In the absence of simultaneous updates and inserts, for a typical DW type
database with nightly loads and queries by day, it looks like16K is
probably a better choice.
Esp, if there are cases where the rows do not fit in one block, this is
very useful. Thank you
On Fri, Jun 12, 2020 at 6:50 PM Orlando L <oralrnr@xxxxxxxxx> wrote:
*THAT* clears up my confusion very well. Thanks a lot Mark.
On Fri, Jun 12, 2020 at 3:02 PM Mark J. Bobak <mark@xxxxxxxxx> wrote:
Hi Orlando,
In terms of block-level contention, we are referring the the Interested
Transaction List (ITL) and the number of ITL entries per block. The
default for ITL entries is 1 for table blocks and 2 for index blocks. You
can increase the INITRANS value, but it will cost you 24 bytes per ITL
entry. So, if you have a table that has 10 rows per block, but you only
have 3 ITL entries per block, then you could end up with block-level
contention due to the shortage of ITL entries. Note that a row that is
being updated points to an ITL entry, and that ITL entry in turn points to
the transaction's UNDO. So, the larger the number of different
transactions that are interested in the data in a particular block, the
higher the chance of block-level contention.
Note that even if you're at the default of 1 ITL entry for a given table,
Oracle will automatically grow the ITL (dynamically add entries) as needed,
as long as there is free space available in the block. So, if you have
plenty of free space in the block, no worries. But ,if you have blocks
with tightly packed rows and didn't plan ahead with additional ITL slots,
you could end up with ITL waits. (Note that on table blocks, only update
and deletes suffer from ITL waits, not inserts, but this is not true for
indexes. An insert on an index must go to a specific block, so, if there's
an ITL shortage, indexes are always subject to ITL waits.
-Mark
On Fri, Jun 12, 2020 at 3:18 PM Orlando L <oralrnr@xxxxxxxxx> wrote:
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
