Percona Server with XtraDB

fsp_reserve_free_extents switches from small to big tblspace handling too early

Reported by Laurynas Biveinis on 2013-04-16
6
This bug affects 1 person
Affects Status Importance Assigned to Milestone
MySQL Server
Unknown
Unknown
Percona Server
Status tracked in 5.6
5.1
Low
Unassigned
5.5
High
Laurynas Biveinis
5.6
High
Laurynas Biveinis

Bug Description

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) {

tags: added: innodb

Setting triage to High because the combination of the bug 1083700 fix and this bug results in more than 10x size increase in small tablespaces, as shown by innodb_wl6347_comp_indx_stat:

@@ -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: 2097152
 # 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: 2097152
 # 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: 2097152
 #cleanup
 DROP TABLE IF EXISTS tab5;
 #reset the stat table before starting next testcase

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