A human-readable log file is likely to have much less activity in it (it was a year ago I was using kafka and we could eat up gigs for the data files but the log files were a few meg). So there's perhaps little to gain.
Also if the power isn't pulled and the OS doesn't crash, log messages will be, I guess, buffered by the OS then written out as a full buffer, or perhaps every nth tick if the buffer fills up very slowly. So it's still reasonably efficient. Adding a few hundred context switches a second for the human log probably isn't a big deal. I remember seeing several tens of thousands/sec when using kafka (although it was other processes running on those multicore machines to be fair). I guess logging overhead is down in the noise, though that's just a guess. Also I remember reading a rather surprising post about mmaping. Just found it <https://lists.freebsd.org/pipermail/freebsd-questions/2004-June/050371.html>. Sniplets: "There are major hardware related overheads to the use of mmap(), on *ANY* operating system, that cannot be circumvented" -and- "you are assuming that copying is always bad (it isn't), that copying is always horrendously expensive (it isn't), that memory mapping is always cheap (it isn't cheap)," A bit vague on my part, but HTH anyway jan On 12/02/2018, YuFeng Shen <v...@hotmail.com> wrote: > Hi jan , > > I think the reason is the same as why index file using memory mapped file. > > As the memory mapped file can avoid the data copy between user and kernel > buffer space, so it can improve the performance for the index file IO > operation ,right? If it is ,why Log file cannot achieve the same performance > improvement as memory mapped index file? > > > Jacky > > > ________________________________ > From: jan <rtm4...@googlemail.com> > Sent: Saturday, February 10, 2018 8:33 PM > To: firstname.lastname@example.org > Subject: Re: why kafka index file use memory mapped files ,however log file > doesn't > > I'm not sure I can answer your question, but may I pose another in > return: why do you feel having a memory mapped log file would be a > good thing? > > > On 09/02/2018, YuFeng Shen <v...@hotmail.com> wrote: >> Hi Experts, >> >> We know that kafka use memory mapped files for it's index files ,however >> it's log files don't use the memory mapped files technology. >> >> May I know why index files use memory mapped files, however log files >> don't >> use the same technology? >> >> >> Jacky >> >