Re: [00/17] Large Blocksize Support V3

On Fri, Apr 27, 2007 at 01:08:17AM +1000, Nick Piggin wrote:
Andy Whitcroft wrote:
Nick Piggin wrote:


I don't understand what you mean at all. A block has always been a
contiguous area of disk.


Lets take Nick's definition of block being a disk based unit for the
moment. That does not change the key contention here, that even with
hardware specifically designed to handle 4k pages that hardware handles
larger contigious areas more efficiently. David Chinner gives us
figures showing major overall throughput improvements from (I assume)
shorter scatter gather lists and better tag utilisation. I am loath to
say we can just blame the hardware vendors for poor design.

So their controllers get double the throughput when going from 512K
(128x4K pages) to 2MB (128x16K pages) requests. Do you really think
it is to do with command processing overhead?

No - it has to do with things like the RAID controller caching behaviour, the
number of disks a single request can keep busy, getting I/os large
enough to avoid partial stripe writes, etc. Remember that this
controller is often on the other side of a HBA so large I/Os are
really desirable here....

Cheers,

Dave.
--
Dave Chinner
Principal Engineer
SGI Australian Software Group
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