Disk Configuration for
The correct configuration of disks for temporary tablespaces is a key performance factor
for data warehouses and other databases that perform intensive batch processing.
Most temporary tablespace I/O operations, including writes, are performed in the foreground,
and so user response times are directly affected by the speed of these I/O operations.
What does temporary tablespace I/O look like?
The temporary tablespace I/O of a single process is typically bursty and logically sequential.
However, when multiple processes in the same instance are using the temporary tablespace simultaneously,
then the I/O is more consistent and pseudo-random at the disks,
although the individual I/O operations may still be large if not split at the file system layer.
However, if multiple processes, each in distinct instances, are using the temporary tablespace simultaneously,
and if disk affinity exists for at least one tempfile (or datafile) of that tablespace in each instance,
then the sequential nature of the I/O may be maintained, because each process will prefer extents in the files for which it has affinity.
How should temporary tablespaces be striped?
In general, temporary tablespaces should be striped to allow for multiple processes in the same instance to use the tablespace simultaneously.
Unfortunately, this defeats disk affinity, but if the user workload is clearly partitioned between instances,
then it may be possible to create a distinct temporary tablespace for each instance on local disk.
The striping of temporary tablespace disks should be for concurrency, rather than transfer rate.
If the files are raw, or if file system direct I/O is available,
then the I/O operations will be large enough to benefit from fine-grained striping to improve the transfer rate.
However, in most environments the benefits of increased concurrency from coarse-grained striping
easily outweigh those that might be obtained by improving the transfer rate.
(In high-end environments it is possible to enjoy both benefits
by layering coarse-grained software-based striping over fine-grained hardware-based striping.)
The stripe breadth for temporary tablespaces should normally be equal to the largest degree of parallelism
expected to be used for parallel query or DML operations against that tablespace,
and the stripe element size should be a small integer divisor of the temporary tablespace extent size.
If such broad striping is not possible, then the stripe element size should be reduced proportionately.
Of course, it should go almost without saying that these disks should be dedicated to just one tablespace.
Temporary tablespace I/O within a single tablespace shows very good locality properties,
and sharing the disks in any way introduces long seeks, and greatly increases the service times.
Do temporary tablespaces need data protection?
It is possible to recover an instance immediately after the loss of a temporary tablespace datafile or tempfile,
without needing to first restore the file itself.
Indeed, it is recommended that tempfiles not even be backed up.
Therefore, data protection is only needed for temporary tablespaces in true 7x24 environments
that cannot afford even a brief outage due to media failure.
If data protection for the temporary tablespaces is deemed necessary,
then it must be provided by the hardware rather than by software,
and preferably via mirroring rather than RAID-5, to ease its performance cost.
File-system based tempfiles
File-system based tempfiles are typically sparse.
This means that although operating system utilities like ls show the intended full size of the tempfile,
no file-system space is allocated to blocks that have never been used.
If the file-system space is then exhausted by other files, it is possible for writes to the tempfile to fail with ORA-07248 errors.
To avoid this, file system based tempfiles should be preallocated as dense files and the REUSE keyword specified during tablespace creation.
Existing sparse tempfiles can be made dense by copying and restoring the file while the instance is down
using standard operating system utilities like cp, dd or tar.
Of course, tempfiles should be raw, so this concern should not arise.
Copyright © Ixora Pty Ltd