10, peak 26 @ 7w0d):
     10 in free list (0 min, 100 max allowed)
     204 hits, 8 misses, 1 trims, 16 created
     0 failures (0 no memory)
Large buffers, 5024 bytes (total 0, permanent 0):
     0 in free list (0 min, 10 max allowed)
     0 hits, 0 misses, 0 trims, 0 created
     0 failures (0 no memory)
Huge buffers, 18024 bytes (total 4, permanent 0, peak 7 @ 7w0d):
     4 in free list (0 min, 4 max allowed)
     42103242 hits, 326390 misses, 652218 trims, 652222 created
     0 failures (0 no memory)
<snip>

The numbers are similar on the other stack members ("remote command 2
show buffers" etc), with big buffers having a very large peak. Could
this lead to lost packets, or would it just mean that forwarding is a
bit slower?

Also, I can try to adjust the buffers (global config "buffers big
permanent <number>") but I am unanware of the consequenses of this. Will
the switches tell me when they don't have enough RAM for the amount of
buffers I try to reserve, or will they just start to perform very badly?

I have tried to search CCO for answers, but either my search-fu is bad
or CCO doesn't have very much information regarding buffers on the 3750.

The switches are interconnects in an iSCSI setup by the way, with jumbo
frames enabled. The switches only to L2.

Regarding Stackwise, I can only find some semi-marketing white papers
and a bit in the documentation for the 3750. Anybody know of somewhere
good to learn about this feature? Is it any good at all? :-)

Thank you,
Peter Rathlev