Techniques for improving the speed of Oracle import (imp or impdp) and Oracle SQL*Loader (sqlldr) jobs.
For professionals only
Of all of the performance techniques for database loading, the most important is the nologging operation, and the undocumented parameter _disable_logging=TRUE.
Note this discussion where Oracle expert Mark Bobak notes that you cannot re-start a failed instance when using _disable_logging=true, and Don Burleson notes that _disable_logging should only be used after taking a full, recoverable backup:
"The main things to consider about NOLOGGING:
- Supported feature of Oracle
- Works only with direct load insert
- Still maintains minimal logging, i.e., space transactions, extent invalidation records for loaded extents, etc.
Contrast with _disable_logging=TRUE:
- Undocumented, unsupported parameter
- all redo writes to log buffer still happen
- writes of redo buffer down to disk are disabled.
- Not limited to direct load insert. All operations are no longer protected by redo on disk.
The big difference is, if your instance dies, (crash or shutdown abort) there's no data in the online redo logs to even do instance recovery. This is very bad. Your database could suffer a corruption from which you can't recover."
All experienced DBA's know that You MUST get approval from Oracle Technical Support for all usage of undocumented parameters, but in the hands of a well-trained DBA, the hidden parameters are often a Godsend, especially for speeding-up rebuilds during tight maintenance windows with limited downtime.
The backup sandwich:
Take a dump (full system backup)
Load everything I can in PARALLEL NOLOGGONG mode (ONLY during a scheduled downtime window).
Take a dump. Take a full system backup right before bringing the system online.
Most DBA's call this high-speed maintenance a "backup sandwich", it's quite common in my world where you have super-tight maintenance windows and every second counts.
Remember, unless you take a full backup after your maintenance, using _disable_logging=TRUE, an instance crash or a shutdown abort WILL trash your Oracle database.
Next, lets examine tips and techniques to improve the performance of all types of data loading, including data pump import (impdp), sql*Loader and SQL INSERT DML.
Maximizing Oracle Insert Performance
Regardless of which options were used when the data was exported, it has no influence on how the data is imported. For example, it is irrelevant to the import process whether it was a direct path export or not, since it is a plain export file, be it generated from direct or conventional means.
Unfortunately, there is no direct option available for imports (only for export and SQL*loader). The import process has more tuning limitations than other utilities. The DBA should consider the following when trying to optimize import performance:
Analyze once after the load - Set analyze=n and analyze with dbms_stats after the load has completed.
Use Solid-state disk - For a fully-tuned import job, only faster devices can speed-up import rates. Many large companies used partitioned tables, and keep the current partition on SSD for fast imports.
Increase recordlength - Many set recordlength to 64k, but it needs to be a multiple of your I/O chunk size and db_block_size (or your multiple block size, e.g. db_32k_block_size).
Set commit=n – For tables that can afford not to commit until the end of the load, this option provides a significant performance increase. Larger tables may not be suitable for this option due to the required rollback/undo space.
Dedicate a single, large rollback segment - Many professionals create a single large rollback segment and take all others offline during theimport.
Set indexes=n – Index creation can be postponed until after import completes, by specifying indexes=n. If indexes for the target table already exist at the time of execution, import performs index maintenance when data is inserted into the table. Setting indexes=n eliminates this maintenance overhead. You can also Use the indexfile parm to rebuild all the indexes once, after the data is loaded.
Defer CBO stats - Using impdp with the parameter exclude=statistics will greatly improve the import speed, but statistics will need to be re-analyzed or imported later.
Use the buffer parameter – By using a larger buffer setting, import can do more work before disk access is performed.
Disable logging - You can also use the hidden parameter _disable_logging = true to reduce redo, but beware that the resulting import will be unrecoverable.
Import Option | Elapsed Time (Seconds) | Time Reduction |
commit=y | 120 | - |
commit=y
buffer=64000 | 100 | 17% |
commit=n
buffer=30720 | 72 | 40% |
commit=N
buffer = 64000 | 67 | 44% |
Also see Metalink Note 155477.1 on improving importing speed:
You may be able to improve performance by increasing the value of the RECORDLENGTH parameter when you invoke a direct path Export.
Your exact performance gain depends upon the following factors:
- DB_BLOCK_SIZE
- the types of columns in your table
- your I/O layout (the drive receiving the export file should be separate from the disk drive where the database files reside)
For example, invoking a Direct path Export with a maximum I/O buffer of 64kb can improve the performance of the Export with almost 50%.
