On 2/7/21 11:06 PM, Chris Murphy wrote:
systemd-journald journals on Btrfs default to nodatacow, upon log
rotation it's submitted for defragmenting with BTRFS_IOC_DEFRAG. The
result looks curious. I can't tell what the logic is from the results.
The journal file starts out being fallocated with a size of 8MB, and
as it grows there is an append of 8MB increments, also fallocated.
This leads to a filefrag -v that looks like this (ext4 and btrfs
nodatacow follow the same behavior, both are provided for reference):
ext4
https://pastebin.com/6vuufwXt
btrfs
https://pastebin.com/Y18B2m4h
Following defragment with BTRFS_IOC_DEFRAG it looks like this:
https://pastebin.com/1ufErVMs
It appears at first glance to be significantly more fragmented. Closer
inspection shows that most of the extents weren't relocated. But
what's up with the peculiar interleaving? Is this an improvement over
the original allocation?
I am not sure how read the filefrag output: I see several lines like
[...]
5: 1691.. 1693: 125477.. 125479: 3:
6: 1694.. 1694: 125480.. 125480: 1: unwritten
[...]
What means "unwritten" ? The kernel documentation [*] says:
[...]
* FIEMAP_EXTENT_UNWRITTEN
Unwritten extent - the extent is allocated but its data has not been
initialized. This indicates the extent's data will be all zero if read
through the filesystem but the contents are undefined if read directly from
the device.
[..]
So it seems that the data didn't touch the platters (!)
My educate guess is that there is something strange in the sequence:
- write
- sync
- close log
- move log
- defrag log
May be the defrag starts before all the data reach the platters ?
For what matters, I create a file with the same fragmentation like your one
$ sudo filefrag -v data.txt
Filesystem type is: 9123683e
File size of data.txt is 25165824 (6144 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 0: 1597171.. 1597171: 1:
1: 1.. 1599: 163433285.. 163434883: 1599: 1597172:
2: 1600.. 1607: 1601255.. 1601262: 8: 163434884:
3: 1608.. 1689: 1604137.. 1604218: 82: 1601263:
4: 1690.. 1690: 1597484.. 1597484: 1: 1604219:
5: 1691.. 1693: 1597465.. 1597467: 3: 1597485:
6: 1694.. 1694: 1597966.. 1597966: 1: 1597468:
7: 1695.. 1722: 1599557.. 1599584: 28: 1597967:
8: 1723.. 1723: 1599211.. 1599211: 1: 1599585:
9: 1724.. 1955: 1648394.. 1648625: 232: 1599212:
10: 1956.. 1956: 1599695.. 1599695: 1: 1648626:
11: 1957.. 2047: 1625881.. 1625971: 91: 1599696:
12: 2048.. 2417: 1648804.. 1649173: 370: 1625972:
13: 2418.. 2420: 1597468.. 1597470: 3: 1649174:
14: 2421.. 2478: 1624667.. 1624724: 58: 1597471:
15: 2479.. 2479: 1596416.. 1596416: 1: 1624725:
16: 2480.. 2482: 1601045.. 1601047: 3: 1596417:
17: 2483.. 2483: 1596854.. 1596854: 1: 1601048:
18: 2484.. 2523: 1602715.. 1602754: 40: 1596855:
19: 2524.. 2527: 1597471.. 1597474: 4: 1602755:
20: 2528.. 2598: 1624725.. 1624795: 71: 1597475:
21: 2599.. 2599: 1596858.. 1596858: 1: 1624796:
22: 2600.. 2607: 1601263.. 1601270: 8: 1596859:
23: 2608.. 2608: 1596863.. 1596863: 1: 1601271:
24: 2609.. 2611: 1601271.. 1601273: 3: 1596864:
25: 2612.. 2612: 1596864.. 1596864: 1: 1601274:
26: 2613.. 2615: 1601274.. 1601276: 3: 1596865:
27: 2616.. 2616: 1596981.. 1596981: 1: 1601277:
