Nicholas D Steeves posted on Wed, 27 Jul 2016 13:19:01 -0400 as excerpted: > Is there any reason why defrag without -t cannot detect and default to > the data chunk size, or why it does not default to 1 GiB?
I don't know the answer, but have wondered that myself. 256 KiB seems a rather small default, to me. I'd expect something in the MiB range, at least, maybe the same 2 MiB that modern partitioners tend to use for alignment, for the same reason, that tends to be a reasonable whole multiple of most erase-block sizes, and if the partition is aligned, should prevent unnecessary read-modify-write cycles on ssd, and help with tiled zones on SMR drives as well. As to the question in the subject line, AFAIK btrfs fi defrag works on extents, not filesize per-se, so using the default 256 KiB target, yes it'll defrag files larger than that, but only for extents that are smaller than that. If all the extents are 256 KiB plus, defrag won't do anything with it without a larger target option or unless the compress option is also used, in which case it rewrites everything it is pointed at, in ordered to recompress it. Talking about compression, it's worth mentioning that filefrag doesn't understand btrfs compression either, and will count each 128 KiB (uncompressed size) compression block as a separate extent. To get the true picture using filefrag, you need to use verbose and either eyeball the results manually or feed them into a script that processes the numbers and combines "extents" if they are reported as immediately consecutive on the filesystem. As such, filefrag, given the regular opportunistic compression, turns out to be a good method of determining whether a file (of over 128 KiB in size) is actually compressed or not, since if it is filefrag will report multiple 128 KiB extents, while if it's not, extent size should be much less regular, likely with larger extents unless the file is often modified and rewritten in-place. -- 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
