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The Linux kernel configuration item CONFIG_AIC7XXX_OLD_TCQ_ON_BY_DEFAULT:
CONFIG_SCSI_AIC7XXX_OLDThis option causes the aic7xxx driver to attempt to use Tagged Command Queueing (TCQ) on all devices that claim to support it.
TCQ is a feature of SCSI-2 which improves performance: the host adapter can send several SCSI commands to a device's queue even if previous commands haven't finished yet. Because the device is intelligent, it can optimize its operations (like head positioning) based on its own request queue. Not all devices implement this correctly.
If you say Y here, you can still turn off TCQ on troublesome devices with the use of the tag_info boot parameter. See the file Documentation/scsi/aic7xxx.txt for more information on that and other aic7xxx setup commands. If this option is turned off, you may still enable TCQ on known good devices by use of the tag_info boot parameter.
If you are unsure about your devices then it is safest to say N here.
However, TCQ can increase performance on some hard drives by as much as 50% or more, so it is recommended that if you say N here, you should at least read the Documentation/scsi/aic7xxx.txt file so you will know how to enable this option manually should your drives prove to be safe in regards to TCQ.
Conversely, certain drives are known to lock up or cause bus resets when TCQ is enabled on them. If you have a Western Digital Enterprise SCSI drive for instance, then don't even bother to enable TCQ on it as the drive will become unreliable, and it will actually reduce performance.
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