Are large RAID stripe sizes useful with FreeBSD?

Scott Long scottl at
Mon Mar 31 15:28:51 PDT 2008

Ivan Voras wrote:
> On 01/04/2008, Scott Long <scottl at> wrote:
>> If you have a RAID controller in front of the disks then the effects of
>>  TCQ are hidden from the OS; it might ultimately make the controller
>>  complete requests faster, but the controller already looks to the OS
>>  like a disk with a really deep queue.  When you're dealing directly with
>>  the disks then TCQ/NCQ is required in order for batching of concurrent
>>  requests to occur.
> Thanks. I thought TCQ is also used between the OS and the controller
> (since the RAID array is presented to the OS as opaque...).

TCQ is a protocol specific to parallel SCSI that involves the use of
protocol messages and control signals found only on a parallel SCSI bus.
PCI RAID controllers that present a SCSI interface to the OS don't use
real TCQ to communicate between the OS and the controller.  Even though
these kinds of controllers use the SCSI command protocol, they still use
a custom device-specific low-level communication protocol that handles
the transfer and queuing of commands.  External RAID
controllers/enclosures do use the appropriate SCSI/SAS/FC/SATA bus
queuing mechanism, but I'm not sure if that's what you're talking about


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