svn commit: r196777 - head/sys/dev/ahci

[Alexander Motin]

Scott Long wrote:
> On Thu, 3 Sep 2009, Alexander Motin wrote:
>> Scott Long wrote:
>>> In this case, set maxio to 64k, not 127.5k.  You'll typically get 
>>> much better i/o performance out of two 64k transfers
>>> than you will out of one 127.k transfer and one 512 bytes transfer, 
>>> which is what the block layer will give you if
>>> you try to send 128k.
>>
>> Couldn't it be somehow handled on that level?
> 
> It's a limitation of the paticular hardware, not the OS.  Plain disks 
> will behave like this, but RAID controllers might now.  But if you want 
> to add this feature to the upper layers, be my guest.

Huh. May be sometimes later.

>> Limiting maxio from 127.5K to 64K is also a penalty for requests with 
>> length in that range.
> 
> Most I/O that goes to a disk comes from one of the VM pagers, and is 
> thus page aligned and multi-of-page sized.  Breaking one of these 
> requests up into sub-page sized requests costs a whole lot more than 
> what you are assuming.  We analyzed exactly this situation a few years 
> ago at Yahoo and came to this conclusion.

Sure, 127.5K is ugly, but what's about 96K or 112K? Is there benefits 
having it strictly in power of 2?

-- 
Alexander Motin