Christoph Anton Mitterer posted on Wed, 16 Dec 2015 22:59:01 +0100 as excerpted:
> I'm a bit unsure how to read filefrag's output... (even in the > uncompressed case). > What would it show me if there was fragmentation /path/to/file: 18 extents found It tells you the number of extents found. Nominally, each extent should be a fragment, but as has been discussed elsewhere, on btrfs compressed files it will interpret each 128 KiB btrfs compression block as its own extent, even if (as seen in verbose mode) the next one begins where the previous one ends so it's really just a single extent. Apparently on ext3/4, it's possible to have multi-gig files as a single extent, thus unfragmented, but as explained in an earlier reply to a point earlier in your post, on btrfs, extents of a GiB are nominally the best you can do as that's the nominal data chunk size, tho in limited circumstances larger extents are still possible on btrfs. In the case above, where I took the 18 extents result from a real file (tho obviously the posted path isn't real), it was 4 MiB in size (I think exactly, it's a 4 MiB BIOS image =:^), so doing the math, extents average 227 KiB. That's on a filesystem that is always mounted with autodefrag, but it's also always mounted with compress, so it's possible some of the reported extents are compressed. Actually, looking at filefrag -v output (which I've never used before but which someone noted could be used to check fragmentation on compressed files, tho it's not as straightforward as you might think), it looks like all but two of the listed extents are 32 blocks long (with 4096 byte blocks), which equates to 128 KiB, the btrfs compression-block size, and the two remaining extents are 224 blocks long or 896 KiB, an exact 7 multiple of 128 KiB, so this file would indeed appear to be compressed except for those two uncompressed extents. (As for figuring out how to interpret the full -v output to know whether the compressed blocks are actually single extents or not, as I said this is my first time trying -v, and I didn't bother going that far with it.) -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
