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

[Scott Long]

On Thu, 3 Sep 2009, Alexander Motin wrote:
> 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
> it strictly in power of 2?
It's convenient.  We've analyzed the combinations.  I'm just trying to 
offer some advice from experience.  I'm going to drop the conversation 
now.

Scott