is writing to slow storage in nohup mode blocking the process?

Question

If I have a process writing to nohup and the storage where it writes to is slow, does it block the process which is running in nohup? For example:

process A is running in nohup, it needs to write 10 lines to console

it takes 2 minutes for disk storage to write these 10 lines into nohup.out (it write 6 lines and then it stuck for a minute waiting for disk)

is the process A blocked until the remaining lines can be written, or not?

Answer 1

nohup mode has no relevance on whether the process is I/O bound. So yes, slow storage will probably slow down the process.

If the process has bursty output (e.g. 10 lines every 1 hour), but throughput lower than your "slow" storage, then you can improve performance by e.g. writing to a pipe or a FIFO (which then gets copied to the slow storage by another process), as long as one burst of output doesn't exceed the pipe's buffer.

mkfifo /tmp/fifo
nohup prog >/tmp/fifo &
cat </tmp/fifo >slow_log_file &

or

nohup prog | buffer -s 16k >slow_log_file &

where buffer is a special program (available on Debian with apt-get install buffer) that maintains a large internal queue to avoid blocking the writer.

Or if you control the source code, you could implement buffering yourself and maybe use a separate logger process to write to the slow storage. I know djbdns (famous for its stability) uses a logger process.

Answer 2

As a normal process which produce huge IO it will be blocked. But usually filesystem blocks are cached in memory. And when OS try to write dirty blocks to disk you will see that wait column in top output become 100%