Am 25.11.2016 um 12:27 hat Alberto Garcia geschrieben: > Hi all, > > The cache-clean-interval setting of qcow2 frees the memory of the L2 > cache tables that haven't been used after a certain interval of time. > > QEMU uses madvise() with MADV_DONTNEED for this. After that call, the > data in the specified cache tables is discarded by the kernel. The > problem with this behavior is that it is Linux-specific. madvise() > itself is not a standard system call and while other implementations > (e.g. FreeBSD) also have MADV_DONTNEED, they don't share the same > semantics. > > POSIX defines posix_madvise(), which has POSIX_MADV_DONTNEED, and > that's what QEMU uses in systems that don't implement madvise(). > However POSIX_MADV_DONTNEED also has different semantics and cannot be > used for our purposes. As a matter of fact, in glibc it is a no-op: > > https://github.molgen.mpg.de/git-mirror/glibc/blob/glibc-2.23/sysdeps/unix/sysv/linux/posix_madvise.c > > So while this all is mentioned in the QEMU documentation, there's > nothing preventing users of other systems from trying to use this > feature. In non-Linux systems it is worse than a no-op: it invalidates > perfectly valid cache tables for no reason without freeing their > memory. > > This series makes Linux a hard requirement for cache-clean-interval > and prints an error message in other systems.
Thanks, applied to the block branch. Kevin
