There is a modestly obscure means to attach *_DONTNEED (to achieve kernel
delayed writes and removing pages when I/O is complete) using the existing
rpm.org rpmfiArchiveReadToFilePsm() Fwrite->write(3) loop calling setvbuf(3)
immediately after Fopen on the fopencookie(3) FILE *.
(aside)
Personally, I think mmap(2) and madvise(2) are easier to implement/maintain
than rpmio fopencookie(3) wrapped stdio(3) abstractions like setvbuf(3): YMMV.
The idea behind *_DONTNEED -- to minimize competition for scarce resources --
is to hint to the kernel that RPM write buffers (actually pages) can be reused
by other processes as soon as written to disk, rather than cached. RPM package
installation is always fire-and-forget for package files, not true in general
for file writes.
Letting the kernel manage its own resources as necessary may be sufficient to
prevent database stalling/starvation as an alternative to attempting explicit
fsync(3) invalidation of cached I/O buffers
which has the unfortunate side effect of blocking RPM until the fsync(3) system
call returns.
(aside)
This *IS* what used to be in RPM, and was a significant win at the time (but
also included directly uncompressing into the mmap(2) buffer avoiding a copy,
and increasing the zlib buffer size from 32K -> 256K, which has long since been
upstreamed). Someone else will have to speak to why mmap(2) was ripped out at
rpm.org.
(hth, off to bed)
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