| It was not only the most stable, but for many applications, especially in networking, it was also the fastest single-threaded operating system, beating easily Linux and Windows XP or 2000. Unfortunately for FreeBSD, the launch of the Intel Pentium 4 with hyper-threading in 2003, then of the AMD dual-core CPUs in 2005 have made quickly FreeBSD 4 completely obsolete. The smaller FreeBSD team has required many years until achieving a decent implementation for multi-threaded CPUs and during that time they have remained long behind Linux and other operating systems. Besides perfect stability (it was normal to not reboot FreeBSD 4 for years) and great networking performance, it also had a much more reliable file system than the competition. Despite the fact that Windows XP used NTFS and Linux had at least 3 file systems with journaling at that time, where journaling was supposed to make the file systems crash-resistant, I have seen at that time (around 1999-2003) many cases of file system corruptions after power outages, on computers without UPSes which used NTFS or Linux file systems with journaling (on Linux EXT2 without journaling any power outage was very likely to require a complete reinstallation). During the same power outages, the computers with FreeBSD that used the UFS file system with "soft updates" never experienced any file system corruption, despite the fact that UFS with "soft updates" was not a journaling file system, but only one where the disk write operations were carefully ordered in such a way as to prevent unrecoverable file system corruption in the case of a crash. |
Many OSes at the time had hitches with SMP. BSD was one of them. FreeBSD had SMP in 4.x but almost everything in the kernel was single-threaded and the kernel thread was a major bottleneck.
FreeBSD wasn't alone in this. Linux suffered from a similar problem at the time, also because of the driver architecture. (The infamous "big kernel lock" wasn't fully eliminated until 2011: https://kernelnewbies.org/BigKernelLock)
This is an area where NT was much better, or VMS, or Solaris. And yes, the SMP issue, in hindsight, does partly explain why both Linux and BSD weren't as historically attractive as they otherwise looked, for large systems.