Re: [Libguestfs] [PATCH libnbd] copy: Set POSIX_FADV_SEQUENTIAL and POSIX_FADV_DONTNEED.

>> +#if defined (HAVE_POSIX_FADVISE) && defined (POSIX_FADV_DONTNEED) >> + /* On Linux this will evict the pages we just read from the page cache. */ >> + posix_fadvise (fd, offset, r, POSIX_FADV_DONTNEED); > > I don't think this is a good idea, since this affects the current page > cache, for > the entire host. > > So if the host is having an image in cache for good reason, running nbdcopy > will drop the cache since nbdcopy does not need it, but maybe the host will > need that cache after running nbdcopy. > > The right way to avoid polluting the page cache is to bypass the cache using > O_DIRECT, so nbdcopy is not using or affecting the page cache. > > This can be useful if the user can enable this with a flag. The trouble with O_DIRECT is it's a pain to use correctly, and I guess doesn't use readahead or the existing cache (kind of the opposite problem). However I take your point that we probably ought not to remove files from the cache that are already there. I somehow thought that POSIX_FADV_DONTNEED only affected the current 'fd', but looking at the Linux impl I see it affects the whole system. This argues for having a flag to enable this.