Re: Are large RAID stripe sizes useful with FreeBSD?

Ivan Voras wrote:
> On 31/03/2008, Scott Long <scottl_at_samsco.org> wrote:
> 
>>  For writes, the performance penalty of smaller I/O's (assuming no RAID-5
>>  effects) is minimal; most caching controllers and drives will batch the
>>  concurrent requests together, so the only loss is in the slight overhead
>>  of the extra transaction setup and completion.  For reads, the penalty
>>  can be greater because the controller/disk will try to execute the first
>>  request immediately and not wait for the second part to be requested,
>>  leading to the potential for extra rotational and head movement delays.
>>  Many caching RAID controllers offer a read-ahead feature to counteract
>>  this.  However, while my testing has shown little measurable benefit to
>>  this, YMMV.
> 
> Thank you, this is the kind of explanation I hoping for. One more
> thing: is TCQ (e.g. the SCSI variant) orthogonal to this?

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.

Scott