Description:
fsp_reserve_free_extents() works by accident or extremely counter-intuitively for tablespaces between 32 and 64 pages.
First, all newly-created tablespaces have their FSP_FREE_LIMIT initialized to 64 in fsp_fill_free_list(). And it is explained next to FSP_FREE_LIMIT define that
"note that in a single-table tablespace where size < 64 pages, this number is 64, i.e., we have initialized the space about the first extent, but have not physically allocted those pages to the file"
But fsp_reserve_free_extents has special handling for small tablespaces that checks for size < 32 pages only:
if (size < FSP_EXTENT_SIZE / 2) {
/* Use different rules for small single-table tablespaces */
*n_reserved = 0;
return(fsp_reserve_free_pages(space, space_header, size, mtr));
}
This means that, whenever size > 32 and < 64, fsp_reserve_free_extents() will end up with very large positive values in its calculations because of
n_free_up = (size - free_limit) / FSP_EXTENT_SIZE;
where free_limit = 64 and size < 64.
If that kind of unsigned math was intended there, it should be explained at least. But the code, including the call to fil_space_reserve_free_extents() with n_free_now close to ULINT_MAX does not seem support that.
Some sample var values with size between 32 and 64:
(gdb) print n_free_up
$3 = 288230376151711743
(gdb) print n_free_up
$4 = 270215977642229759
(gdb) print n_free
$6 = 270215977642229759
2808 if (n_free <= reserve + n_ext) {
(gdb) print reserve
$9 = 2
(gdb) print n_ext
$10 = 3
(gdb) n
2825 success = fil_space_reserve_free_extents(space, n_free, n_ext);
My first fix idea would be to cut off small tablespace handling at 64 pages instead of 32 pages. But I am not familiar with this code enough to suggest that this would be correct.
How to repeat:
innodb_wl6347_comp_indx_stat.
Note that my suggested fix perturbs the resulting size of tablespaces as checked by the testcase:
@@ -987,7 +987,7 @@
AND table_name='tab5' AND database_name='test'
AND index_name like 'idx%' ;
compress_stat 1
-The size of the tab5.ibd file: 159744
+The size of the tab5.ibd file: 163840
# fetch the compressed page and check the stats
===============
Fetch Records
@@ -1013,7 +1013,7 @@
AND table_name='tab5' AND database_name='test'
AND index_name like 'idx%' ;
compress_stat 1
-The size of the tab5.ibd file: 159744
+The size of the tab5.ibd file: 163840
# fetch the compressed same page once again and check the stats
# the stat figures should be same as above query
===============
@@ -1040,7 +1040,7 @@
AND table_name='tab5' AND database_name='test'
AND index_name like 'idx%' ;
compress_stat 1
-The size of the tab5.ibd file: 159744
+The size of the tab5.ibd file: 163840
#cleanup
DROP TABLE IF EXISTS tab5;
#reset the stat table before starting next testcase
Suggested fix:
=== modified file 'storage/innobase/fsp/fsp0fsp.cc'
--- storage/innobase/fsp/fsp0fsp.cc 2012-09-28 06:10:51 +0000
+++ storage/innobase/fsp/fsp0fsp.cc 2013-04-16 09:05:18 +0000
@@ -2681,7 +2681,7 @@
ulint n_used;
ut_a(space != 0);
- ut_a(size < FSP_EXTENT_SIZE / 2);
+ ut_a(size < FSP_EXTENT_SIZE);
descr = xdes_get_descriptor_with_space_hdr(space_header, space, 0,
mtr);
@@ -2761,7 +2761,7 @@
try_again:
size = mtr_read_ulint(space_header + FSP_SIZE, MLOG_4BYTES, mtr);
- if (size < FSP_EXTENT_SIZE / 2) {
+ if (size < FSP_EXTENT_SIZE) {
/* Use different rules for small single-table tablespaces */
*n_reserved = 0;
return(fsp_reserve_free_pages(space, space_header, size, mtr));
@@ -2776,6 +2776,7 @@
some of them will contain extent descriptor pages, and therefore
will not be free extents */
+ ut_ad (size >= free_limit);
n_free_up = (size - free_limit) / FSP_EXTENT_SIZE;
if (n_free_up > 0) {
