Hey Ted. Thanks for your detailed reply. I took the liberty to forward that to linux-btrfs just in case they wouldn't know already.
As for this bug... as you prefer, we could either let it open until this should ever get resolved... or we could mark it wontfix till then. Cheers, Chris On Fri, 2015-12-18 at 18:25 -0500, Theodore Ts'o wrote: > On Fri, Dec 18, 2015 at 01:16:14AM +0100, Christoph Anton Mitterer > wrote: > > > > It would be nice if filefrag supports btrfs compression. > > Right now, it seems to assume that each 128KiB compression "block" > > is one extent, though, AFAIU discussion on linux-btrfs, > > it's in realliy one extent. > > How, exactly, do you propose that filefrag understand this? It is > getting the information from the fiemap ioctl: > > https://www.kernel.org/doc/Documentation/filesystems/fiemap.txt > > and the problem is that fiemap returns the logical length of the > extent, but not the *physical* length of the extent. So this is why > the filefrag is show what appears to be overlapping extents: > > > ext: logical_offset: physical_offset: > > length: expected: flags: > > 0: 0.. 31: 3072.. 3103: 32: > > encoded > > 1: 32.. 63: 3078.. 3109: 32: 310 > > 4: encoded > > Presumably, the first extent for logical blocks 0 -- 31 is taking > only > 6 blocks --- but there's no way filefrag can know that, not having > any > magical or psychic abilities. It could be that that the blocks only > take 5 blocks, and there is a one block "hole" meaning there was true > fragmentation --- but there's no way to tell that, and having > filefrag > send e-mail to start a discussion on linux-btrfs is obviously not > something that is going to work given the current state of Artificial > Intelligence. :-) > > We could display: > > ext: logical_offset: physical_offset: > length: expected: flags: > 0: 0.. 31: 3072.. ????: 32: > encoded > 1: 32.. 63: 3078.. ????: 32: 3104 > : encoded > ... > > ... since the encoded flag means that it is possible that physical > length is^H^H^H might be different from the logical length. But even > that is not guaranteed; it could be that the blocks are encrypted, > and > the physical length and logical lengths of the extent are the same -- > - > all FM_ENCODED means is that it is not safe to try to read the data > blocks directly if you expect to get the actual data. (One of the > uses of fiemap is for bootloaders who want to store the block list so > they can boot the system without understanding the low-level file > system.) > > So I'm afraid there's not much I can do here. My suggestion is that > you kick off a discussion on linux-btrfs about adding a new, expanded > ioctl to the kernel that provides a superset of the FIEMAP ioctl, > which also returns the physical length. > > Best regards, > > - Ted
smime.p7s
Description: S/MIME cryptographic signature