28: 2617.. 2691: 1649174.. 1649248: 75: 1596982:
29: 2692.. 2696: 1597475.. 1597479: 5: 1649249:
30: 2697.. 2756: 1634995.. 1635054: 60: 1597480:
31: 2757.. 2758: 1597480.. 1597481: 2: 1635055:
32: 2759.. 2762: 1601351.. 1601354: 4: 1597482:
33: 2763.. 2764: 1597482.. 1597483: 2: 1601355:
34: 2765.. 2837: 1649249.. 1649321: 73: 1597484:
35: 2838.. 2838: 1597038.. 1597038: 1: 1649322:
36: 2839.. 2855: 1601538.. 1601554: 17: 1597039:
37: 2856.. 2856: 1597045.. 1597045: 1: 1601555:
38: 2857.. 2904: 1624547.. 1624594: 48: 1597046:
39: 2905.. 2926: 1600795.. 1600816: 22: 1624595:
40: 2927.. 2942: 1602034.. 1602049: 16: 1600817:
41: 2943.. 2963: 1600817.. 1600837: 21: 1602050:
42: 2964.. 2979: 1602183.. 1602198: 16: 1600838:
43: 2980.. 3001: 1600927.. 1600948: 22: 1602199:
44: 3002.. 3043: 1621164.. 1621205: 42: 1600949:
45: 3044.. 3053: 1599231.. 1599240: 10: 1621206:
46: 3054.. 3066: 1601952.. 1601964: 13: 1599241:
47: 3067.. 3067: 1597056.. 1597056: 1: 1601965:
48: 3068.. 3084: 1602375.. 1602391: 17: 1597057:
49: 3085.. 3094: 1599290.. 1599299: 10: 1602392:
50: 3095.. 3096: 1601355.. 1601356: 2: 1599300:
51: 3097.. 3107: 1600717.. 1600727: 11: 1601357:
52: 3108.. 3156: 1642892.. 1642940: 49: 1600728:
53: 3157.. 3157: 1597059.. 1597059: 1: 1642941:
54: 3158.. 3251: 1649322.. 1649415: 94: 1597060:
55: 3252.. 3254: 1599241.. 1599243: 3: 1649416:
56: 3255.. 3304: 1645466.. 1645515: 50: 1599244:
57: 3305.. 3305: 1597100.. 1597100: 1: 1645516:
58: 3306.. 3312: 1601357.. 1601363: 7: 1597101:
59: 3313.. 3319: 1599300.. 1599306: 7: 1601364:
60: 3320.. 3331: 1601611.. 1601622: 12: 1599307:
61: 3332.. 3339: 1600838.. 1600845: 8: 1601623:
62: 3340.. 3343: 1601419.. 1601422: 4: 1600846:
63: 3344.. 3351: 1600846.. 1600853: 8: 1601423:
64: 3352.. 3432: 1649416.. 1649496: 81: 1600854:
65: 3433.. 3433: 1597109.. 1597109: 1: 1649497:
66: 3434.. 3489: 1649497.. 1649552: 56: 1597110:
67: 3490.. 3491: 1599227.. 1599228: 2: 1649553:
68: 3492.. 3521: 1619348.. 1619377: 30: 1599229:
69: 3522.. 3523: 1599307.. 1599308: 2: 1619378:
70: 3524.. 3530: 1601688.. 1601694: 7: 1599309:
71: 3531.. 3539: 1600949.. 1600957: 9: 1601695:
72: 3540.. 3579: 1629356.. 1629395: 40: 1600958:
73: 3580.. 3580: 1597124.. 1597124: 1: 1629396:
74: 3581.. 3601: 1604219.. 1604239: 21: 1597125:
75: 3602.. 3603: 1599585.. 1599586: 2: 1604240:
76: 3604.. 3614: 1602636.. 1602646: 11: 1599587:
77: 3615.. 3616: 1599587.. 1599588: 2: 1602647:
78: 3617.. 3677: 1649553.. 1649613: 61: 1599589:
79: 3678.. 3680: 1599692.. 1599694: 3: 1649614:
80: 3681.. 3723: 1647818.. 1647860: 43: 1599695:
81: 3724.. 3726: 1599821.. 1599823: 3: 1647861:
82: 3727.. 3756: 1622218.. 1622247: 30: 1599824:
83: 3757.. 3759: 1600630.. 1600632: 3: 1622248:
84: 3760.. 3766: 1603288.. 1603294: 7: 1600633:
85: 3767.. 3768: 1600633.. 1600634: 2: 1603295:
86: 3769.. 3950: 76053306.. 76053487: 182: 1600635:
87: 3951.. 3958: 1600958.. 1600965: 8: 76053488:
88: 3959.. 3986: 1619921.. 1619948: 28: 1600966:
89: 3987.. 3995: 1600966.. 1600974: 9: 1619949:
90: 3996.. 4036: 1649614.. 1649654: 41: 1600975:
91: 4037.. 4045: 1600975.. 1600983: 9: 1649655:
92: 4046.. 4050: 1601423.. 1601427: 5: 1600984:
