Hi,

Can anyone lucidly explain the whys and hows of this change? Is this a
SCSI emulation thing or what? I know that Ubuntu 7.04 introduced this,
which became a problem and broke a lot of people's /etc/fstab. If you
were lucky enough to have drives referenced by their UUID, you were
safe.

A server that we installed with 7.04 though came up with the hd as
/dev/hda. When we did the 7.10 gutsy upgrade though it became /dev/sda
(luckily the UUID was the reference, though the upgrade doesn't change
the comments written in /etc/fstab.

Anyway under 7.04 I had used hdparm to force on DMA as well as set the
udma level from 2 to 4 - to optimise disk I/O performance. However
hdparm now under 7.10 seems not to want to talk to /dev/sda properly.
I get the following result that used to work

[EMAIL PROTECTED]:~$ sudo hdparm -d1 /dev/sda

/dev/sda:
 setting using_dma to 1 (on)
 HDIO_SET_DMA failed: Inappropriate ioctl for device
[EMAIL PROTECTED]:~$ sudo hdparm -X udma4 /dev/sda

/dev/sda:
 setting xfermode to 68 (UltraDMA mode4)
SG_IO: bad/missing ATA_16 sense data::  70 00 05 00 00 00 00 0a 00 00
00 00 24 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 00
 HDIO_DRIVE_CMD(setxfermode) failed: Input/output error


The problem is that I notice that I get a heck of a lot of CPU WAIT
time happening when doing I/O to the disk, presumably because DMA
isn't working. (BTW I looked at "sdparm" but it seems to be a totally
different animal)

I have added a comment to https://bugs.launchpad.net/bugs/96693.

It seems that there is no definitive technical description of this
migration. I sort of wonder how thoroughly it has been thought through
or tested. Anyone here care to comment?

Regards, Martin


Martin Visser
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