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Re: Filesystem fragmentation and scatter-gather DMA



Jon Forrest wrote:
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.

You should keep in mind that big servers also have higher end storage systems (or at least multiple devices). Heads don't tend to move about randomly - they will normally try to read (or write) in a specific order. Normally, that order is in increasing sector order.

Every level of the the system tries to guess how to combine and read ahead, all the way from the file system down to the internal firmware in the storage.

The best way to get read-ahead to work is to use really obvious patterns - sequential, increasing and large IO's work best ;-)


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 would not spend any time worrying about the sector remapping. SMART can tell you how many sectors have been remapped, but even with a really large disk the maximum number of remapped sectors is tiny (say 2000 or so for a 500GB disk). Your chances of hitting them are tiny, especially since most drives end up with very, very few remapped sectors before they get tossed. Those with more than 100 sectors, for example, tend to complain a lot.

The short answer is to look at the sector level order of your file and assume (pretend) that it reflects the media layout as well.

Note that the whole deal changes when you have multi-drive RAID devices (software or hardware).

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.


I think that fragmentation is a bad performance hit, but that we actually do relatively well in keeping our files contiguous in normal cases.

I have a simple bit of c code that uses fibmap to dump the sectors/blocks for a specific file. If you like, I can send it over to you.

Regards,

Ric


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