Re: [PATCH] Avoiding fragmentation through different allocator

[James Bottomley <jejb@steeleye.com>]

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


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