Are large RAID stripe sizes useful with FreeBSD?
scottl at samsco.org
Mon Mar 31 15:01:35 PDT 2008
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.
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