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btrfs-defrag-plugin: Check fragmentation level before filling up disk #43

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StefanBruens opened this issue Jan 23, 2018 · 5 comments
Open

btrfs-defrag-plugin: Check fragmentation level before filling up disk #43

StefanBruens opened this issue Jan 23, 2018 · 5 comments
Labels
enhancement
Milestone
v0.4.1

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@StefanBruens
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The defragmentation is run unconditionally, even if the majority of extens is still large, e.g.:
` /usr/sbin/filefrag -v /usr/lib/sysimage/rpm/Packages

Filesystem type is: 9123683e
File size of /usr/lib/sysimage/rpm/Packages is 129085440 (31515 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:   47234564..  47234564:      1:             shared
   1:        1..       1:   47234562..  47234562:      1:   47234565: shared
   2:        2..    2140:   47243301..  47245439:   2139:   47234563: shared
   3:     2141..    4031:   47291194..  47293084:   1891:   47245440: shared
   4:     4032..    4369:   47340585..  47340922:    338:   47293085: shared
   5:     4370..    4370:   47332867..  47332867:      1:   47340923: shared
   6:     4371..    5895:   47340924..  47342448:   1525:   47332868: shared
   7:     5896..    7626:   47527000..  47528730:   1731:   47342449: shared
   8:     7627..    9317:   47759384..  47761074:   1691:   47528731: shared
   9:     9318..   10923:   47809669..  47811274:   1606:   47761075: shared
  10:    10924..   12474:   48474629..  48476179:   1551:   47811275: shared
  11:    12475..   14655:   41523831..  41526011:   2181:   48476180: shared
  12:    14656..   14658:   41509577..  41509579:      3:   41526012: shared
  13:    14659..   14800:   41526015..  41526156:    142:   41509580: shared
  14:    14801..   17127:   44118314..  44120640:   2327:   41526157: shared
  15:    17130..   18564:   14822118..  14823552:   1435:   44120643: shared
  16:    18565..   19997:   15854971..  15856403:   1433:   14823553: shared
  17:    19998..   21397:   35984576..  35985975:   1400:   15856404: shared
  18:    21398..   22713:   36143758..  36145073:   1316:   35985976: shared
  19:    22714..   24330:   44172104..  44173720:   1617:   36145074: shared
  20:    24331..   24331:   44122071..  44122071:      1:   44173721: shared
  21:    24332..   25342:   44173722..  44174732:   1011:   44122072: shared
  22:    25343..   26656:   44174906..  44176219:   1314:   44174733: shared
  23:    26657..   27870:   46696520..  46697733:   1214:   44176220: shared
  24:    27871..   29692:   47173682..  47175503:   1822:   46697734: shared
  25:    29693..   31514:   47203363..  47205184:   1822:   47175504: last,shared,eof
/usr/lib/sysimage/rpm/Packages: 26 extents found

This is after installing a single package, with deactivated defrag plugin

Defragmentation causes high disk usage (man btrfs-filesystem):

Warning
Defragmenting with Linux kernel versions < 3.9 or ≥ 3.14-rc2 as well as with Linux stable
kernel versions ≥ 3.10.31, ≥ 3.12.12 or ≥ 3.13.4 will break up the reflinks of COW data
(for example files copied with cp --reflink, snapshots or de-duplicated data). This may
cause considerable increase of space usage depending on the broken up reflinks.
@kdave
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kdave commented Jan 23, 2018

The defrag in kernel will check if the extent size is smaller than the desired size (which is 32MiB as set by the plugin) so the reflink breaking should not happen for the large extents.

@StefanBruens
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$> /usr/sbin/filefrag -v /usr/lib/sysimage/rpm/Packages 

Filesystem type is: 9123683e
File size of /usr/lib/sysimage/rpm/Packages is 119873536 (29266 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..       0:   41431285..  41431285:      1:             shared
   1:        1..       1:   41431284..  41431284:      1:   41431286: shared
   2:        2..    2005:   41418336..  41420339:   2004:   41431285: shared
   3:     2006..    3869:   41485147..  41487010:   1864:   41420340: shared
   4:     3870..    5660:   42789245..  42791035:   1791:   41487011: shared
   5:     5661..    7351:   43204908..  43206598:   1691:   42791036: shared
   6:     7352..    7827:   47230750..  47231225:    476:   43206599: shared
   7:     7828..    7828:   47202769..  47202769:      1:   47231226: shared
   8:     7829..    8957:   47231227..  47232355:   1129:   47202770: shared
   9:     8958..   10529:   47431169..  47432740:   1572:   47232356: shared
  10:    10530..   11910:   48537624..  48539004:   1381:   47432741: shared
  11:    11911..   12945:   48681080..  48682114:   1035:   48539005: shared
  12:    12946..   14499:   48885660..  48887213:   1554:   48682115: shared
  13:    14500..   16045:   41145954..  41147499:   1546:   48887214: shared
  14:    16046..   16046:   41134733..  41134733:      1:   41147500: shared
  15:    16047..   16053:   41147501..  41147507:      7:   41134734: shared
  16:    16057..   17063:   41427289..  41428295:   1007:   41147511: shared
  17:    17064..   18062:   41595470..  41596468:    999:   41428296: shared
  18:    18063..   19060:   41601520..  41602517:    998:   41596469: shared
  19:    19061..   20019:   42776324..  42777282:    959:   41602518: shared
  20:    20020..   20939:   42868357..  42869276:    920:   42777283: shared
  21:    20940..   21855:   43950672..  43951587:    916:   42869277: shared
  22:    21856..   22752:   44058986..  44059882:    897:   43951588: shared
  23:    22753..   24496:   41403185..  41404928:   1744:   44059883: shared
  24:    24497..   25368:   44114935..  44115806:    872:   41404929: shared
  25:    25369..   26237:   44153880..  44154748:    869:   44115807: shared
  26:    26238..   27749:   47353469..  47354980:   1512:   44154749: shared
  27:    27750..   28505:   47336920..  47337675:    756:   47354981: shared
  28:    28506..   29261:   47384779..  47385534:    756:   47337676: shared
  29:    29262..   29265:   47365639..  47365642:      4:   47385535: last,shared,eof
/usr/lib/sysimage/rpm/Packages: 30 extents found

