sparse files

[Alexander 'Leo' Bergolth]

Hi!

I am trying to create a sparse qemu qcow2 image on xfs. (Centos 6.3)
Unfortunately this doesn't produce the expected results:
Creating a 10G file results in 16GB of disk usage!
Doing the same on ext4 produces a 10G sparse file with only 1.8M allocated...
(See the block bitmaps below.)

Is this related to dynamic speculative preallocation?
http://serverfault.com/questions/406069/why-are-my-xfs-filesystems-suddenly-consuming-more-space-and-full-of-sparse-file

Are there any recommended filesystem options that optimize xfs behavior for hosting (sparse) virtual disk images?

Cheers,
--leo

-------------------- 8< --------------------
$  qemu-img create -f qcow2 -o 'preallocation=metadata' onxfs.img 10G
Formatting 'onxfs.img', fmt=qcow2 size=10737418240 encryption=off cluster_size=65536 preallocation='metadata' 
$ du -hs onxfs.img 16G     onxfs.img

$ ls -ls onxfs.img 16253692 -rw-r--r-- 1 leo users 10739318784 Oct  2 14:22 onxfs.img

# xfs_bmap -v onxfs.img onxfs.img:
 EXT: FILE-OFFSET           BLOCK-RANGE          AG AG-OFFSET              TOTAL
   0: [0..1023]:            168294344..168295367  3 (11007944..11008967)    1024
   1: [1024..1049215]:      hole                                         1048192
   2: [1049216..1049727]:   168295368..168295879  3 (11008968..11009479)     512
   3: [1049728..2097919]:   hole                                         1048192
   4: [2097920..3146495]:   165668600..166717175  3 (8382200..9430775)   1048576
   5: [3146496..3146623]:   hole                                             128
   6: [3146624..5243775]:   163571448..165668599  3 (6285048..8382199)   2097152
   7: [5243776..5244159]:   hole                                             384
   8: [5244160..9438463]:   159377144..163571447  3 (2090744..6285047)   4194304
   9: [9438464..9439103]:   hole                                             640
  10: [9439104..17827711]:  52616968..61005575    1 (188168..8576775)    8388608
  11: [17827712..17828991]: hole                                            1280
  12: [17828992..20975231]: 105886128..109032367  2 (1028528..4174767)   3146240

# xfs_info /srv
meta-data=/dev/mapper/vg_1-lv_srv isize=256    agcount=4, agsize=6553600 blks
         =                       sectsz=512   attr=2
data     =                       bsize=4096   blocks=26214400, imaxpct=25
         =                       sunit=0      swidth=0 blks
naming   =version 2              bsize=4096   ascii-ci=0
log      =internal               bsize=4096   blocks=12800, version=2
         =                       sectsz=512   sunit=0 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0

-------------------- 8< --------------------
$ qemu-img create -f qcow2 -o 'preallocation=metadata' onext4.img 10G
Formatting 'onext4.img', fmt=qcow2 size=10737418240 encryption=off cluster_size=65536 preallocation='metadata' 
$ du -h onext4.img 1.8M    onext4.img

$ ls -ls onext4.img 1744 -rw-r--r-- 1 leo users 10739318784 Oct  2 14:25 onext4.img

debugfs:  stat /leo/onext4.img
Inode: 1048578   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 3370152928    Version: 0x00000000:00000001
User:   501   Group:   100   Size: 10739318784
File ACL: 0    Directory ACL: 0
Links: 1   Blockcount: 3488
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x506add38:7909fe14 -- Tue Oct  2 14:25:28 2012
 atime: 0x506add38:731c2ccc -- Tue Oct  2 14:25:28 2012
 mtime: 0x506add38:7909fe14 -- Tue Oct  2 14:25:28 2012
crtime: 0x506add36:5320b76c -- Tue Oct  2 14:25:26 2012
Size of extra inode fields: 28
EXTENTS:
(0): 33856, (16): 33872, (32-79): 33888-33935, (131152-131167): 34896-34911, (262240-262255): 36960-36975, (393328-393343): 39024-39039, (524416-524447): 41088-41119, (655520-655535): 43168-43183, (786608-786623): 45232-45247, (917696-917711): 47296-47311, (1048784-1048815)
: 49360-49391, (1179888-1179903): 51440-51455, (1310976-1310991): 53504-53519, (1442064-1442079): 55568-55583, (1573152-1573183): 57632-5
7663, (1704256-1704271): 59712-59727, (1835344-1835359): 61776-61791, (1966432-1966447): 63840-63855, (2097520-2097551): 65904-65935, (22
28624-2228639): 67984-67999, (2359712-2359727): 70048-70063, (2490800-2490815): 72112-72127, (2621887-2621903): 74175-74191


-- 
e-mail   ::: Leo.Bergolth (at) wu.ac.at   fax      ::: +43-1-31336-906050
location ::: IT-Services | Vienna University of Economics | Austria