This post is to document my findings. There is a lot of guesswork and interpretation here, so be careful if you use or refer to any information here.
The whole testcase is on 12.1.0.2.170418 BundlePatch EE, Linux, 2 node RAC.
I start with a simple test table:
create table test_blobs (
id number(5),
comments varchar2(80),
file_content blob
)
SEGMENT CREATION IMMEDIATE
NOCOMPRESS
NOPARALLEL
TABLESPACE USERS
lob (file_content) store as SECUREFILE(
disable storage in row
chunk 16K
tablespace users
)
;
let's get the LOB segment:
col column_name for A20 col segment_name for A30 SELECT column_name, segment_name FROM DBA_LOBS where owner='BERX' and table_name='TEST_BLOBS'; COLUMN_NAME SEGMENT_NAME -------------------- ------------------------------ FILE_CONTENT SYS_LOB0000454368C00003$$
and it's extent:
col owner for A10 col segment_name for A30 select owner, segment_name, file_id, block_id, blocks from dba_extents where owner='BERX' and segment_name='SYS_LOB0000454368C00003$$'; OWNER SEGMENT_NAME FILE_ID BLOCK_ID BLOCKS ---------- ------------------------------ ---------- ---------- ---------- BERX SYS_LOB0000454368C00003$$ 4 255360 16
this segment only has 16 blocks, so it should be easy to have a look at it:
ALTER SESSION SET TRACEFILE_IDENTIFIER = "EMPTY_BLOB"; alter system dump datafile 4 block min 255360 block max 255375;
What different blocktypes are there:
grep -E 'Block dump from disk|buffer tsn|frmt' EMPTY_BLOB.trc Block dump from disk: buffer tsn: 4 rdba: 0x0103e580 (4/255360) frmt: 0x02 chkval: 0xfec6 type: 0x45=NGLOB: Lob Extent Header Block dump from disk: buffer tsn: 4 rdba: 0x0103e581 (4/255361) frmt: 0x02 chkval: 0x12dd type: 0x3f=NGLOB: Segment Header Block dump from disk: buffer tsn: 4 rdba: 0x0103e582 (4/255362) frmt: 0x02 chkval: 0x86ba type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e583 (4/255363) frmt: 0x02 chkval: 0x86bb type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e584 (4/255364) frmt: 0x02 chkval: 0x86be type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e585 (4/255365) frmt: 0x02 chkval: 0xbb23 type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e586 (4/255366) frmt: 0x02 chkval: 0x86be type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e587 (4/255367) frmt: 0x02 chkval: 0x86be type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e588 (4/255368) frmt: 0x02 chkval: 0x86b6 type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e589 (4/255369) frmt: 0x02 chkval: 0x86b6 type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e58a (4/255370) frmt: 0x02 chkval: 0x86b6 type: 0x3d=NGLOB: Hash Bucket Block dump from disk: buffer tsn: 4 rdba: 0x0103e58b (4/255371) frmt: 0x02 chkval: 0x54e9 type: 0x06=trans data Block dump from disk: buffer tsn: 4 rdba: 0x0103e58c (4/255372) frmt: 0x02 chkval: 0x91be type: 0x06=trans data Block dump from disk: buffer tsn: 4 rdba: 0x0103e58d (4/255373) frmt: 0x02 chkval: 0x3ba9 type: 0x06=trans data Block dump from disk: buffer tsn: 4 rdba: 0x0103e58e (4/255374) frmt: 0x02 chkval: 0x7817 type: 0x06=trans data Block dump from disk: buffer tsn: 4 rdba: 0x0103e58f (4/255375) frmt: 0x02 chkval: 0x3418 type: 0x06=trans data
Let's go into detail of these blocks:
NGLOB: Lob Extent Header
Block dump from disk:
buffer tsn: 4 rdba: 0x0103e580 (4/255360)
scn: 0xa9c.1cf87e98 seq: 0x02 flg: 0x04 tail: 0x7e984502
frmt: 0x02 chkval: 0xfec6 type: 0x45=NGLOB: Lob Extent Header
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x00007F6247D60800 to 0x00007F6247D62800
7F6247D60800 0000A245 0103E580 1CF87E98 04020A9C [E........~......]
7F6247D60810 0000FEC6 0103E58B 00000005 00000000 [................]
7F6247D60820 59A53020 59A53020 00000000 00000000 [ 0.Y 0.Y........]
7F6247D60830 00000000 0006EF63 00000000 00000001 [....c...........]
7F6247D60840 0103E58B 00000005 00000000 00000000 [................]
7F6247D60850 00000000 00000000 00000000 00000000 [................]
