Tag Archives: BASH

CLI

Blow away ZFS snapshots and watch the progress

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For the last month I have had a testing system (FreeNAS) take ZFS snapshots of sample datasets every five minutes. As you can imagine, the snapshot count has risen quite dramatically. I am currently at over 12,000 snapshots.

In testing a backup script I’m working on I’ve discovered that replicating 12,000 snapshots takes a while. The initial data transfer completes in a reasonable time frame but copying each subsequent snapshot takes more time than the original data. Consequently, I decided to blow away all my snapshots. It took a while! I devised this fun little way to watch the progress.

Open two terminal windows. In terminal #1, enter the following:

bash
while [ true ]; do zfs list -H -t snapshot | wc -l; sleep 6; done

The above loads BASH and the runs a simple loop to count the total number of snapshots on the system. The sleep command is only there because it takes a few seconds to return the results when you have more than 10,000 snapshots.

Alternatively you could make the output a little prettier by entering the following:

while [ true ]; do REMAINING="`zfs list -H -t snapshot | wc -l`"; echo "Snapshots remaining: $REMAINING" ; sleep 6; done

In terminal #2, enter the following (taken from here):

bash
for snapshot in `zfs list -H -t snapshot | cut -f 1`
do
zfs destroy $snapshot
done

You can now hide terminal#2 and observe terminal #1. It will show you how many snapshots are left, refreshing the number every 6 seconds. Neat.

CLI

Watch a zpool resilver in freeNAS

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In my experiments with freeNAS and RaidZ I have come to miss some functionality I enjoyed with Linux and mdadm. One such function was being able to watch an array rebuild, or in ZFS parlance, a pool resilvering.

My inability to watch the resilvering stems from the difference between what the watch command in Linux does and what it does in FreeBSD. Watch in BSD snoops on a tty line whereas watch in Linux executes a command repeatedly.

One option is to install a watch utility for BSD that behaves as the Linux watch command; however, freeNAS is a small read only image so installing things isn’t an option.

The way to do it in freeNAS is to use a while loop in the command line. After 20 minutes of googling I realized that there is no easy way to do this in one line like you can in bash (something about things requiring to be on a new line), so I had to settle for a quick script like one outlined here.

My familiarity with scripts comes from BASH, but I quickly found out freeNAS doesn’t ship with BASH.

echo $shell
/bin/csh

edit: It turns out freeNAS does indeed ship with bash! It’s just not the default shell. Simply execute “bash” in the shell and use your familiar bash shell syntax to your heart’s content. The BASH equivalent of the script below is:

while [ true ]; do clear; zpool status; sleep 1; done

I’ll leave the rest in for reference sake.

I did some digging on how to write CSH scripts and thanks to this website was able to write a simple CSH script to execute a given command at a given interval indefinitely.

Here is my C style watch script:

#!/bin/csh

#A simple script to replace the Linux watch functionality. The first input it takes is how many seconds to refresh; the second, the command to run. If the command has arguments (spaces), it must be passed in quotes.

set INTERVAL = "$1"
set COMMAND = "$2"

while ( 1 )
        clear
        $COMMAND
        sleep $INTERVAL
end

I placed this script in the /tmp directory, made it executable by running chmod +x, and then executing it by running ,/script.sh 1 “command”