What command do I use to see the start and end block of a file in the file system?
hdparm
I'm not 100% sure this is what you're looking for but I believe you can do this using the command hdparm, specifically with its --fibmap switch.
excerpt
--fibmap
When used, this must be the only option given. It requires a
file path as a parameter, and will print out a list of the block
extents (sector ranges) occupied by that file on disk. Sector
numbers are given as absolute LBA numbers, referenced from sector
0 of the physical device rather than from the partition or
filesystem. This information can then be used for a variety of
purposes, such as examining the degree of fragmenation of larger
files, or determining appropriate sectors to deliberately corrupt
during fault-injection testing procedures.
This option uses the new FIEMAP (file extent map) ioctl() when
available, and falls back to the older FIBMAP (file block
map) ioctl() otherwise. Note that FIBMAP suffers from a 32-bit
block-number interface, and thus not work beyond 8TB or 16TB.
FIBMAP is also very slow, and does not deal well with
preallocated uncommitted extents in ext4/xfs filesystems, unless a
sync() is done before using this option.
Example
Say we have a sample file.
$ echo "this is a test file" > afile
Now when we run hdparm.
$ sudo hdparm --fibmap afile
afile:
filesystem blocksize 4096, begins at LBA 0; assuming 512 byte sectors.
byte_offset begin_LBA end_LBA sectors
0 282439184 282439191 8
filefrag
Another nice method for finding out a file's beginning & ending blocks is filefrag. You'll need to use appropriate switches though, to get the desired output. One upside of this tool over hdparm is that any user can run it, so no sudo is required. You'll need to use the -b512 switch so that outputs are displayed in 512 byte blocks. Also we need to tell filefrag to be verbose.
Example
$ filefrag -b512 -v afile
Filesystem type is: ef53
File size of afile is 20 (8 block of 512 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 7: 282439184.. 282439191: 8: eof
afile: 1 extent found
debugfs
A third method for getting a file's LBAs is to make use of debugfs. This method will require a little math, but I thought it important to show how one can convert from the extents value reported by debugfs to LBAs, for those that might be curious.
So let's start with the file's inode.
$ ls -i afile
6560281 afile
NOTE: We could also use the file's name within debugfs but for this demonstration I'm going to use the inode instead.
Now let's get the stat information via debugfs about our inode.
$ sudo debugfs -R "stat <6560281>" /dev/mapper/fedora_greeneggs-home
debugfs 1.42.7 (21-Jan-2013)
Inode: 6560281 Type: regular Mode: 0664 Flags: 0x80000
Generation: 1999478298 Version: 0x00000000:00000001
User: 1000 Group: 1000 Size: 20
File ACL: 0 Directory ACL: 0
Links: 1 Blockcount: 8
Fragment: Address: 0 Number: 0 Size: 0
ctime: 0x52be10c3:a640e994 -- Fri Dec 27 18:44:03 2013
atime: 0x52bff8a1:a9f08020 -- Sun Dec 29 05:25:37 2013
mtime: 0x52be0fe7:18a2f344 -- Fri Dec 27 18:40:23 2013
crtime: 0x52be0dd8:64394b00 -- Fri Dec 27 18:31:36 2013
Size of extra inode fields: 28
Extended attributes stored in inode body:
selinux = "unconfined_u:object_r:user_home_t:s0\000" (37)
EXTENTS:
(0):35304898
The important information is in the extents section. These are actually filesystem blocks that are being used by this inode. We just need to convert them to LBA. We can do this through the following equation.
NOTE: Assuming that our filesystem uses 4k block sizes and that underlying hardware uses 512 byte units, we need to multiply the exents by 8.
beginning LBA = (BEGIN EXTENT) * 8
ending LBA = (((ENDING EXTENT) + 1) * 8) - 1
Example
So in our example our beginning and ending extent is the same, since our file fits within a single extent.
beginning LBA = 35304898 * 8 = 282439184
ending LBA = ((35304898 + 1) * 8) - 1 = 282439191
So our LBAs are 282439184..282439191.
References
- Finding what hard drive sectors occupy a file
- Identifying file associated with unreadable disk sector
- Bad block HOWTO for smartmontools
- C5170 Lecture notes -- Internal Representation of Files - The Unix File System
- Logical block addressing
- Ext4 Disk Layout
Sector number relative to the block device holding the FS (not whole disk)
(Note that hdparm --fibmap is relative to the whole disk, not the partition or whatever other blockdev holds the FS. It also requires root.)
filefrag -e works well, and uses the generic and efficient FIEMAP ioctl, so it should work on pretty much any filesystem (including the often-weird BTRFS, even for BTRFS-compressed files). It will fall back to FIBMAP for filesystems / kernels without FIEMAP support.
$ filefrag xpsp3.vdi # some old sparse disk image I had lying around
xpsp3.vdi: 110 extents found
$ filefrag -e xpsp3.vdi
Filesystem type is: 58465342
File size of xpsp3.vdi is 5368730112 (1310726 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 5: 1322629241..1322629246: 6:
1: 13.. 13: 1322620799..1322620799: 1: 1322629247:
2: 15.. 47: 1323459271..1323459303: 33: 1322620800:
...
