标签：lob空间重用
As records are deleted, the LOB data segments are marked as psuedo-deleted and they can only be reused when the ‘readLSN‘ of the table progresses past the LSN of when the deletes were performed(you can think of this as allowing read-stability isolation for any older open read transactions in case the pseudo-deleted LOB data is still needed by it). So, a new insert statement must constantly search the entire LOB space looking for free segments without being able to reuse the pseudo-delete LOB segments it comes by (not until whoever is holding up the readLSN is gone).
窗口1(session 1):
db2 "create table lob01 like sysibm.systables long in userspace1"
rm lob01*
db2 inspect check table name lob01 schema e97q7a results keep lob01.out
db2inspf lob01.out lob01.txt
cat lob01.txt
LOB Object Summary: Total Pages 0 - Used Pages 0
LBA Object Summary: Total Pages 1 - Used Pages 0
插入数据:
db2 "insert into lob01 select * from sysibm.systables"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "select count(1) from lob01"
LOB Object Summary: Total Pages 15168 - Used Pages 15160
LBA Object Summary: Total Pages 16 - Used Pages 16
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from lob01"
db2 "select count(1) from lob01"
LOB Object Summary: Total Pages 15168 - Used Pages 15045
LBA Object Summary: Total Pages 16 - Used Pages 16
db2 "insert into lob01 select * from sysibm.systables"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "select count(1) from lob01"
LOB Object Summary: Total Pages 15168 - Used Pages 15160
LBA Object Summary: Total Pages 16 - Used Pages 16
Used Pages没涨, delete的lob空间这里重用了.
窗口2(session 2):
readlsn(读操作hold住lob01表当前的lsn)
db2 +c "declare c1 cursor for select * from lob01"
db2 +c "open c1"
db2 +c "fetch from c1"
窗口1(session 1):
观察锁的情况.
db2pd -d sample -locks
Locks:
Address TranHdl Lockname Type Mode Sts Owner Dur HoldCount Att ReleaseFlg rrIID
0x0700000040FF9980 12 00000001000000010001652056 Internal V ..S G 12 1 0 0x00000000 0x40000000 0
0x0700000040FF9B80 12 4141414141424263C34A726E41 Internal P ..S G 12 1 0 0x00000000 0x40000000 0
0x0700000040FF9A80 12 00030021000000000000000054 Table .IS G 12 1 0 0x00003000 0x40000000 0
lob01表上了一个IS锁.
继续删除数据:
delete操作会更新lowTranLSN.
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from lob01"
db2 "select count(1) from lob01"
插入数据:
db2 "insert into lob01 select * from sysibm.systables"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "select count(1) from lob01"
LOB Object Summary: Total Pages 30528 - Used Pages 30512
LBA Object Summary: Total Pages 31 - Used Pages 31
窗口2 hold住的readLSN< delete操作的LSN, readLSN之后的空间都不会被重用.
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from (select * from lob01 fetch first 2000 rows only)"
db2 "delete from lob01"
db2 "select count(1) from lob01"
db2 "insert into lob01 select * from sysibm.systables"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "insert into lob01 select * from lob01"
db2 "select count(1) from lob01"
LOB Object Summary: Total Pages 45888 - Used Pages 45864
LBA Object Summary: Total Pages 46 - Used Pages 46
窗口3(Session 3):
在readLSN未释放的情况下, 对lob01表做reorg操作. 在U锁convert to Z锁的时候会发生锁等待, 因为z锁和IS锁冲突, 锁转换不受locktimeout参数控制(working as designed, 另外一个话题了), 导致reorg会hang住.
Locks being waited on :
AppHandl [nod-index] TranHdl Lockname Type Mode Conv Sts CoorEDU AppName AuthID AppID
28 [000-00028] 12 00030021000000000000000054 Table .IS G 7713 db2bp E97Q7A *LOCAL.e97q7a.150423030505
7 [000-00007] 2 00030021000000000000000054 Table ..U ..Z C 2572 db2bp E97Q7A *LOCAL.e97q7a.150423025731
标签：lob空间重用
原文地址：http://masaly.blog.51cto.com/9302075/1637504