Description: fsp_reserve_free_extents() works by accident or extremely counter-intuitively for tablespaces between 32 and 64 pages. First, all newly-created tablespaces have their FSP_FREE_LIMIT initialized to 64 in fsp_fill_free_list(). And it is explained next to FSP_FREE_LIMIT define that "note that in a single-table tablespace where size < 64 pages, this number is 64, i.e., we have initialized the space about the first extent, but have not physically allocted those pages to the file" But fsp_reserve_free_extents has special handling for small tablespaces that checks for size < 32 pages only: if (size < FSP_EXTENT_SIZE / 2) { /* Use different rules for small single-table tablespaces */ *n_reserved = 0; return(fsp_reserve_free_pages(space, space_header, size, mtr)); } This means that, whenever size > 32 and < 64, fsp_reserve_free_extents() will end up with very large positive values in its calculations because of n_free_up = (size - free_limit) / FSP_EXTENT_SIZE; where free_limit = 64 and size < 64. If that kind of unsigned math was intended there, it should be explained at least. But the code, including the call to fil_space_reserve_free_extents() with n_free_now close to ULINT_MAX does not seem support that. Some sample var values with size between 32 and 64: (gdb) print n_free_up $3 = 288230376151711743 (gdb) print n_free_up $4 = 270215977642229759 (gdb) print n_free $6 = 270215977642229759 2808 if (n_free <= reserve + n_ext) { (gdb) print reserve $9 = 2 (gdb) print n_ext $10 = 3 (gdb) n 2825 success = fil_space_reserve_free_extents(space, n_free, n_ext); My first fix idea would be to cut off small tablespace handling at 64 pages instead of 32 pages. But I am not familiar with this code enough to suggest that this would be correct. How to repeat: innodb_wl6347_comp_indx_stat. Note that my suggested fix perturbs the resulting size of tablespaces as checked by the testcase: @@ -987,7 +987,7 @@ AND table_name='tab5' AND database_name='test' AND index_name like 'idx%' ; compress_stat 1 -The size of the tab5.ibd file: 159744 +The size of the tab5.ibd file: 163840 # fetch the compressed page and check the stats =============== Fetch Records @@ -1013,7 +1013,7 @@ AND table_name='tab5' AND database_name='test' AND index_name like 'idx%' ; compress_stat 1 -The size of the tab5.ibd file: 159744 +The size of the tab5.ibd file: 163840 # fetch the compressed same page once again and check the stats # the stat figures should be same as above query =============== @@ -1040,7 +1040,7 @@ AND table_name='tab5' AND database_name='test' AND index_name like 'idx%' ; compress_stat 1 -The size of the tab5.ibd file: 159744 +The size of the tab5.ibd file: 163840 #cleanup DROP TABLE IF EXISTS tab5; #reset the stat table before starting next testcase Suggested fix: === modified file 'storage/innobase/fsp/fsp0fsp.cc' --- storage/innobase/fsp/fsp0fsp.cc 2012-09-28 06:10:51 +0000 +++ storage/innobase/fsp/fsp0fsp.cc 2013-04-16 09:05:18 +0000 @@ -2681,7 +2681,7 @@ ulint n_used; ut_a(space != 0); - ut_a(size < FSP_EXTENT_SIZE / 2); + ut_a(size < FSP_EXTENT_SIZE); descr = xdes_get_descriptor_with_space_hdr(space_header, space, 0, mtr); @@ -2761,7 +2761,7 @@ try_again: size = mtr_read_ulint(space_header + FSP_SIZE, MLOG_4BYTES, mtr); - if (size < FSP_EXTENT_SIZE / 2) { + if (size < FSP_EXTENT_SIZE) { /* Use different rules for small single-table tablespaces */ *n_reserved = 0; return(fsp_reserve_free_pages(space, space_header, size, mtr)); @@ -2776,6 +2776,7 @@ some of them will contain extent descriptor pages, and therefore will not be free extents */ + ut_ad (size >= free_limit); n_free_up = (size - free_limit) / FSP_EXTENT_SIZE; if (n_free_up > 0) {