Re: [PATCH] Avoiding fragmentation through different allocator

James Bottomley
2005-01-24T20:43:10Z

On Mon, 2005-01-24 at 13:49 -0200, Marcelo Tosatti wrote:
> So is it valid to affirm that on average an operation with one SG element pointing to a 1MB
> region is similar in speed to an operation with 16 SG elements each pointing to a 64K
> region due to the efficient onboard SG processing?

it's within a few percent, yes. And the figures depend on how good the
I/O card is at it. I can imagine there are some wildly varying I/O
cards out there.

However, also remember that 1MB of I/O is getting beyond what's sensible
for a disc device anyway. The cable speed is much faster than the
platter speed, so the device takes the I/O into its cache as it services
it. If you overrun the cache it will burp (disconnect) and force a
reconnection to get the rest (effectively splitting the I/O up anyway).
This doesn't apply to arrays with huge caches, but it does to pretty
much everything else. The average disc cache size is only a megabyte or
so.

James