The three primary bottlenecks you should worry about are CPU, disk I/O, and the network. For most machines, CPUs are rarely a bottleneck. A single Sun SPARC 10 CPU can start (and complete) between 700 and 800 I/O operations a second, giving approximately 5,600 to 6,400KB/s of throughput when the data averages around 8KBs (a common buffer size). A single Intel Pentium 133 can do less only because of somewhat slower cache and bus interfaces, not due to lack of CPU power. Purpose-designed Pentium servers, like some Compaq servers, will be able to start 700 operations per CPUs, on up to four CPUs.
Too little memory, on the other hand, can easily be a bottleneck; each Samba process will use between 600 and 800KB on Intel Linux, and more on RISC CPUs. Having less will cause an increase in virtual memory paging and therefore a performance hit. On Solaris, where it has been measured,
smbd will use 2.6 MB for program and shared libraries, plus 768KB for each connected client.
nmbd occupies 2.1 MB, plus 496KB extra for its (single) auxiliary process.
Hard disks will always bottleneck at a specific number of I/O operations per second: for example, each 7200 RPM SCSI disk is capable of performing 70 operations per second, for a throughput of 560KB/s; a 4800 RPM disk will perform fewer than 50, for a throughput of 360KB/s. A single IDE disk will do still fewer. If the disks are independent, or striped together in a RAID 1 configuration, they will each peak out at 400 to 560KB/s and will scale linearly as you add more. Note that this is true only of RAID 1. RAID levels other than 1 (striping) add extra overhead.
Ethernets (and other networks) are obvious bottleneck: a 10 Mb/s (mega
bits/second) Ethernet will handle around 1100KB/s (kilo
bytes/s) using 1500-byte packets A 100 Mb/s Fast Ethernet will bottleneck below 65,000KB/s with the same packet size. FDDI, at 155 Mb/s will top out at approximately 6,250KB/s, but gives good service at even 100 percent load and transmits much larger packets (4KB).
ATM should be much better, but as of the writing of this book it was too new to live up to its potential; it seems to deliver around 7,125 Mb/s using 9KB packets.
Of course, there can be other bottlenecks: more than one IDE disk per controller is not good, as are more than three 3600 SCSI-I disks per slow/narrow controller, or more than three 7200 SCSI-II disks per SCSI-II fast/wide controller. RAID 5 is also slow, as it requires twice as many writes as independent disks or RAID 1.
After the second set of Ethernets and the second disk controller, start worrying about bus bandwidth, especially if you are using ISA/EISA buses.
