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
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