On 04Mar2015 18:27, Rick Stevens <ricks@xxxxxxxxxxxxxx> wrote:
On 03/04/2015 05:01 PM, Tom Horsley wrote:
Here's a weird one: A system at work has (God knows why) a
gazillion symlinks directly under / pointing to NFS mountpoints
for filesystems (some of which might well have high latency).
If I run "df -l", using the -l option in the apparently
vain hope that it might not timeout forever on some NFS
mount, it hangs for a long time.
"Walks"? This is df, not du. Consult mount table, do fstats.
[...]
Since strace requires it to report what it's doing, it's getting
interrupted a lot, so it doesn't hang. I mean, it's still waiting on
the I/O to complete from NFS, but rather than waiting an interminate
time, it's getting interrupts (signals) from strace rather than hanging
on the one from the NFS system.
I'm fairly certain that strace does not work that way. The traced process is
not doing work for the tracer.
Just a wild guess.
I think so too.
Tom:
- _after_ a fast straced df, is un unstraced df slow again?
(thinking about cached answers to call, caches in the OS, possibly quite
briefly)
- see if the result of strace's -T option is informative.
- since df's output is line buffered on a terminal, the presentation of the
lines should tell you where it is hanging.
df makes pleasingly few system calls on a handy RHEL5 host. It opens /etc/mtab
and essentially just calls statfs() on each name. This implies that determining
localness is done based entirely on the contents of /etc/mtab.
Also, this shows that there are no other system calls between the write()
reporting the prior filesystem and the statfs() inquiring about the next, so
watching an unstraced on-a-terminal df should pinpoint the place of stallness.
Is it similar on your fedora box?
Cheers,
Cameron Simpson <cs@xxxxxxxxxx>
Against stupidity....the Gods themselves contend in vain!
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