On 12/27/2010 11:17 AM, Sunil Mushran wrote: > On 12/27/2010 06:47 AM, Filipe David Manana wrote: >> Hi, >> >> I have been playing around with fallocate to preallocate space for a >> file with the mode FALLOC_FL_KEEP_SIZE. >> I'm running with Linux kernel 2.6.35-24 and ext4 as the fs. >> >> I'm allocating 1Gb for a newly created file and then in a loop I write >> 1Gb of data into that file in chunks of 1Kb. >> fallocate is returning me 0, therefore it was successful. >> However I don't see any performance gains compared to a version of >> that same code that doesn't call fallocate. >> >> The test code which does this is: http://friendpaste.com/2UR0n2U851u4IXmubeLZh0 >> >> Am I doing something wrong? > > fallocate() gives users the ability to allocate space instantly. One way > to compare would be to time just fallocate() with another program > writing zeros for that length. > > But that's not the aim of the syscall. The aim is to allow the fs to > allocate > the space in as large chunks as possible to allow for better read > performance. Well, all fallocate is really -supposed- to do is guarantee that the space will be available for a future write. "After a successful call to posix_fallocate(), subsequent writes to bytes in the specified range are guaranteed not to fail because of lack of disk space." A practical side effect is that it is often more contiguous, but that is not guaranteed. It -could- return your allocated space in very fragmented extents. > If you don't do fallocate() and allow writes to allocate in small chunks, > as you are doing, the allocations on disks could be interleaved in face of > multiple processes doing the same. Fragmented allocations can only hurt > read performance. As you followed up in later emails, the original test case isn't going to show much if any difference; a 1G write is so small that it may well all turn into a single delalloc write anyway. Since ext4 maxes out at 128MB extents that's still several extents to allocate but it's not that much overhead. A more interesting test might be to do random writes into a large file, and compare preallocated vs. not-preallocated. Ext4 leaves physical gaps for logical gaps though, so even that may not show a huge difference in performance, esp. when you consider that the random writes will cause "fragmentation" anyway in terms of written- and unwritten-extents which must be converted ... -Eric -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
