On Wed, Apr 03, 2024 at 05:49:42PM +1030, Qu Wenruo wrote: > > > 在 2024/4/3 16:32, Sweet Tea Dorminy 写道: > >>> This means, we will emit a entry that uses the end to the physical > >>> extent end. > >>> > >>> Considering a file layout like this: > >>> > >>> item 6 key (257 EXTENT_DATA 0) itemoff 15816 itemsize 53 > >>> generation 7 type 1 (regular) > >>> extent data disk byte 13631488 nr 65536 > >>> extent data offset 0 nr 4096 ram 65536 > >>> extent compression 0 (none) > >>> item 7 key (257 EXTENT_DATA 4096) itemoff 15763 itemsize 53 > >>> generation 8 type 1 (regular) > >>> extent data disk byte 13697024 nr 4096 > >>> extent data offset 0 nr 4096 ram 4096 > >>> extent compression 0 (none) > >>> item 8 key (257 EXTENT_DATA 8192) itemoff 15710 itemsize 53 > >>> generation 7 type 1 (regular) > >>> extent data disk byte 13631488 nr 65536 > >>> extent data offset 8192 nr 57344 ram 65536 > >>> extent compression 0 (none) > >>> > >>> For fiemap, we would got something like this: > >>> > >>> fileoff 0, logical len 4k, phy 13631488, phy len 64K > >>> fileoff 4k, logical len 4k, phy 13697024, phy len 4k > >>> fileoff 8k, logical len 56k, phy 13631488 + 8k, phylen 56k > >>> > >>> [HOW TO CALCULATE WASTED SPACE IN USER SPACE] > >>> My concern is on the first entry. It indicates that we have wasted > >>> 60K (phy len is 64K, while logical len is only 4K) > >>> > >>> But that information is not correct, as in reality we only wasted 4K, > >>> the remaining 56K is still referred by file range [8K, 64K). > >>> > >>> Do you mean that user space program should maintain a mapping of each > >>> utilized physical range, and when handling the reported file range > >>> [8K, 64K), the user space program should find that the physical range > >>> covers with one existing extent, and do calculation correctly? > >> > >> My goal is to give an unprivileged interface for tools like compsize > >> to figure out how much space is used by a particular set of files. > >> They report the total disk space referenced by the provided list of > >> files, currently by doing a tree search (CAP_SYS_ADMIN) for all the > >> extents pertaining to the requested files and deduplicating extents > >> based on disk_bytenr. > >> > >> It seems simplest to me for userspace for the kernel to emit the > >> entire extent for each part of it referenced in a file, and let > >> userspace deal with deduplicating extents. This is also most similar > >> to the existing tree-search based interface. Reporting whole extents > >> gives more flexibility for userspace to figure out how to report > >> bookend extents, or shared extents, or ... > >> > >> It does seem a little weird where if you request with fiemap only e.g. > >> 4k-16k range in that example file you'll get reported all 68k > >> involved, but I can't figure out a way to fix that without having the > >> kernel keep track of used parts of the extents as part of reporting, > >> which sounds expensive. > >> > >> You're right that I'm being inconsistent, taking off extent_offset > >> from the reported disk size when that isn't what I should be doing, so > >> I fixed that in v3. > > > > Ah, I think I grasp a point I'd missed before. > > - Without setting disk_bytenr to the actual start of the data on disk, > > there's no way to find the location of the actual data on disk within > > the extent from fiemap alone > > Yes, that's my point. > > > - But reporting disk_bytenr + offset, to get actual start of data on > > disk, means we need to report a physical size to figure out the end of > > the extent and we can't know the beginning. > > disk_bytenr + offset + disk_num_bytes, and with the existing things like > length (aka, num_bytes), filepos (aka, key.offset) flags > (compression/hole/preallocated etc), we have everything we need to know > for regular extents. > > For compressed extents, we also need ram_bytes. > > If you ask me, I'd say put all the extra members into fiemap entry if we > have the space... > > It would be u64 * 4 if we go 1:1 on the file extent items, otherwise we > may cheap on offset and ram_bytes (u32 is enough for btrfs at least), in > that case it would be u64 * 2 + u32 * 2. > > But I'm also 100% sure, the extra members would not be welcomed by other > filesystems either. That's probably right, too many btrfs-specific information in the generic FIEMAP, but we may also do our own enhanced fiemap ioctl that would provide all the information you suggest and we'd be free to put the compression information there too.
