This is part 2 of a number of blogposts about huge Oracle database IO’s.
If you landed on this blogpost and did not read part 1, please read part 1 here.
In part 1 I showed how database IOs of a full table scan could be bigger than 1MB by increasing the db_file_multiblock_read_count parameter to a number beyond 1MB expressed in Oracle blocks. These bigger IOs only happen with direct path reads, not with buffered multiblock reads.
But how much bigger can these IOs be? In part 1 I showed Oracle IOs of 1020 blocks. Is that the limit? To investigate this, I created a much bigger table (table T2 in part 1 had a maximum extent size of 1024 blocks, which meant that the 1020 is the biggest IO possible from this table).
For the sake of this investigation I created a much bigger table to get larger extents:
EXTENT_ID BLOCKS ---------- ---------- 0 8 ... 263 8192 264 8192 265 8192 266 rows selected.
So with my new, big table I got extents up to 8192 blocks. Let's do the same test on this table as I did in the previous post, which means setting db_file_multiblock_read_count to 2000 with SQL trace at level 8 with throttled IOPS to get waits!
The IOs with the "small" extents of 8 blocks and 128 blocks are done exactly the same as the previous post. The IOs done against the extents with a size of 1024 blocks are the same too, simply because reading the 1020 blocks in one IO is as efficient as it could be! But let's take a look at the point where the extents become 8192 blocks:
WAIT #140281084141152: nam='direct path read' ela= 7996378 file number=5 first dba=174468 block cnt=1020 obj#=76227 tim=1373104660882677 WAIT #140281084141152: nam='direct path read' ela= 7995820 file number=5 first dba=175489 block cnt=1023 obj#=76227 tim=1373104668882345 WAIT #140281084141152: nam='direct path read' ela= 7996472 file number=5 first dba=176520 block cnt=632 obj#=76227 tim=1373104676882677 WAIT #140281084141152: nam='direct path read' ela= 7998049 file number=5 first dba=177152 block cnt=1024 obj#=76227 tim=1373104684883512 WAIT #140281084141152: nam='direct path read' ela= 7995472 file number=5 first dba=178176 block cnt=1024 obj#=76227 tim=1373104692882932 WAIT #140281084141152: nam='direct path read' ela= 7993677 file number=5 first dba=179200 block cnt=1024 obj#=76227 tim=1373104700880106 WAIT #140281084141152: nam='direct path read' ela= 7996969 file number=5 first dba=180224 block cnt=1024 obj#=76227 tim=1373104708880891 WAIT #140281084141152: nam='direct path read' ela= 5998630 file number=5 first dba=181248 block cnt=1024 obj#=76227 tim=1373104714882889 WAIT #140281084141152: nam='direct path read' ela= 9996459 file number=5 first dba=182272 block cnt=1024 obj#=76227 tim=1373104724882545
I've included one IO of 1020 blocks, after which Oracle issues an IO request of 1023 blocks, then 632 blocks, and then it issues a number of requests for 1024 blocks.
It seems that the maximal amount of blocks of multiblock IO requests done by Oracle "unbuffered" or "direct path" is 1024 blocks, which is 8MB! Please mind I have tested this ONLY with Oracle version 22.214.171.124 64 bits on Linux X64 on ASM, with a database and segment blocksize of 8kB.
There's another peculiarity I want to introduce. Let's take a look at the dba and block cnt only, together with block_id and blocks (number of blocks in the extent, taken from dba_extents):
BLOCK_ID BLOCKS EXTENT_ID dba=174468 block cnt=1020 174464 1024 197 dba=175489 block cnt=1023 175488 1024 198 dba=176520 block cnt=632 176512 8192 199 dba=177152 block cnt=1024 dba=178176 block cnt=1024 dba=179200 block cnt=1024 dba=180224 block cnt=1024 dba=181248 block cnt=1024 dba=182272 block cnt=1024 dba=183296 block cnt=1024
There are two anomalies here: the first one is at dba 175489. It seems that dba 175489/extent id 198 has only one BMB, so Oracle can read 1023 blocks of the extent, instead of the 1020 blocks in the other extents with the size of 1024 blocks.
The second anomaly is much more interesting: extent id 199 starts at block id 176512. This is an extent consisting of 8192 blocks. The first IO starts at block id 176520. This means there are 176520-176512=8 blocks not read, which are probably BMB's. But this is in line with my expectation. What is odd, is the first IO issued for datablocks in this extent which is 8192 blocks in length is "only" 632 blocks!
After the oddly sized IO request at the beginning of the extent (632), the other IO requests are the full 1024 blocks, which probably is the maximum size of direct path Oracle IO requests.
I wondered about the oddly sized request at the beginning. I looked at the dba's of the requests, and noticed the dba's of the 1024 blocks sized requests looked vaguely familiar. Then I spotted the familiarity and a possible reason for the "small" request at the beginning of the extent:
SYS@v11203 AS SYSDBA> select mod(176520,1024) from dual; MOD(176520,1024) ---------------- 392 SYS@v11203 AS SYSDBA> select mod(177152,1024) from dual; MOD(177152,1024) ---------------- 0 SYS@v11203 AS SYSDBA> select mod(178176,1024) from dual; MOD(178176,1024) ---------------- 0
ONLY when the direct path read requests can potentially read "the full" 1024 blocks in one IO, Oracle first issues an IO to align the later requests to a block id being a multiplication of 1024, after which it will issue IO requests of 1024 blocks.
I don't know why the alignment takes place. I can imagine this alignment can line up with ASM, thus avoiding a single Oracle IO request touching more than one ASM allocation unit. But I am open to other people's ideas and suggestions about potential reasons!
Stay tuned for further updates about extra huge database IOs!