Oracle disk I/O tuning: ATA tuning in Linux, part 2

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.

Mike Ault Mike Ault is one of SearchOracle.com's Oracle Internals experts. Mike is senior Oracle consultant with Burleson Consulting, and one of the leading names in Oracle technology. To view Mike's expert responses or to ask him a question, click here.

This tip, listing hdparm parameters K-S, is a continuation of ATA tuning in Linux, part 1.

• -k - Get/set the keep_settings_over_reset flag for the drive. When this flag is set, the driver will preserve the -dmu options over a soft reset, as done during the error recovery sequence. This flag defaults to off, to prevent drive reset loops, which could be caused by combinations of -dmu settings. The -k flag should therefore only be set after one has achieved confidence in correct system operation with a chosen set of configuration settings. In practice, all that is typically necessary to test a configuration (prior to using k) is to verify the drive can be read/written, and no error logs (kernel messages) are generated in the process (look in /var/adm/messages on most systems).

• -K - Set the drive's keep_features_over_reset flag. Setting this enables the drive to retain the settings for -APSWXZ over a soft reset (as done during the error recovery sequence). Not all drives support this feature.

• -m - Get/set sector count for multiple sector I/O on the drive. A setting of 0 disables this feature. Multiple sector mode (IDE Block Mode) is a feature of most modern IDE hard drives, permitting the transfer of multiple sectors per I/O interrupt, rather than the usual one sector per interrupt. When this feature is enabled, it typically reduces operating system overhead for disk I/O by 30-50%. On many systems, it also provides increased data throughput of anywhere from 5% to 50%. HOWEVER, some drives, most notably the WD Caviar series, seem to run slower with multiple mode enabled. Your mileage may vary. Most drives support the minimum settings of 2, 4, 8, or 16 (sectors). Larger settings may also be possible, depending on the drive. A setting of 16 or 32 seems optimal on many systems. Western Digital recommends lower settings of 4 to 8 on many of their drives, due to tiny (32kB) drive buffers and non-optimized buffering algorithms. The -i flag can be used to find the maximum setting supported by an installed drive (look for MaxMultSect in the output). Some drives claim to support multiple mode, but lose data at some settings. Under rare circumstances, such failures can result in massive file system corruption.

• -M - Get/set Automatic Acoustic Management (AAM) setting. Most modern disk drives have the ability to speed down the head movements to reduce their noise output. The possible values are between 0 and 254. The quietest setting is 128 (and therefore slowest) and 254 is the fastest (and loudest). Some drives have only two levels (quiet/fast), while others may have different levels between 128 and 254. THIS FEATURE IS EXPERIMENTAL AND NOT WELL TESTED. USE AT YOUR OWN RISK. The faster the head, the better the performance at a cost of a noisier drive.

• -p - Attempt to reprogram the IDE interface chipset for the specified PIO mode, or attempt to auto-tune for the "best" PIO mode supported by the drive. This feature is supported in the kernel for only a few "known" chipsets, and even then, the support is iffy at best. Some IDE chipsets are unable to alter the PIO mode for a single drive, in which case this flag may cause the PIO mode for both drives to be set. Many IDE chipsets support either fewer or more than the standard six (0 to 5) PIO modes, so the exact speed setting that is actually implemented will vary by chipset/driver sophistication. Use with extreme caution! This feature includes zero protection for the unwary, and an unsuccessful outcome may result in severe file system corruption!

• -P - Set the maximum sector count for the drive's internal pre-fetch mechanism. Not all drives support this feature.

• -Q - Set tagged queue depth (1 or greater), or turn tagged queuing off (0). This only works with the newer 2.5.xx (or later) kernels, and only with the few drives that currently support it.

• -r - Get/set read-only flag for device. When set/ write operations are not permitted on the device.

• -S - Set the standby (spindown) timeout for the drive. This value is used by the drive to determine how long to wait (with no disk activity) before turning off the spindle motor to save power. Under such circumstances, the drive may take as long as 30 seconds to respond to a subsequent disk access, though most drives are much quicker. The encoding of the timeout value is somewhat peculiar. A value of zero means "off". Values from 1 to 240 specify multiples of 5 seconds for timeouts from 5 seconds to 20 minutes. Values from 241 to 251 specify from 1 to 11 units of 30 minutes for timeouts from 30 minutes to 5.5 hours. A value of 252 signifies a timeout of 21 minutes, 253 sets a vendor-defined timeout and 255 is interpreted as 21 minutes plus 15 seconds.

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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!