This can be achieved by specifying the additional Export parameters DIRECT and RECORDLENGTH. e.g.:
exp userid=system/manager full=y direct=y recordlength=65535 file=exp_full.dmp log=exp_full.log
imp userid=system/manager full=y recordlength=65535 file=exp_full.dmp log=imp_full.log
Maximizing SQL*Loader Performance
Oracle SQL*Loader is flexible and offers many options that should be considered to maximize the speed of data loads. These include:
1. Use Direct Path Loads - The conventional path loader essentially loads the data by using standard insert statements. The direct path loader (direct=true) loads directly into the Oracle data files and creates blocks in Oracle database block format. The fact that SQL is not being issued makes the entire process much less taxing on the database. There are certain cases, however, in which direct path loads cannot be used (clustered tables). To prepare the database for direct path loads, the script $ORACLE_HOME/rdbms/admin/catldr.sql.sql must be executed.
2. Disable Indexes and Constraints. For conventional data loads only, the disabling of indexes and constraints can greatly enhance the performance of SQL*Loader.
3. Use a Larger Bind Array. For conventional data loads only, larger bind arrays limit the number of calls to the database and increase performance. The size of the bind array is specified using the bindsize XE "bindsize" parameter. The bind array's size is equivalent to the number of rows it contains (rows=) times the maximum length of each row.
4. Use ROWS=n to Commit Less Frequently. For conventional data loads only, rows specifies the number of rows per commit. Issuing fewer commits will enhance performance.
5. Use Parallel Loads. Available with direct path data loads only, this option allows multiple SQL*Loader jobs to execute concurrently.
$ sqlldr control=first.ctl parallel=true direct=true
$ sqlldr control=second.ctl parallel=true direct=true
6. Use Fixed Width Data. Fixed width data format saves Oracle some processing when parsing the data. The savings can be tremendous, depending on the type of data and number of rows.
7. Disable Archiving During Load. While this may not be feasible in certain environments, disabling database archiving can increase performance considerably.
8. Use unrecoverable. The unrecoverable option (unrecoverable load data) disables the writing of the data to the redo logs. This option is available for direct path loads only.
Optimizing Oracle INSERT performance
When using standard SQL statements to load Oracle data tables, there are several tuning approaches:
a - Manage segment header contention for parallel inserts - Make sure to define multiple freelist (or freelist groups) to remove contention for the table header. Multiple freelists add additional segment header blocks, removing the bottleneck. You can also use Automatic Segment Space Management (bitmap freelists) to support parallel DML, but ASSM has some limitations.
b - Parallelize the load - You can invoke parallel DML (i.e. using the PARALLEL hint) to have multiple inserts into the same table. For this INSERT optimization, make sure to define multiple freelists or freelist groups. Mark Bobak notes that if you submit parallel jobs to insert against the table at the same time, using the APPEND hint may cause serialization, removing the benefit of parallel jobstreams.
c - APPEND into tables - By using the APPEND hint, you ensure that Oracle always grabs "fresh" data blocks by raising the high-water-mark for the table. If you are doing parallel insert DML, the Append mode is the default and you don't need to specify an APPEND hint. Mark Bobak notes "Also, if you're going w/ APPEND, consider putting the table into NOLOGGING mode, which will allow Oracle to avoid almost all redo logging."
insert /*+ append */ into customer values ('hello',';there');
d - Use a large blocksize - By defining large (i.e. 32k) blocksizes for the target table, you reduce I/O because more rows fit onto a block before a "block full" condition (as set by PCTFREE) unlinks the block from the freelist.
e - Disable/drop indexes - It's far faster to rebuild indexes after the data load, all at-once. Also indexes will rebuild cleaner, and with less I/O if they reside in a tablespace with a large block size.
Using SSD for insert tablespaces
For databases that require high-speed loads, some shops define the insert table partition on solid-state disk (later moving it to platter disk). Mike Ault notes in his book "Oracle Solid-State Disk Tuning", a respectable 30% improvement in load speed:
“In the SSD verses ATA benchmark the gains for insert and update processing as shown in the database loading and index build scenarios was a respectable 30%.
This 30% was due to the CPU overhead involved in the insert and update activities.
If the Oracle level processing for insert and update activities could be optimized for SSD, significant performance gains might be realized during these activities."
NOTE : Also see my notes on tuning and improving Oracle export (expdp) performance speed.
SOURCE : http://dba-oracle.com/oracle_tips_load_speed.htm
