Now that ext4 is the default filesystem, it would be nice to see more programs taking advantage of its preallocation features. But here's my question: what is the best way to actually do this? In recent glibc, posix_fallocate() will first try to use fallocate(), and then fall back to writing nulls to fill up the requested range if fallocate() fails. The problem is, posix_fallocate()'s fallback behavior effectively results in writing the file twice. If we expect that the process of writing the file's actual content will take a long time (e.g., I'm using a BitTorrent client to download some Fedora DVD ISOs), then that's not a big deal. But if I'm using a utility like rsync or cp, the performance penalty of writing every single file twice is unacceptable. Additionally, the semantics that result from using fallocate() with FALLOC_FL_KEEP_SIZE (which posix_fallocate() does NOT do) are arguably more intuitive, because FALLOC_FL_KEEP_SIZE (essentially) creates a sparse file: $ fallocate-test foo $[1024*1024*512] && ls -lsa foo fallocate-test: allocating 536870912 bytes for new file foo 524292 -rw-r--r-- 1 root root 0 2009-04-02 13:59 foo If I'm (e.g.) downloading a large file via a P2P network, then I can see the progress of the download by eyeballing the file's actual size versus its allocated size. I see that at one point, people were kicking around patches to use ioctl() and/or fcntl() to perform preallocation. Do any of those interfaces actually exist, or were they deprecated in favor of fallocate()? If the latter, is there an interface for fallocate() that doesn't have quite as much FAIL as posix_fallocate()? -- fedora-devel-list mailing list fedora-devel-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/fedora-devel-list
