Re: [SUGGESTION]: drop virtual merge accounting in I/O requests

[Mikulas Patocka]

> > > > As you mentioned ESP driver, it declares .sg_tablesize = SG_ALL, so
> > > > BIO_VMERGE_BOUNDARY has no effect on the operation of this driver. Any
> > > > other driver where BIO_VMERGE_BOUNDARY does matter?
> > >
> > > When BIO_VMERGE_BOUNDARY is turned on, requests that would not
> > > otherwise fit into the device's limits, can.
> >
> > Why would someone want to overshoot SG_ALL? ... and, shouldn't the
> > constant be increased then --- instead of making buggy BIO_VMERGE_BOUNDARY
> > expectations?
>
> I'm talking about other devices, not about the ESP, here.

And which ones are those important drivers that need merge accounting?

A100U2W is an old card, I got it for $8.5 in bazaar, it does 38MB/s. This 
virtual merge accounting helps to stuff on average 4 more segments into 
the 32-entry table.

So the question is: to reduce number of requests by 12% on an outdated 
SCSI card, it is sensible to maintain complicated merge accounting logic 
in the core block layer? To me, it doesn't seem sensible.

Mikulas
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