Apple OSes and IDE DMA Support?

KFox wishes to get to the core of this particular issue: "I just recently purchased an iBook and I have noticed that even in Jaguar, the system gets choppy from disk I/O. It appears that Mac OS 9 and Mac OS X don't support DMA access for hard drives, even if the drives are initialized in a UDMA mode. Wintel has had support in this area for a long time (since Win95b). Has anyone in the Mac world had any experience with DMA support on hard drives? Is it supported on iBooks? If so, which models?"

Comments

Drive Setup v1.1

[brejc8]

The ChangeLog seems to have added UDMA in version 1.1 and later updated in 1.7 .
Try searching for UDMA Ultra-ATA ATA33/66/100 because DMA doesnt allways show up.

Choppy Disk or VM switching out

[Anonymous Coward]

Are you sure it's the disk causing the problem vs delays due to VM switching in and out. There's a memory utility (can't remember name, Memory Tracker or Memory Monitor?) that will beep each time memory is paged in or out (or you could use top if you can interpret the numbers).

I've found it can be pretty easy to use up all free RAM if you have a number of programs open or they leak memory. I've found at times I've had over 2GB of switch files.

This is with 896 MB of RAM, iBooks can be under spec'd in that department.

Re:Choppy Disk or VM switching out

[coolgeek]

Me too. Esp. if running OS X w/256MB (or less), you need the WindowServer compression hack or else all your available RAM goes to window buffers, which inevitably causes swapping.

Re:Choppy Disk or VM switching out

[frankie]

Note that window buffer compression is enabled by default in Jaguar. I still wouldn't run OS X with less than 256MB though, and 512+ is much faster.

try this

[austad]

# sysctl -a | grep dma

Look for a setting that's related to DMA and set it from 0 to 1 with "sysctl -w". You should be able to place the setting in /etc/sysctl.conf to make it enable it on every boot also.

Unfortunately, I don't have an OSX box to test it on right now, so I can't give you the exact settings. OSX is based on FreeBSD, and FreeBSD enables DMA by default, however, it's possible that apple found a need to disable it by default. Some drive/chipset combos have problems with DMA, and if they were unsure of which brands of drives or chipsets they would be using, disabling it by default would be the safe way to go.

Re:DMA check for your ibook

[OttoM]

Since 10.0 it is Command-V (V was the public beta). The information in the daemon news article is quite old. Jaguar seems a lot less verbose. It does not say anything about DMA transfers for disk or CD. Which of course doesn't mean it does not do DMA.

A quick and dirty test of disk I/O

[tokki]

This isn't the most comprehensive test of a disk's I/O, but it's easy and will help determine is something is potentially wrong.

Run this command from the Mac OS X shell:

dd if=/dev/zero of=testfile bs=1024 count=102400

This should make you a 100 MB file. dd on some systems will give you how long it took to write out the file in seconds (I don't have access to a Mac OS X box, just a FreeBSD box right now). If it doesn't, just put the time command in front of it:

time dd if=/dev/zero of=testfile bs=1024 count=102400

Devide the number of seconds it took to write the file out by 100, and you'll have a rough estimate of how many megabytes per second it wrote to disk.

Again, not the most comprehensive metric of course, but it gives you a rough idea. For an ATA system, it should probably do between 15 and 25 MB/s without too much trouble for a fast drive. The interface can be much faster (ATA/33, ATA/66, ATA/100), but the UFS and HFS+ file systems aren't optimized for sequntial writing, and/or there are other factors involved, so about 20 MB/s is a good mark I think.

Not a DMA issue

[h0tblack]

Mac's have had DMA support for a long time in both hardware and software, see this technote and the Developer Notes on hardware here for details. This includes the original clamshell iBooks. The iBooks may not have the fastest drives or interfaces but I don't think this is the problem. It's much more likely a Virtual Memory issue with lots of caching going on. check the contents of /private/var/vm/ for the number of 80Mb swapfiles you have, also try issuing vm_stat from the CLI to see whats going on. Two suggestions, either get more RAM or setup a 'clean' swap-drive partition.