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(ITS#7772) liblmdb sub-page growth when adding existing data
Full_Name: Hallvard B Furuseth
Version: mdb.master, 4c8f57615c5ca7b014c038e59c1045182e74f5ad
OS: Linux x86_64
Submission from: (NULL) (184.108.40.206)
Submitted by: hallvard
A non-LEAF2 sub-page grows if you put() a data item which already
exists. The space remains unused while the sub-page exists.
(A LEAF2 page grows reusable space, and only if it is full.)
Demo program enclosed. Edit mdb.c to set mdb_debug=1 and build
with -DMDB_DEBUG. Watch the 1st DPRINTF() in mdb_node_add():
bash$ ./a.out 2>&1 | perl -lne '/add to (.*) key size/ && print $1'
leaf page 2 index 0, data size 404 # node with 1st item
leaf page 2 index 0, data size 840 # convert to sub-page for 2nd item
leaf sub-page 2 index 0, data size 0
leaf sub-page 2 index 1, data size 0
leaf page 2 index 0, data size 1250 # space for re-adding 2nd item
leaf page 2 index 0, data size 1660 # space for re-adding 2nd item
leaf page 2 index 0, data size 48 # ...too big, moved to sub-DB.
Fix: lmdb could position cursor+xcursor fully before spill/touch,
with an MDB_GET_BOTH variant which returns different codes for "no
such key" and "no such data". And position it before MDB_MULTIPLE
loops up. The current comparisons to see if the data item exists,
go away. Maybe mdb_cursor_put() gets split in several functions,
since much of the work in the recursive put()s will be unneeded.