I am doing large DB imports on a Postgres instance running in a Proxmox 1.9 OpenVZ virtualized Ubuntu 11.04. For some reason, the executable that initiates and controls the import (or Postgres), always stops at some point, typically, when there is high I/O load on the system (i.e. the proxmox host or in any of the VM instances) caused in addition to that of the importing process. I am trying to figure out the root issue and whether I can potentially avoid it. Ps tells me that wchan = n_tty_ for the importing process (which is sleeping). First of all what does this mean and what are good practices in how to debug further?


Try strace.

You can invoke it like strace -p pid

Or, better yet, strace your_prog_here

  • strace -f -v -p 27662 Process 27662 attached - interrupt to quit read(0, Process 27662 attached - interrupt to quit read(0, ^C <unfinished ...> Process 27662 detached' Hm, I am not any wiser now.. – user462982 Jan 20 '12 at 0:26
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    file descriptor 0 is stdin, it sounds like this process is reading something from it's terminal or a pipe, and it is waiting for that. Are you sure you are running the command properly (passing input to it on stdin when necessary)? Is there a command in a pipe in front of this that maybe is hanging and not sending data to this process? – Sean Reifschneider Jan 20 '12 at 4:37
  • The program is a c program compiled on the same system and it talks to postgres via pipe, so this could be the issue here I guess (it does not require user intervention). Postgres indicates the program still as connected, though idle for more than 24hs. To me, this looks like a problem with IPC. As I have the source of the c program (I am not not a c developer, though), is it maybe possible (and would it help) to include some kind of debug symbols/compile it with debug flags (without changing the source and with gcc) that could provide more information? – user462982 Jan 20 '12 at 8:10

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