Filesystem fragmentation and scatter-gather DMA

[Jon Forrest]

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
94720-1460
510-643-1032
jlforrest at berkeley.edu