Repeat 505 times
7F6247D627F0 00000000 00000000 00000000 7E984502 [.............E.~]
Dump of Extent Header Block
--------------------------------------------------------
sdba: 0x0103e58b len:5 flag:0x0 synctime:1503997984 updtime:1503997984
objd:454499 inc:0 total:1 opcode:0 xid: 0x0000.000.00000000
entry 0: sdba: 0x0103e58b len:5 fdba: 0x00000000
--------------------------------------------------------
There is some information already:
| sdba | I don't know what this stands for, but it points to the 1st block of type 0x06=trans data. |
| len | probably a length of something - as it's 5 and the extent has 5 blocks of type 0x06=trans data, I guess it's the length (count) of blocks capable of data. |
| flag | unknown to me right now |
| synctime | this looks like an unix epoc time - it matches to GMT: Tuesday, August 29, 2017 9:13:04 AM when I created the table. |
| objid | is the OBJECT_ID (or DATA_OBJECT_ID? I don't know) of the LOB segment. |
| inc | unknown to me right now |
| total | unknown to me right now |
| opcode | unknown to me right now |
| xid | might be a transaction ID when used. |
| entry 0 | same as sdba and len from above - there might be multiple entries in the future? |
NGLOB: Segment Header
Block dump from disk:
buffer tsn: 4 rdba: 0x0103e581 (4/255361)
scn: 0xa9c.1cf87e98 seq: 0x04 flg: 0x04 tail: 0x7e983f04
frmt: 0x02 chkval: 0x12dd type: 0x3f=NGLOB: Segment Header
Hex dump of block: st=0, typ_found=1
Dump of memory from 0x00007F6247D60800 to 0x00007F6247D62800
...
NGLOB Segment Header
Extent Control Header
-----------------------------------------------------------------
Extent Header:: spare1: 0 spare2: 0 #extents: 1 #blocks: 16
last map 0x00000000 #maps: 0 offset: 8168
Highwater:: 0x0103e590 ext#: 0 blk#: 16 ext size: 16
#blocks in seg. hdr's freelists: 0
#blocks below: 16
mapblk 0x00000000 offset: 0
Disk Lock:: Locked by xid: 0x01b3.015.00002ca0
Map Header:: next 0x00000000 #extents: 1 obj#: 454499 flag: 0x40000000
Extent Map
-----------------------------------------------------------------
0x0103e580 length: 16
---------------
CFS hbbcnt:7 hbbmx:7 Mxrng:7 UFS hbbcnt:2 PUA cnt:0 Flag:2
Segment Stats
-------------
Retention: -1
Free Space: 5
PUA Batchsize: 50
UFS Array
---------
DBA: 0x0103e582 Inst:1
DBA: 0x0103e583 Inst:1
Ufs off:152 sz:512
CFS Array
---------
Range: 0 DBA: 0x0103e584
Range: 1 DBA: 0x0103e585
Range: 2 DBA: 0x0103e586
Range: 3 DBA: 0x0103e587
Range: 4 DBA: 0x0103e588
Range: 5 DBA: 0x0103e589
Range: 6 DBA: 0x0103e58a
Cfs off:664 sz:576
PUA Array
---------
pua off:1240 sz:8
--------------------------------------------------------
There is nothing of great interest for me in Extent Control Header. The first interesting part is CFS.
CFS means Committed Free Space (blocks) - there are different "counts" - all of them 7 right now.
UFS means Uncommitted Free Space (blocks) - it seems to have a count of 2.
PUA might mean Persistent Undo Array (my guessing!) - no count right now.
In Segment Stats, Free Space: is 5 which matches the 5 trans data blocks available. They are where the real data will go.
UFS Array contains 2 blocks, so it matches the count above.
CFS Array contains 7 blocks, which matches the count above also.
PUA Array is empty - there are no NGLOB: Persistent Undo blocks right now.
I have no idea what the different off: and sz: mean. Offset and Size?
I start with a simple
1st NGLOB: Hash Bucket
(part of UFS)Block dump from disk: buffer tsn: 4 rdba: 0x0103e582 (4/255362) scn: 0xa9c.1cf87e98 seq: 0x02 flg: 0x04 tail: 0x7e983d02 frmt: 0x02 chkval: 0x86ba type: 0x3d=NGLOB: Hash Bucket Hex dump of block: st=0, typ_found=1 Dump of memory from 0x00007F6247D60800 to 0x00007F6247D62800 ... Dump of Hash Bucket Block -------------------------------------------------------- Hash Bucket Header Dump Inst:1 Flag:6 Total:0 FSG_COUNT:0 OBJD:454499 Inc:0 Self Descriptive fsg_count:0 Head:0x 0x00000000 Tail:0x 0x00000000 Opcdode:0 Locker Xid: 0x0000.000.00000000 Fsbdba: 0x0 Fbrdba: 0x0 Head Cache Entries ------------------- ------------------- Tail Cache Entries ------------------- ------------------- Free Space Chunk Summary Inst:0 Minc:0 Maxc:0 Count:0 Tot:0 MAXC:0 UFS List --------------
it seems quite empty.
