Re: max request size 1024KiB by default?

[Johannes Stezenbach <js@convergence.de>]

On Sat, Jul 24, 2004 at 06:58:02PM -0400, Lee Revell wrote:
> On Sat, 2004-07-24 at 07:27, Rudo Thomas wrote:
> > > HD info:
> > > /dev/hdc:
> > > 
> > >  Model=Maxtor 6Y160P0, FwRev=YAR41BW0, SerialNo=Y44K8TZE
> > >  Config={ Fixed }
> > >  RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=57
> > >  BuffType=DualPortCache, BuffSize=7936kB, MaxMultSect=16, MultSect=16
> > >  CurCHS=4047/16/255, CurSects=16511760, LBA=yes, LBAsects=268435455
> > >  IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
> > >  PIO modes:  pio0 pio1 pio2 pio3 pio4 
> > >  DMA modes:  mdma0 mdma1 mdma2 
> > >  UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6 
> > >  AdvancedPM=yes: disabled (255) WriteCache=enabled
> > >  Drive conforms to: (null): 
> > 
> 
> Your disk controller must not support that.  It looks like the default
> is 1024KiB or whatever the max your controller supports is:

For drives smaller than 128GiB the "48-bit Address feature set" is optional,
hence many older drives do not support it (since the old 28-bit LBA
addressing is suffient).

With 28-bit addressing the number sectors which can be transferred
in one request is limited to 256. Linux 2.4 limited this further
to 128 (i.e. 64KiB). With 48-bit addressing up to 2^16 sectors
could be transferred in one request, but Linux 2.6 puts an arbitrarily
chosen limit of 1024KiB on it.

'hdparm -I /dev/hda' will tell you whether your drive supports
48-bit addressing:

  Commands/features:
	Enabled	Supported:
	   *	48-bit Address feature set 

For full details see the ATA/ATAPI-7 spec at http://www.t13.org/.

Johannes