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First, finding posts on DMA is tough because the search engine appears to not search on short terms.
Second, I am using Slackware 14.1 on my systems.
Here's the problem....apparently the optical drive does not come up as a DMA device. This can be a problem when booting off optical, but it is a bigger issue if frequent backups are done via the optical drive.
I am open to any suggestions as to how to proceed in understanding this issue.
It would be very good to have optical drive IO done via DMA.
The first thing I would try is a session with hdparm on the drive investigating. Does it do dma? does that improve the read times. man hdparm for options. hdparm -i and -I are interesting. Mine says SATA ≤ 3.0, udma ≤ 5, and ATA/ATAPI ≤7, and that's a recent drive. You should not get higher than ata ≥ 4 without DMA.
So it may not say dma, but if it's running a fast mode it's doing it.
I think you are right. It doesn't actually matter a hoot on the cd. The disk surface seems to affect read speeds more than anything else. read speeds are all ≤ 3MB/S here with various cds, cdrs curiously being faster than pressed disks.
The sata drive will make a difference if it is not dma. Usually it loses dma because it has been grabbed by the wrong driver, and this was a principal reason for me building my own kernel in days gone by.