Weekly edition	Kernel	Security	Distributions	Contact Us	Search
Archives	Calendar	Subscribe	Write for LWN	LWN.net FAQ	Sponsors

I/O scheduler performance: not good enough!

Posted Aug 24, 2010 21:53 UTC (Tue) by mhelsley (subscriber, #11324)
In reply to: I/O scheduler performance: not good enough! by wmf
Parent article: A systemd status update

I don't think FIEMAP provides block numbers. Block numbers would be unique on the partition/device whereas I think FIEMAP provides "extents" (and FIBMAP provides bits) which effectively describe offsets within the file -- not block numbers.

http://lwn.net/Articles/260803/

(Log in to post comments)

Posted Aug 25, 2010 11:28 UTC (Wed) by etienne (subscriber, #25256) [Link]

Both FIEMAP and FIBMAP report block numbers, i.e. the position of the file on the device.
You have to guess yourself the position of the device on the hardware device, simple if they are the same of there is a simple partition table - difficult if there is LVM or MD(RAID) in between.
FIEMAP is a lot quicker than FIBMAP, noticeable when getting the mapping of ISO file images.
As an example:

$ wget http://www.mirrorservice.org/sites/download.sourceforge.n...
$ gcc -O2 showmap.c -o showmap
$ ./showmap ./showmap.c
File "./showmap.c" of size 15013 (32 blocks512) is on filesystem 0x302.
According to /proc/diskstats, file './showmap.c' is on device '/dev/hda2'
/dev/hda2: Permission denied
$ su
Password:
# ./showmap ./showmap.c
File "./showmap.c" of size 15013 (32 blocks512) is on filesystem 0x302.
According to /proc/diskstats, file './showmap.c' is on device '/dev/hda2'
Device block size: 512, FS block size: 4096, device size: 61432560 blocks
Device length: 31453470720 bytes
The device start at 61432560 sectors, C/H/S: 65535/16/63.
First FIEMAP says 1 extents
second FIEMAP success, 1 extents filled:
0: logical offset 0 (0 * 4096), physical offset 25904218112 (6324272 * 4096),
length 16384 (4 * 4096) flags 0x1
flags meaning: FIEMAP_EXTENT_LAST 0x1, FIEMAP_EXTENT_UNKNOWN 0x2,
FIEMAP_EXTENT_DELALLOC 0x4, FIEMAP_EXTENT_ENCODED 0x8,
FIEMAP_EXTENT_DATA_ENCRYPTED 0x0, FIEMAP_EXTENT_NOT_ALIGNED 0x100,
FIEMAP_EXTENT_DATA_INLINE 0x200, FIEMAP_EXTENT_DATA_TAIL 0x400,
FIEMAP_EXTENT_UNWRITTEN 0x800, FIEMAP_EXTENT_MERGED 0x1000.
FIGETBSZ: block size 4096 bytes
File (4 blocks of 4096) start at block 6324272 for 4 blocks,
last block 6324275 and file has 1 fragments.
FIBMAP succeeded after end of file, block index 4 give block 0
#

I/O scheduler performance: not good enough!

Posted Aug 26, 2010 12:48 UTC (Thu) by renox (subscriber, #23785) [Link]

Interesting, thanks.
It didn't work on my computer though:
>>
./showmap ./showmap
Boot record of size 512 bytes read successfully from './showmap'
First bytes 0x7F 0x45 0x4C, signature 0x804, FAT16 sig 0x0, FAT32 sig 0x4,
No FAT signature recognised, cannot analyse header.
Partition table: WindowsNTmarker 0x3C, Unknown 0x0, signature 0x804
0: indicator 0x0 i.e. non bootable, start 5570560 length 0
(i.e. end at 5570560); start 513/0/43 end 0/0/18 name 'empty'
1: indicator 0x0 i.e. non bootable, start 3014656 length 0
(i.e. end at 3014656); start 768/0/29 end 0/0/18 name 'empty'
2: indicator 0x0 i.e. non bootable, start 6750208 length 0
(i.e. end at 6750208); start 0/0/54 end 0/0/18 name 'empty'
3: indicator 0x0 i.e. non bootable, start 4587520 length 2438987776
(i.e. end at 2443575296); start 256/0/60 end 0/0/18 name 'empty'
<<
Probably a not recent enough version..

I/O scheduler performance: not good enough!

Posted Aug 26, 2010 13:33 UTC (Thu) by etienne (subscriber, #25256) [Link]

Sorry, said to compile showmap.c and gave the link to showmbr.c, the good link is:
http://www.mirrorservice.org/sites/download.sourceforge.n...

I/O scheduler performance: not good enough!

Posted Aug 29, 2010 10:38 UTC (Sun) by renox (subscriber, #23785) [Link]

Thanks, I should have seen this but the long URL was shortened which hid the issue.
I'll try again.