[LU-16843] ext4 extent tree should be able to shrink

• Type: Improvement
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.16.0
• Severity: 3
• Assignee: Alex Zhuravlev
• Reporter: Alex Zhuravlev

Description

ext4's extent code doesn't handle cases when extents merge/collapse. for example, one can create a sparse file like this

EXTENTS:
(ETB0):33409
 (0):34816
 (2):34818
 (4):34820
 (6):34822
 (8):34824
 (10):34826
 (12):34828
 (14):34830
 (16):34832
 (18):34834
 (20):34836
 (22):34838
 (24):34840
 (26):34842
 (28):34844
 (30):34846
 (32):34848
 (34):34850
 (36):34852
 (38):34854
 (40):34856
 (42):34858
 (44):34860
 (46):34862
 (48):34864
 (50):34866

then fill the holes and get an extent tree like this one:

EXTENTS:
(ETB0):33409
 (0-1):34816-34817
 (ETB0):33411
 (2-679):34818-35495
 (ETB0):33410
 (680-998):35496-35814

where 2 blocks are used to store 2 mergable extents.

Issue Links

• is related to: LU-17187 ldiskfs extent merge can release incorrect block
• is related to: LU-17337 ldiskfs extent merging may need more revoke credits
• is related to: LU-17136 ext4 extent tree: increase max depth