m5@lynx.UUCP (Mike McNally ) (05/05/88)
Could one of the cognoscenti fill me in on this ``virtual memory file system'' of ``4.4''BSD? Is it like the Multics file system (about which I know almost nothing)? How does it handle multiple connections to the same file? Why is it better than open(), close(), read(), and write()? I'm just curious. -- Mike McNally of Lynx Real-Time Systems uucp: lynx!m5 (maybe pyramid!voder!lynx!m5 if lynx is unknown)
chris@mimsy.UUCP (Chris Torek) (05/06/88)
In article <3711@lynx.UUCP> m5@lynx.UUCP (Mike McNally ) writes: >Could one of the cognoscenti More like Illuminati :-) >fill me in on this ``virtual memory file system'' of ``4.4''BSD? It is just a file system (presumably /tmp might be one of these) in which files are not written to any sort of backing store unless the system `feels like it', and if and when they *are* written, they are written in a way that does not recover from crashes. In particular, this file system will use free memory pages, unless there are few; then the pages may get pushed to swap space. A major reason for the existence of this file system is so that swap space has a name in the file system: fd = new_temp_file(); res = mmap(addr, len, PROT_READ|PROT_WRITE, MAP_PRIVATE, fd, off); would get what amounts to zero-filled space, a la the existing sbrk(). This obviates the need for a `magic' file descriptor and/or a MAP_SWAP flag when all you want is blank swap-backed memory. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
barnett@vdsvax.steinmetz.ge.com (Bruce G. Barnett) (05/06/88)
In article <3711@lynx.UUCP> m5@lynx.UUCP (Mike McNally (Man from Mars)) writes: |Could one of the cognoscenti fill me in on this ``virtual memory file |system'' of ``4.4''BSD? Another 'feature' is that commands like cp humongeous-file /dev/null will take 0 seconds. Imagine the benchmarks!!! -- Bruce G. Barnett <barnett@ge-crd.ARPA> <barnett@steinmetz.UUCP> uunet!steinmetz!barnett
guy@gorodish.Sun.COM (Guy Harris) (05/07/88)
> Another 'feature' is that commands like > cp humongeous-file /dev/null > will take 0 seconds. Almost: Script started on Fri May 6 11:49:27 1988 gorodish$ cat /etc/motd SunOS Release 4.0 (GORODISH) #267: Thu May 5 14:49:36 PDT 1988 gorodish$ time cp /vmunix /dev/null real 0m0.13s user 0m0.03s sys 0m0.08s You *do* have to spend some time starting up the program, opening the file, "mmap"ping the pages, etc..