Oracle RAC block size & cache size Configuration tips
Oracle Tips by Burleson Consulting
January 27, 2004 - Updated October 4, 2014
Block Size and RAC
The choice of the
database block size can have an impact on the Cluster Interconnect. One normally
considers the block size parameter, db_block_size in relation to disk I/O operations. For Oracle RAC
databases, Cache Fusion will also be performing cross-instance I/O as well. It
should come as no surprise that the database block size can impact global cache
transfers as well. See these notes on
NOTE: RAC likes small blocksizes: A 2k db_block_size
is recommended for all OLTP RAC databases.
The general rule of
thumb is that databases in support of data warehouses should use large blocks
sizes and databases in support of online transaction systems should use smaller
block sizes. Oracle RAC database often follow this convention as well.
The smaller the
database block, the quicker a block can be read from disk. Just like disk I/O,
the smaller the database block, the quicker it is transferred from one instance
to another via Cache Fusion. This is just a matter of physics. No matter how
fast the Cluster Interconnect is, 2 kilobytes of data can transfer across the
network faster than 8 kilobytes. There are studies found which indicate transfer
rates similar to the following.
Table 3.1 Block Size
The actual transfer
times do not matter and may vary considerably from environment to environment.
The numbers in the table above are just meant to illustrate the effect of larger
block sizes on global cache transfers.
Initially, one might
look at the table above and conclude that for Oracle RAC databases, we should
always choose the smaller block size. While a 2KB block transfers faster than a
4KB block, the amount of data being transferred per millisecond is improved as
the block size increases. Using the figures from the table above, easy
mathematics calculates the transfer rate as shown in the next table.
0.00667 KB/ μ s
0.020000 KB/ μ s
0.01142 KB/ μ s
0.026667 KB/ μ s
0.01778 KB/ μ s
0.040000 KB/ μ s
3.2 Block Size Throughput
The values in the
table above were derived by simple math, i.e. block size divided by time in
microseconds. In any case, the larger the block size, the more data per
microsecond is transferred.
So based on the
information in both tables, which block size should be used? The answer all
depends on the application and how it accesses data. If the application accesses
few rows at a time, then the smaller block size is preferred as it returns the
data back to the application quicker than larger block sizes. If the application
regularly accesses multiple rows or large amounts of data then a larger block
size is preferred.
To illustrate the
concept using the example times above, consider an online transaction processing
system where a query selects one row from the table and the table has an average
row length of 148 bytes. A database block size of 8KB would contain, on average,
55 rows of data. A database block size of 2KB would contain, on average, 13 rows
of data. If the application is seeking 1 row, the 8KB block size would force a
read of 54 unneeded rows of data while the 2KB block size would force a read of
12 unneeded rows of data. The one row would be returned in 300 microseconds if
the 2KB block size is used or 450 microseconds for the 8KB block size. For
applications that exhibit this behavior, a smaller block size is preferred. The
application would benefit from the metrics seen in the first table of this
On the flip side is
the application that processes many rows of data and each row is larger than as
described in the previous paragraph. Assume the application selects thousand
rows of the table at a time and each row in the table averages 533 bytes. The
typical data access pattern would seek 533,000 bytes at a time. For the 2KB
block size, 261 blocks would be needed whereas for the 8KB block size, 66 blocks
would be needed. In this example, the application would benefit from the metrics
in the second table of this section.
The examples above
assumed that each data block was completely full, i.e. no space reserved for
future update or insert operations. The examples further assume that all rows
were the average size and they packed nicely into the block. Data in blocks
rarely cooperate in this fashion in the real world. The examples are used to
illustrate the points made in this section. However, the main ideas of each
example will hold true even if a parameter like
pctfree is used to reserve space for future updates. In the end, the
only thing that matters is end user application performance. If you are
considering the block size to be used, perform adequate testing of the
application with different block sizes to determine the most optimal setting.
Remember that Oracle has one default block size but you can also employ
non-default block sizes as well to have the best of all worlds. Many of today?s
applications do not exhibit true OLTP or DW behavior but have characteristics of
both. One could choose a block size that is in the middle of allowable values or
one could deploy multiple block sizes for their application.
All Oracle DEFAULT caches and block sizes in a RAC environment (as
defined by db_block_size) must be identical, but there are other
important differences between the configuration of a RAC database and a
single-instance Oracle database, especially when using grid server
Beginning with Oracle RAC Oracle
allowed sub-caches to be configured that have different blocksizes than
the default db_cache_size, allowing db_2K_cache_size,
db_4k_cache_size, db_8k_cache_size, db_16k_cache_size and
db_32k_cache_size (not available on all platforms) as sub-caches.
example, if your default db_block_size is 4K, you could define a
db_2k_cache_size to hold additional objects. However, all RAC
nodes must have the sub-cache defined.
We must remember that the rules for caching in a
RAC instance are very different than for a standard Oracle system. For
numerous studies have shown that Oracle indexes often benefit from
using the largest supported blocksize. This is not always true in RAC
With Oracle system now employing
RAM-SAN instead of disk, and systems having giant db_cache_size
regions, it is tempting to configure RAC with large data buffers.
However, we must remember that multi-instance Oracle is very different
from a single-instance system:
Blade clusters have small buffer caches: Most
blade servers have only 2-gig or 4-gig of RAM. Hence, each RAC node is
limited in the available db_cache_size.
RAC likes small blocksizes: Because of the
inter-instances block transfer via Cache Fusion, smaller block sizes
minimize pinging of blocks between instances. Most RAC DBA's will
define the default db_block_buffers to 2k and then add a 4k buffer to
isolate other data objects, such as indexes. See MOSC Note:
422844.1 for details.
RAC scales by the sum of RAM caches:
Unlike a single Oracle database with a 32 gigabyte RAM data cache,
RAC system achieve high caching by the sum of the individual RAM
caches. Hence, a 32 node RAC cluster with a 2 gigabyte RAM cache on each node would
effectively have a total cache of 128 gigabytes.
See my related notes on RAC caching and block sizes:
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