Thanks again.
"If it's transactional (OLTP) with simultaneous inserts and updates from
many processes, then the increased number of rows per blocks comes into
play." If we are having, say 10 insert/updates happening to one 8K block,
then with a 16K block, there could be 20 insert/updates. If that is the
case, what is 'contention'? Underneath an oracle block are multiple OS
blocks (512b or 1K blocksize?). Plus locking also should not be an issue
since locks are at the row level. Trying to understand, thank you for your
time in advance.
On Thu, Jun 11, 2020 at 12:57 PM Neil Chandler <neil_chandler@xxxxxxxxxxx>
wrote:
Orlando,
The contention relates to DML (not queries) on the blocks so it would
depend on how data is populated.
- If it's bulk loads and manipulations there is less opportunity for a
problem.
- If it's transactional (OLTP) with simultaneous inserts and updates
from many processes, then the increased number of rows per blocks comes
into play. The more rows per leaf block, the more opportunity for
contention in this scenario.
regards
Neil.
------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx> on
behalf of Orlando L <oralrnr@xxxxxxxxx>
*Sent:* 11 June 2020 18:29
*To:* jlewisoracle@xxxxxxxxx <jlewisoracle@xxxxxxxxx>
*Cc:* Mladen Gogala <gogala.mladen@xxxxxxxxx>; Oracle L <
oracle-l@xxxxxxxxxxxxx>
*Subject:* Re: Block size qn
Thank you all. I am not talking about mixing 8K and 16K. It is either one
or the other. Looks like we have less to worry about row chaining with 16K
where data for one row cannot fit in a 8K block. I read the link provided
by Jonathan. It looks like 16K are tested thoroughly by Oracle corp
("thoroughly tested just like 8k blocks"). No reason not to consider 16K if
that is going to be the only size used in the DB. It will be a typical DW
with loads in the nights and queries during days.
Neil, can you please explain "Larger block sizes will increase index block
contention". Why would there be contention for an index block? If the data
in the index blocks are accessed frequently, wouldn't the chances of the
index blocks being cached go up, hence producing better results. I think
the assumption here is that the index blocks can contain way more 'rows'
than data blocks. If the individual rows in the index blocks are accessed
simultaneously and if they are in memory, how would there be contention? Is
it because in DW the queries are typically performing range scans and
multiple queries could be doing range scans simultaneously on frequently
accessed blocks. If that is the case, if there are 2 queries accessing the
same block and if they are both readers, can they not read the block
simultaneously? Am I understanding this correctly
Thanks for your time.
Orlando
On Thu, Jun 11, 2020 at 4:05 AM Jonathan Lewis <jlewisoracle@xxxxxxxxx>
wrote:
A critical reference document about blocksizes is this one from Roger
McNical (Mr. Tablescan):
https://blogs.oracle.com/smartscan-deep-dive/random-thoughts-on-block-sizes
Read it all, but here's an important pair of myth-buster points:
1. A quick scan of the data layer regression tests showed a very large
number running on 16k blocks
2. Oracle typically runs it DW stress tests on 16k blocks
From my own experience:
a) There are a couple of boundary cases where a 16KB or 32KB block size
has a negative effect (typically due to bugs)
b) Using multiple block sizes in a single database is probably sub-optimal
- but it is possible to find special cases.
c) For RAC systems a block size that fits a single message on the
interconnect is a nice idea
d) Creating realistic tests of the effects of different block sizes is
very hard and very time-consuming
Regards
Jonathan Lewis
On Thu, Jun 11, 2020 at 4:24 AM Mladen Gogala <gogala.mladen@xxxxxxxxx>
wrote:
Also, Oracle software is tested on 8K database so a 16K specific bug may
remain undiscovered until the lucky DBA in search of an adventure discovers
it on his production database. Then the life will turn into the support
nightmare on the elm street. However, if you have a non-Exadata box with
huge amount of memory and fast flash storage that can read more than 1MB in
one operation, then I would consider 16K.
On 6/10/20 8:37 PM, Herring, Dave (Redacted sender HerringD for DMARC)
wrote:
We have a medium db (about 80TB) sitting on a 4-node X-8 system. Oracle
reviewed performance as part of our move from X-4 system and one of their
recommendations was to move to an 8K block size (currently it's 16K).
Their reasoning? When you use a non-8K block size on Exadata you bypass
various optimizations that Exadata could perform. Unfortunately they
didn't elaborate on what those optimizations are related to 8K block size
and we didn't have a downtime window that would allow us to change the
block size. Still, if you're on Exadata it's something to factor.
Regards,
Dave
*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx> *On Behalf Of *Neil Chandler
*Sent:* Wednesday, June 10, 2020 11:09 AM
*To:* oracle-l@xxxxxxxxxxxxx; oralrnr@xxxxxxxxx
*Subject:* Re: Block size qn
*CAUTION:* This email originated from outside of D&B. Please do not click
links or open attachments unless you recognize the sender and know the
content is safe.
General rule of thumb: stick with 8k unless you have a very good reason,
and have tested and proven that 16k provides tangible benefits over 8k.
Larger block sizes will increase index block contention but may reduce
block chaining and is generally better for LOBs.
The overriding reason not to use 32K block sizes: Oracle standard
regression tests do not test 32K block sizes. It mostly tests 8K with some
16K testing. For this reason alone, I'd avoid 32k block sizes - they aren't
testing their code against it.
You probably shouldn't mix block sizes in the same database.
Of course, your milage may vary. Test test test. Then stick with 8k 🙂
Neil Chandler
Database Guy
------------------------------
*From:* oracle-l-bounce@xxxxxxxxxxxxx <oracle-l-bounce@xxxxxxxxxxxxx>
<oracle-l-bounce@xxxxxxxxxxxxx> on behalf of Orlando L <oralrnr@xxxxxxxxx>
<oralrnr@xxxxxxxxx>
*Sent:* 10 June 2020 16:14
*To:* oracle-l@xxxxxxxxxxxxx <oracle-l@xxxxxxxxxxxxx>
<oracle-l@xxxxxxxxxxxxx>
*Subject:* Block size qn
Hi,
We are in planning stages for a big DW. Do any of the listers have a block
size bigger than the default 8k in their site? Is there a need for
anything like that? Any advantages or pitfalls?
Thanks,
Orlando
--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
