The following is part of a series on the different aspects of disk I/O performance and optimization for Oracle
databases. Each tip is excerpted from the not-yet-released Rampant TechPress book, "Oracle disk I/O tuning," by Mike Ault. Check back to the main series page for upcoming installments.
- -T - Perform timings of cache reads for benchmark and comparison purposes. For meaningful results, this operation should be repeated 2-3 times on an otherwise inactive system (no other active processes) with at least a few megabytes of free memory. This displays the speed of reading directly from the Linux buffer cache without disk access. This measurement is essentially an indication of the throughput of the processor, cache, and memory of the system under test. If the -t flag is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t operation.
- -t - Perform timings of device reads for benchmark and comparison purposes. For meaningful results, this operation should be repeated 2-3 times on an otherwise inactive system (no other active processes) with at least a couple of megabytes of free memory. This displays the speed of reading through the buffer cache to the disk without any prior caching of data. This measurement is an indication of how fast the drive can sustain sequential data reads under Linux, without any file system overhead. To ensure accurate measurements, the buffer cache is flushed during the processing of -t using the BLKFLSBUF ioctl. If the -T flag is also specified, then a correction factor based on the outcome of -T will be incorporated into the result reported for the -t operation.
- -u - Get/set interrupt-unmask flag for the drive. A setting of 1 permits the driver to unmask other interrupts during processing of a disk interrupt, which greatly improves Linux's responsiveness and eliminates "serial port overrun" errors. Use this feature with caution: some drive/controller combinations do not tolerate the increased I/O latencies possible when this feature is enabled, resulting in massive files system corruption. In particular, CMD-640B and RZ1000 (E) IDE interfaces can be unreliable (due to a hardware flaw) when this option is used with kernel versions earlier than 2.0.13. Disabling the IDE pre-fetch feature of these interfaces, (usually a BIOS/CMOS setting) provides a safe fix for the problem for use with earlier kernels.
- -v - Display all settings, except -i (same as -acdgkmnru for IDE, -gr for SCSI or -adgr for XT). This is also the default behavior when no flags are specified.
- -W - Disable/enable the IDE drive's write-caching feature (default state is undeterminable; manufacturer/model specific).
- -X - Set the IDE transfer mode for newer (E) IDE/ATA drives. This is typically used in combination with -d1 when enabling DMA to/from a drive on a supported interface chipset. X mdma2 is used to select multiword DMA mode2 transfers and -X sdma1 is used to select simple mode 1 DMA transfers. With systems which support UltraDMA burst timings, -X udma2 is used to select UltraDMA mode2 transfers (you'll need to prepare the chipset for UltraDMA beforehand). Apart from that, use of this flag is seldom necessary since most/all modern IDE drives default to their fastest PIO transfer mode at power-on. Fiddling with this can be both needless and risky. On drives which support alternate transfer modes, -X can be used to switch the mode of the drive only. Prior to changing the transfer mode, the IDE interface should be jumpered or programmed (see -p flag) for the new mode setting to prevent loss and/or corruption of data. Use this with extreme caution! For the PIO (Programmed Input/Output) transfer modes used by Linux, this value is simply the desired PIO mode number plus 8. Thus, a value of 09 sets PIO mode1, 10 enables PIO mode2, and 11 selects PIO mode3. Setting 00 restores the drive's "default" PIO mode, and 01 disables IORDY. For multiword DMA, the value used is the desired DMA mode number plus 32. For UltraDMA, the value is the desired UltraDMA mode number plus 64.
- -z - Force a kernel re-read of the partition table of the specified device(s).
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About the author
Mike Ault is a SearchOracle.com expert and a senior Oracle consultant with Burleson Consulting, and one of the leading names in Oracle technology. The author of more than 20 Oracle books and hundreds of articles in national publications, Mike Ault has five Oracle Masters Certificates and was the first popular Oracle author with his landmark book "Oracle7 administration and management." Mike also wrote several of the "Exam Cram" books, and enjoys a reputation as a leading author and Oracle consultant. Ask Mike a question today!