Varnish commands

Setting up Varnish

Varnish has a set of command line tools and utilities to monitor and administer Varnish. These are:

• varnishncsa: Displays the varnishd shared memory logs in Apache / NCSA combined log format
• varnishlog: Reads and presents varnishd shared memory logs.
• varnishstat: Displays statistics from a running varnishd instance.
• varnishadm: Sends a command to the running varnishd instance.
• varnishhist: Reads varnishd shared memory logs and presents a continuously updated histogram showing the distribution of the last N requests by their processing.
• varnishtop: Reads varnishd shared memory logs and presents a continuously updated list of the most commonly occurring log entries.
• varnishreplay: Parses varnish logs and attempts to reproduce the traffic.

For further information and example of usage, please refer to the man-pages.

varnishtop

This command shows the most often-made requests to the backend:

It’s excellent for spotting often-requested items that are currently not being cached. The “-b” flag filters for requests made to the backend. “-i TxURL” filters for the request URL that triggered the request to the backend. Its output looks something like this:

Top of the list, most often-requested URL from the backend. A prime candidate for caching.

varnishhist

This command hows a histogram for the past 1000 requests, whether they were cache hits (denoted by a ‘|’) or misses (denoted by a ‘#’), and how long the requests took to process (further to the right, longer time). It’s good for a high-level view of how the server is doing under load.

varnishlog

This command displays all varnish traffic for a specific client. It’s helpful for seeing exactly what a particular page or request is doing. Set it to your workstation IP, load the page, see everything Varnish does with your connection including hit/miss/pass status. Varnishlog is really useful, but it puts out an overwhelmingly-large amount of data that isn’t easily filtered. The “-o” option groups all of the entries for a specific request together (without it all entries from all requests are displayed fifo) and it accepts a tag (”ReqStart” in this example) and regex (the IP address in this case) to filter for only requests associated with that tag & regex. It’s the only way I’ve found to filter down the firehose of log entries into something useful.

varnishstat

This command provides an overview of the stats for the current Varnish instance. It shows hit/miss/pass rates and ratios, lots of other gory internal details.

Watch that RAM, or “vmstat, oh how I love thee!”

Varnish can eat RAM like there’s no tomorrow. Be careful and be sure to configure its max memory to be something less than your available RAM. I forgot when I first set things up. The system worked great for a while, and then took a nosedive as the Varnish cache ate up all the available RAM and pushed the system into a swap death spiral.