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. Regards, Berto Alberto Garcia (4): qcow2: Make qcow2_cache_table_release() work only in Linux qcow2: Allow 'cache-clean-interval' in Linux only qcow2: Remove stale comment docs: Specify that cache-clean-interval is only supported in Linux block/qcow2-cache.c | 6 +++--- block/qcow2.c | 8 ++++++++ docs/qcow2-cache.txt | 5 +++-- 3 files changed, 14 insertions(+), 5 deletions(-) -- 2.10.2