$> sudo btrfs filesystem defragment -v -f -r -t $((64*1024*1024)) /usr/lib/sysimage/rpm/Packages
$> /usr/sbin/filefrag -v /usr/lib/sysimage/rpm/Packages Filesystem type is: 9123683e

File size of /usr/lib/sysimage/rpm/Packages is 119873536 (29266 blocks of 4096 bytes)
 ext:     logical_offset:        physical_offset: length:   expected: flags:
   0:        0..    1002:   47803913..  47804915:   1003:             shared
   1:     1003..    2001:   48325808..  48326806:    999:   47804916: shared
   2:     2002..    2999:   48514434..  48515431:    998:   48326807: shared
   3:     3000..    3958:   48682115..  48683073:    959:   48515432: shared
   4:     3959..    4878:   48685174..  48686093:    920:   48683074: shared
   5:     4879..    5746:   48732345..  48733212:    868:   48686094: shared
   6:     5747..    6600:   48750367..  48751220:    854:   48733213: shared
   7:     6601..    7438:   48826925..  48827762:    838:   48751221: shared
   8:     7439..    8226:   48961438..  48962225:    788:   48827763: shared
   9:     8227..    8951:   49060434..  49061158:    725:   48962226: shared
  10:     8952..    9668:   12734017..  12734733:    717:   49061159: shared
  11:     9669..   10375:   15088029..  15088735:    707:   12734734: shared
  12:    10376..   11059:   15090851..  15091534:    684:   15088736: shared
  13:    11060..   11725:   15470301..  15470966:    666:   15091535: shared
  14:    11726..   13049:   37246799..  37248122:   1324:   15470967: shared
  15:    13050..   13711:   37296116..  37296777:    662:   37248123: shared
  16:    13712..   14882:   37327447..  37328617:   1171:   37296778: shared
  17:    14883..   16053:   37371038..  37372208:   1171:   37328618: shared
  18:    16057..   16715:   44053314..  44053972:    659:   37372212: shared
  19:    16716..   17371:   46584234..  46584889:    656:   44053973: shared
  20:    17372..   18017:   46725185..  46725830:    646:   46584890: shared
  21:    18018..   18655:   47581014..  47581651:    638:   46725831: shared
  22:    18656..   19289:   47685701..  47686334:    634:   47581652: shared
  23:    19290..   20552:   39101005..  39102267:   1263:   47686335: shared
  24:    20553..   21183:   39267296..  39267926:    631:   39102268: shared
  25:    21184..   22438:   36158632..  36159886:   1255:   39267927: shared
  26:    22439..   23065:   37296778..  37297404:    627:   36159887: shared
  27:    23066..   24319:   41290100..  41291353:   1254:   37297405: shared
  28:    24320..   24946:   41350627..  41351253:    627:   41291354: shared
  29:    24947..   25572:   41432985..  41433610:    626:   41351254: shared
  30:    25573..   26823:   41213704..  41214954:   1251:   41433611: shared
  31:    26824..   27433:   41438670..  41439279:    610:   41214955: shared
  32:    27434..   28349:   16016876..  16017791:    916:   41439280:
  33:    28350..   29265:   31404890..  31405805:    916:   16017792: last,eof
/usr/lib/sysimage/rpm/Packages: 34 extents found

@kdave
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kdave commented Jan 23, 2018

So the target size of 64M (I was mistaken about 32M, that was default of defrag itself) is too much on an average aged filesystem and leads to overwriting the whole file and breaking the reflinks. At mininimum we can reduce it to 4M. An extent layout analysis or smarter defrag would be better in the long run.

@kdave
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kdave commented Jan 30, 2018

I've checked one of my rolling distro installations where the system gets updated frequently and the rpm database is usually fragmented. There were way more fragments of all sizes. The post-update defragmentation was able to suqeeze the whole file to larger blocks. So I think the fragmentation itself is not enough to determine if the defragmentation should be done or not. Changing the target extent size to 32M could produce a similar result so I don't think this would make the situation worse in the above case.

kdave added a commit that referenced this issue Mar 6, 2018
@kdave
RPM defrag plugin: lower the target extent size from 64 MiB to 32 MiB
82974d7 
The RPM database becomes fragmented in any case and the free space tends
to be fragmented as well over time, so the 64MB target is too ambitious.

Besides the fragmentation itself, the shared extents caused by
snapshotting of the underlying subvolume will become unshared after
defragmentation. This will consume more space so the lower target will
also possibly reduce the unsharing.

We'd need a better extent analysis, the defrag still seems too rough.

Issue: #43
Signed-off-by: David Sterba <dsterba@suse.com>
@kdave kdave added this to the v0.4.1 milestone Mar 6, 2018
@kdave
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kdave commented Mar 6, 2018

I'm going to set the target size to 32MiB and keep the issue open as the extent analysis is a better solution.

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