93: 4051.. 4052: 1600854.. 1600855: 2: 1601428:
94: 4053.. 4055: 1601555.. 1601557: 3: 1600856:
95: 4056.. 4056: 1597129.. 1597129: 1: 1601558:
96: 4057.. 4059: 1601745.. 1601747: 3: 1597130:
97: 4060.. 4060: 1597134.. 1597134: 1: 1601748:
98: 4061.. 4063: 1602050.. 1602052: 3: 1597135:
99: 4064.. 4064: 1597137.. 1597137: 1: 1602053:
100: 4065.. 4079: 1604297.. 1604311: 15: 1597138:
101: 4080.. 4088: 1600987.. 1600995: 9: 1604312:
102: 4089.. 4095: 1603295.. 1603301: 7: 1600996:
103: 4096.. 4106: 1600996.. 1601006: 11: 1603302:
104: 4107.. 4117: 1622600.. 1622610: 11: 1601007:
105: 4118.. 4119: 1601007.. 1601008: 2: 1622611:
106: 4120.. 4129: 1622611.. 1622620: 10: 1601009:
107: 4130.. 4131: 1601009.. 1601010: 2: 1622621:
108: 4132.. 4141: 1622621.. 1622630: 10: 1601011:
109: 4142.. 4145: 1601011.. 1601014: 4: 1622631:
110: 4146.. 4155: 1622986.. 1622995: 10: 1601015:
111: 4156.. 4157: 1601015.. 1601016: 2: 1622996:
112: 4158.. 4168: 1622996.. 1623006: 11: 1601017:
113: 4169.. 4170: 1601017.. 1601018: 2: 1623007:
114: 4171.. 4180: 1623007.. 1623016: 10: 1601019:
115: 4181.. 4182: 1601019.. 1601020: 2: 1623017:
116: 4183.. 4192: 1624473.. 1624482: 10: 1601021:
117: 4193.. 4195: 1601021.. 1601023: 3: 1624483:
118: 4196.. 4205: 1624796.. 1624805: 10: 1601024:
119: 4206.. 4207: 1601024.. 1601025: 2: 1624806:
120: 4208.. 4217: 1624806.. 1624815: 10: 1601026:
121: 4218.. 4220: 1601026.. 1601028: 3: 1624816:
122: 4221.. 4230: 1625972.. 1625981: 10: 1601029:
123: 4231.. 4408: 1648626.. 1648803: 178: 1625982:
124: 4409.. 4411: 1602199.. 1602201: 3: 1648804:
125: 4412.. 4434: 1601328.. 1601350: 23: 1602202:
126: 4435.. 4437: 1602647.. 1602649: 3: 1601351:
127: 4438.. 4439: 1601029.. 1601030: 2: 1602650:
128: 4440.. 4442: 1602755.. 1602757: 3: 1601031:
129: 4443.. 4480: 1601650.. 1601687: 38: 1602758:
130: 4481.. 4491: 1629530.. 1629540: 11: 1601688:
131: 4492.. 4560: 1624404.. 1624472: 69: 1629541:
132: 4561.. 4571: 1629541.. 1629551: 11: 1624473:
133: 4572.. 4582: 1601031.. 1601041: 11: 1629552:
134: 4583.. 4586: 1603302.. 1603305: 4: 1601042:
135: 4587.. 4620: 1602537.. 1602570: 34: 1603306:
136: 4621.. 4631: 1629716.. 1629726: 11: 1602571:
137: 4632.. 4634: 1601042.. 1601044: 3: 1629727:
138: 4635.. 6143: 156004864.. 156006372: 1509: 1601045: last,eof
data.txt: 139 extents found
the I tried to defrag it
$ btrfs fi defra data.txt
$ sudo filefrag -v data.txt
Filesystem type is: 9123683e
File size of data.txt is 25165824 (6144 blocks of 4096 bytes)
ext: logical_offset: physical_offset: length: expected: flags:
0: 0.. 6143: 164002967.. 164009110: 6144: last,eof
data.txt: 1 extent found
So it seems that the defrag works
[*] https://www.kernel.org/doc/Documentation/filesystems/fiemap.txt
https://pastebin.com/1ufErVMs
If I unwind the interleaving, it looks like all the extents fall into
two localities and within each locality the extents aren't that far
apart - so my guess is that this file is also not meaningfully
fragmented, in practice. Surely the drive firmware will reorder the
reads to arrive at the least amount of seeks?
--
gpg @keyserver.linux.it: Goffredo Baroncelli <kreijackATinwind.it>
Key fingerprint BBF5 1610 0B64 DAC6 5F7D 17B2 0EDA 9B37 8B82 E0B5