2nd NGLOB: Hash Bucket
(part of UFS)Block dump from disk: buffer tsn: 4 rdba: 0x0103e583 (4/255363) scn: 0xa9c.1cf87e98 seq: 0x02 flg: 0x04 tail: 0x7e983d02 frmt: 0x02 chkval: 0x86bb type: 0x3d=NGLOB: Hash Bucket Hex dump of block: st=0, typ_found=1 Dump of memory from 0x00007F6247D60800 to 0x00007F6247D62800 ... Dump of Hash Bucket Block -------------------------------------------------------- Hash Bucket Header Dump Inst:1 Flag:6 Total:0 FSG_COUNT:0 OBJD:454499 Inc:0 Self Descriptive fsg_count:0 Head:0x 0x00000000 Tail:0x 0x00000000 Opcdode:0 Locker Xid: 0x0000.000.00000000 Fsbdba: 0x0 Fbrdba: 0x0 Head Cache Entries ------------------- ------------------- Tail Cache Entries ------------------- ------------------- Free Space Chunk Summary Inst:0 Minc:0 Maxc:0 Count:0 Tot:0 MAXC:0 UFS List --------------
Also empty.
3rd NGLOB: Hash Bucket
(part of CFS)Block dump from disk: buffer tsn: 4 rdba: 0x0103e584 (4/255364) scn: 0xa9c.1cf87e98 seq: 0x02 flg: 0x04 tail: 0x7e983d02 frmt: 0x02 chkval: 0x86be type: 0x3d=NGLOB: Hash Bucket Hex dump of block: st=0, typ_found=1 Dump of memory from 0x00007F6247D60800 to 0x00007F6247D62800 ... Dump of Hash Bucket Block -------------------------------------------------------- Hash Bucket Header Dump Range: 2k to 32k Inst:1 Flag:5 Total:0 FSG_COUNT:0 OBJD:454499 Inc:0 Self Descriptive fsg_count:0 Head:0x 0x00000000 Tail:0x 0x00000000 Opcdode:0 Locker Xid: 0x0000.000.00000000 Fsbdba: 0x0 Fbrdba: 0x0 Head Cache Entries ------------------- ------------------- Tail Cache Entries ------------------- ------------------- Free Space Chunk Summary Inst:1 Minc:0 Maxc:0 Count:0 Tot:0 MAXC:0 CFS Chunk List --------------
Again an empty block, but it contains a Range: 2k to 32k!
the next NGLOB: Hash Bucket blocks are similar (empty) but with different ranges:
| 0x0103e584 | Range: 2k to 32k |
| 0x0103e585 | Range: 32k to 64k |
| 0x0103e586 | Range: 64k to 128k |
| 0x0103e587 | Range: 128k to 256k |
| 0x0103e588 | Range: 256k to 512k |
| 0x0103e589 | Range: 512k to 1m |
| 0x0103e58a | Range: 1m to 64m |
trans data
is empty right now:Block dump from disk:
buffer tsn: 4 rdba: 0x0103e58b (4/255371)
scn: 0xa5d.aa1c5d7d seq: 0x02 flg: 0x04 tail: 0x5d7d0602
frmt: 0x02 chkval: 0x54e9 type: 0x06=trans data
Hex dump of block: st=0, typ_found=1
...
Block header dump: 0x0103e58b
Object id on Block? Y
seg/obj: 0x1c334 csc: 0xa5d.aa1c5d7c itc: 1 flg: E typ: 5 - LOCAL LOBS
fsl: 0 fnx: 0xffffffff ver: 0x01
Itl Xid Uba Flag Lck Scn/Fsc
0x01 0x0014.019.0002bdd9 0x00000000.0000.00 -B-- 0 fsc 0x0000.00000000
========
bdba [0x0103e58b]
kdlich [0x7f6247d6084c 56]
flg0 0x20 [ver=0 typ=data lock=n]
flg1 0x00
scn 0x0a5d.aa1c5d7c
lid 000000010000147f95a5
rid 0x00000000.0000
kdlidh [0x7f6247d60864 24]
flg2 0x00 [ver=0 lid=short-rowid hash=n cmap=n pfill=n]
flg3 0x00
pskip 0
sskip 0
hash 0000000000000000000000000000000000000000
hwm 8060
spr 0
data [0x7f6247d60880 52 8060]
It's of typ: 5 - LOCAL LOBS.I don't think there is anything of interest in the empty block right now. Maybe more later.
These are the 16 blocks we have.
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