>
> We don't disable caching on other platforms. And unless you modified
> run-readtest.sh, we aren't disabling it on OS X either in the test (that
> requires an extra -D flag to be passed to readtest).
Caching was disabled, but I had tried a lot of different tests, one of the was
to remove lookahead reads and inadvertently it’s still in there. So this
explains the more ‘deterministic’ results. But I think you’re going to need
this also, since what you’re doing is writing your own file cache, and you
don’t want the filesystem to think it knows better.
if (fcntl(fd, F_RDAHEAD, 0) == -1) {
fprintf (stderr, "Cannot set F_RDAHEAD on file #%d\n", n);
}
> Then a message shows up saying "disk system cannot keep up". Then you start
> digging ... and digging .. and digging ... and you find that for 8 seconds
> the disk was wasn't managing more than 5MB/sec. And eventually you end up
> with readtest.c :)
Ok, that clarifies the issue drastically. One thing to check is possibly
something else hitting the disk, like spotlight. Anything else touching the
disk at that seekrate, and the bandwidth is destroyed. You can disable
spotlight for that drive (System preferences -> Spotlight -> Privacy). If it’s
not spotlight, a utility called fseventer will show system wide file access,
but it doesn’t look like mavericks is supported yet. But there has got to be a
command line util that can watch fsevent.
Something to try anyway.
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