160: 899498.. 915839: 1325792977..1325809318: 16342: 1325725438:
161: 1307294.. 1307391: 1323938199..1323938296: 98: 1325809319: last
xpsp3.vdi: 110 extents found
XFS-only
If you are using xfs, then xfs_bmap has nicer output: It shows you where there are holes, while filefrag just has the next extent starting at a later sector. It uses 512B blocks, not whatever the filesystem blocksize actually is. (typically 4k on Linux). It shows you which allocation-group each extent is in, and how it is aligned on RAID stripe boundaries.
$ xfs_bmap -vvpl xpsp3.vdi # the extra -v prints a key to the flags
xpsp3.vdi:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..47]: 10581033928..10581033975 13 (83912..83959) 48 01111
1: [48..103]: hole 56
2: [104..111]: 10580966392..10580966399 13 (16376..16383) 8 01010
3: [112..119]: hole 8
...
322: [10458352..10459135]: 10591505592..10591506375 13 (10555576..10556359) 784 01111
323: [10459136..10485807]: hole 26672
FLAG Values: # this part is only here with -vv
010000 Unwritten preallocated extent
001000 Doesn't begin on stripe unit
000100 Doesn't end on stripe unit
000010 Doesn't begin on stripe width
000001 Doesn't end on stripe width
-l is redundant when -v is used, but for some reason I always type -vpl. -pl is more compact output.
Both filefrag and xfs_bmap show you preallocated extents.
$ fallocate --length $((1024*1024*8)) prealloced_file
$ filefrag -e prealloced_file
Filesystem type is: 58465342
File size of prealloced_file is 8388608 (2048 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 2047: 1325371648..1325373695: 2048: last,unwritten,eof
prealloced_file: 1 extent found
$ xfs_bmap -vvpl prealloced_file
prealloced_file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..16383]: 10602973184..10602989567 13 (22023168..22039551) 16384 10010
FLAG Values:
010000 Unwritten preallocated extent
001000 Doesn't begin on stripe unit
000100 Doesn't end on stripe unit
000010 Doesn't begin on stripe width
000001 Doesn't end on stripe width
$ dd if=/dev/zero of=prealloced_file conv=notrunc bs=4k count=10 seek=10000
40960 bytes (41 kB) copied, 0.000335111 s, 122 MB/s
$ xfs_bmap -vpl prealloced_file
prealloced_file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..16383]: 10602973184..10602989567 13 (22023168..22039551) 16384 10010
1: [16384..79999]: hole 63616
2: [80000..80895]: 10603013120..10603014015 13 (22063104..22063999) 896 00111
# oops, wrote past EOF and extended the file, instead of in the middle of the preallocated extent
$ dd if=/dev/zero of=prealloced_file conv=notrunc bs=4k count=10 seek=100
40960 bytes (41 kB) copied, 0.000212986 s, 192 MB/s
$ xfs_bmap -vpl prealloced_file
prealloced_file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..16383]: 10602973184..10602989567 13 (22023168..22039551) 16384 10010
1: [16384..79999]: hole 63616
2: [80000..80895]: 10603013120..10603014015 13 (22063104..22063999) 896 00111
# If you check *right away*, XFS's delayed allocation hasn't happened yet.
# FIEMAP on xfs only reflects allocations, which lag behind completed writes. fsync first if you need it, IIRC.
$ xfs_bmap -vpl prealloced_file
prealloced_file:
EXT: FILE-OFFSET BLOCK-RANGE AG AG-OFFSET TOTAL FLAGS
0: [0..799]: 10602973184..10602973983 13 (22023168..22023967) 800 10111
1: [800..879]: 10602973984..10602974063 13 (22023968..22024047) 80 01111
2: [880..16383]: 10602974064..10602989567 13 (22024048..22039551) 15504 11010
3: [16384..79999]: hole 63616
4: [80000..80895]: 10603013120..10603014015 13 (22063104..22063999) 896 00111
$ filefrag -e prealloced_file
Filesystem type is: 58465342
File size of prealloced_file is 41000960 (10010 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 99: 1325371648..1325371747: 100: unwritten
1: 100.. 109: 1325371748..1325371757: 10:
2: 110.. 2047: 1325371758..1325373695: 1938: unwritten
3: 10000.. 10111: 1325376640..1325376751: 112: 1325373696: last,eof
prealloced_file: 2 extents found
hdparm --fibmap is only useful if you want a sector number relative to the entire hard drive, not within the partition the filesystem is on. It doesn't work on top of software RAID (or presumably anything else between the filesystem and a hard drive). It also requires root. Despite the name of the option, it actually uses FIEMAP when available (the newer extent-map ioctl, not the old slow block-map ioctl).
# hdparm --fibmap ..../xpsp3.vdi
Unable to determine start offset LBA for device, aborting.