IDE performance drop between 2.4.23 and 2.6.0

[Jean-Luc Fontaine <jfontain@free.fr>]

Using hdparm -Tt /dev/hdb:

on 2.4:
  Timing buffer-cache reads:   168 MB in  2.01 seconds =  83.58 MB/sec
  Timing buffered disk reads:   44 MB in  3.12 seconds =  14.10 MB/sec
on 2.6:
  Timing buffer-cache reads:   172 MB in  2.02 seconds =  84.95 MB/sec
  Timing buffered disk reads:   34 MB in  3.08 seconds =  11.04 MB/sec

Note the big drop of of 3 MB/sec on disk reads.

dmesg (edited) gives:

on 2.4:
  VP_IDE: VIA vt82c586b (rev 41) IDE UDMA33 controller on pci00:07.1
  hdb: IC35L040AVVN07-0, ATA DISK drive
  hdb: 80418240 sectors (41174 MB) w/1863KiB Cache, CHS=79780/16/63, 
UDMA(33)
on 2.6:
  VP_IDE: VIA vt82c586b (rev 41) IDE UDMA33 controller on pci0000:00:07.1
  hdb: IC35L040AVVN07-0, ATA DISK drive
  hdb: 80418240 sectors (41174 MB) w/1863KiB Cache, CHS=65535/16/63, 
UDMA(33)

hdparm -i output is identical between the 2 kernels and hdparm settings 
are identical as follows (except cylinders as above):

  Model=IC35L040AVVN07-0, FwRev=VA2OAG0A, SerialNo=VNP210B2RAP0TB
  Config={ HardSect NotMFM HdSw>15uSec Fixed DTR>10Mbs }
  RawCHS=16383/16/63, TrkSize=0, SectSize=0, ECCbytes=52
  BuffType=DualPortCache, BuffSize=1863kB, MaxMultSect=16, MultSect=16
  CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=80418240
  IORDY=on/off, tPIO={min:240,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
  AdvancedPM=yes: disabled (255) WriteCache=enabled
  Drive conforms to: ATA/ATAPI-5 T13 1321D revision 1:

  multcount    = 16 (on)
  IO_support   =  1 (32-bit)
  unmaskirq    =  1 (on)
  using_dma    =  1 (on)
  keepsettings =  0 (off)
  readonly     =  0 (off)
  readahead    =  8 (on)
  geometry     = 79780/16/63, sectors = 80418240, start = 0

I noticed a difference in dmesg:
on 2.4:
  Uniform Multi-Platform E-IDE driver Revision: 7.00beta4-2.4
on 2.6:
  Uniform Multi-Platform E-IDE driver Revision: 7.00alpha2

Could that make a difference?

Please let me know if you'd like me to run more tests, patch kernels, 
..., anything to regain the performance.


Many thanks in advance,

-- 
Jean-Luc Fontaine