Oracle Advanced Compression
Frequently Asked Questions
What is Advanced Compression?
Advanced Compression
is an option introduced in Oracle Database 11g. It
includes compression for structured data (numbers, characters, etc.),
unstructured data (documents, images, etc.), backups (RMAN and Data
Pump) and network transport (redo log transport during Data Guard gap
resolution).
What are the benefits of using Advanced
Compression?
Advanced Compression
provides storage reduction through compression of
all data types. It also provides performance improvements in several
cases as compressed blocks result in more data being moved per I/O. It
further improves memory efficiency as Oracle Database keeps data
compressed in memory. All these benefits cascade through all the
environments such as staging, QA, development, standby, backup, etc.,
magnifying the benefits several times. Advanced Compression includes
enhanced compression capabilities for backups and also helps in faster
synchronization of databases using Data Guard during gap resolution
process.
I am already using the Table Compression
feature
introduced in Oracle9i. What additional benefits do I get
from Advanced Compression option?
With Table
Compression feature of Oracle9i Database, one could only
compress table data during bulk load operations. With Advanced
Compression, new and modified data is also compressed. Further,
Advanced Compression includes compression and deduplication of
SecureFiles. This can provide significant savings for applications that
were storing unstructured data as LOBs in previous versions. Improved
compression for backups and compression of redo logs are the other
capabilities available only with Advanced Compression option.
I am already using the RMAN backup
compression
feature in Oracle Database 10g Release 2. In what way is the RMAN
backup compression included in Advanced Compression superior to this?
RMAN backup
compression available in Advanced Compression provides much
faster backups. It significantly reduces the duration for compressed
backups with very little impact on compression ratios, when compared
with RMAN backup compression available in Oracle Database 10g Release
2. During lab tests conducted at Oracle, it provided a 2.5X performance
improvement i.e. backup time was reduced to 40% of the time taken by
Oracle Database 10g Release 2.
What are SecureFiles? What is the
relationship between Advanced Compression and SecureFiles?
Oracle Database 11g
has introduced SecureFiles as the next generation
infrastructure for managing unstructured data. Using SecureFiles,
applications can get the high level of performance earlier available
only with file systems, and also attain the same level of security,
scalability, transaction consistency and high availability as
structured data within Oracle Database. Advanced Compression provides
capabilities for compression and deduplication of SecureFiles data.
Click here
for additional information on SecureFiles.
What kind of technology is used to compress
data?
For compression of
table data, Advanced Compression extends industry
standard algorithms to compress data at a block level. Repeating data
patterns are stored in the block header as symbols, and occurrences of
such data in the block are replaced with pointers to symbols. For
unstructured data compression, backup compression and network
compression, Advanced Compression uses industry standard algorithms
that work on a similar concept of eliminating data redundancies.
What optimizations has Oracle done to
minimize compression overhead for OLTP applications?
Oracle compresses a
block in batch mode rather than compressing data
every single time a write operation takes place. When a transaction
causes the data in the block to reach an internally controlled
threshold, all contents of the block are compressed. Only the
transactions that trigger compression of the block will experience the
minimal compression overhead. Therefore, a majority of OLTP
transactions on compressed blocks will have the exact same performance
as they would with uncompressed blocks.
Does table data get decompressed before it
is read?
No. Oracle Database
does not need to decompress table blocks when
reading data. Oracle can keep blocks compressed in memory and read them
directly. Hence, more data can be packed in memory which results in
improved cache hit ratio and reduced I/O.
How much compression can I expect by using
Advanced Compression?
Compression ratios
are dependent on the nature of data being compressed
and can vary from a dataset to another. A two to four times reduction
in size could be expected in most of the cases.
What is the performance impact of using OLTP Table Compression?
For DML operations
on a compressed table, Oracle Database’s
specialized batch algorithm keeps the performance overhead to a
minimum. Internal tests at Oracle showed a minimal overhead of less
than 3% for a DML workload. In many cases, query performance will
improve due to improved I/O efficiencies. During internal benchmarking
tests, table scans on compressed table provided a three times
improvement in performance i.e. scan on a compressed table completed in
one third the time when compared to table scan on the corresponding
uncompressed table.
With cost of disk storage falling, why do I
still need to care about compression?
While cost of
storage on a per unit (GB) basis is falling, the data
volumes are growing at a much faster rate. By industry estimates,
databases are growing to twice or thrice their original size every
couple of years. Thus, on a total basis, storage expenditure for
organizations is still increasing. Further, data center overheads such
as cooling, power, floor space, storage administration, etc. continue
to grow with hardware getting added to handle higher data volumes. By
reducing the need to add new hardware, Advanced Compression helps in
managing these overheads. Another important benefit is in the
performance area. A major bottleneck for many systems is I/O bandwidth.
Advanced Compression can help alleviate that bottleneck in several
cases by reducing the amount of data that needs to be transferred
across I/O channel and also further boost performance through improved
memory efficiencies.
How much will I save using Advanced
Compression option?
To estimate savings
from Advanced Compression, the following cost
elements need to be considered:
- Size of
production database
- Size of other
copies (staging, standbys, QA, development, backup, etc.)
- Excess storage
capacity provisioned for
future growth (this should also go down with compression –
excess capacity provisioned for a 6 TB database will be much more than
excess capacity provisioned for a 2TB compressed database)
- Data center
overheads (cooling, power, floor space, administration, etc.)
Apart from storage
cost savings, Advanced Compression also improves
memory efficiency and boosts query performance in many cases.
Can I compress data at a partition level?
Yes. Compression can
be done at a tablespace, table or partition level.
How do I turn on various features under
Advanced Compression option?
For table
compression, click here.
For SecureFiles compression and deduplication, click here.
For RMAN compression, click here.
For Data Pump table compression, click here.
For Data Guard network compression, click here.
How do I compress an existing table?
There are multiple
options available to compress existing tables. For
offline compression, one could use ALTER TABLE Table_Name MOVE COMPRESS
statement. A compressed copy of an existing table can be created by
using CREATE TABLE Table_Name COMPRESS FOR ALL OPERATIONS AS SELECT *.
For online compression, Oracle’s online redefinition utility
can
be used. More details for online redefinition are available here.
In addition to using Advanced Compression,
what else can I do to reduce my storage costs?
Oracle
Database’s Information Lifecycle Management (ILM)
capabilities provide a comprehensive strategy for storage cost
reduction. More information on ILM can be found here.
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