Filesystem fragmentation and scatter-gather DMA

Jon Forrest jlforrest at
Mon Mar 17 16:52:04 UTC 2008

David Schwartz wrote:

> That's not really the issue. The issue is whether a read of a chunk of a
> file can take place without any extra seeks or whether it does require extra
> seeks. Further, for the vast majority of cases, there is only one I/O stream
> going on at a time. The disk will read ahead. If that can satisfy even a
> small fraction of the subsequent I/Os the OS issues, that's a big win.

Maybe on a single user PC, some of the time there is only one I/O
stream going on a time. But, once you start doing anything in parallel,
or have multiple users, the number of sources (and destinations) of I/O
goes way up. This, the arm is going to have to be moving around randomly
even if the files involved aren't fragmented. Some (most?) OSs sort
I/Os so that the movement is minimized but it still occurs.

>> 3) Modern disks do all kind of internal block remapping so there's
>> no guarantee that what appears to be contiguous to the operating
>> system is actually really and truly contiguous on the disk. I have
>> no idea how often this possibility occurs, or how bad the skew is
>> between "fake" blocks and "real" blocks. But, it could happen.
> Not bad enough to make a significant difference on any but a nearly-failing
> drive.

It would be interesting to see what I'm calling the skew between
the true sector layout and what an O/S sees on modern SATA drives.
I'm not aware of any way to see this. Does anybody know?

I stand by my assertion that while disk fragmentation is in no way
a good thing, it isn't something to fear, at least not in the way
shown in the advertisements for defragmentation products.

Jon Forrest
Research Computing Support
College of Chemistry
173 Tan Hall
University of California Berkeley
Berkeley, CA
jlforrest at

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