[ On Monday, May 29, 2000 at 10:16:03 (-0700), Roeland Meyer (E-mail) wrote: ]
Subject: RE: pop server in an ISP environment
The theoretical limit, on most Unix kernels, is 64K users. This is only because the internal representation of the uuid is usually an int.
You're about six years behind the times, I think! ;-) I'd bet that 99.9% of running Unix and Unix-like kernels that are capable of running TCP/IP and would be used in production in an ISP setting today are good to at least 2^31 users, if not 2^32. The first widely used 32-bit system, Unix 32V, used a "short" to represent the UID in the kernel (in struct proc, for example). IIRC a short was indeed still 16 bits on a VAX (and thus a UID was effectively restricted to 15 bits). By the time AT&T UNIX SysIII came along it was a "ushort", and so definitely 16 bits. 4.3net2 still uses a u_short, but I doubt any ISPs are using any such kernels in production. By the time 4.4BSD is available (1994) uid_t is an "unsigned long", so even on a 32-bit machine that's 2^32 users! ;-) AT&T System Vr4 (and thus SunOS-5.x) still calls uid_t just a "long" so it's only good for 2^31 users on 32-bit system, making 32-bit 4.4BSD boxes ~2 billion (and that's an American Billion!) times better than 32-bit SunOS-5 boxes! ;-) [and 64-bit 4.4BSD boxes are "about" 16140901064495857664 times better than 64-bit SunOS-5 boxes! 2^ ;-)]
Your 5000 user limit, based on /etc/passwd, is bogus. For example, on Intel hardware, you will never approach even 500 concurrent shell users (developers) without the silicon melting down. However, a Sun e10K can handle 5000 of such users easily. That same Intel box can handle over 10000 mailboxen, if you give it enough disk space (RAID0 spool) and memory (RAM cache) [but not on a single 56Kbps port <g>].
On machines which do not use a hashed database (dbm, db, etc.) for /etc/passwd (eg. un-adorned SunOS-5.x not using NIS+) there may be some issue with having more than say 20-30 thousand users. More CPU and RAM will offset this limit somewhat of course. -- Greg A. Woods +1 416 218-0098 VE3TCP <gwoods@acm.org> <robohack!woods> Planix, Inc. <woods@planix.com>; Secrets of the Weird <woods@weird.com>