high iowait

[Jorge]

You are running linux, check that the path where the SHM log is written is
tmpfs mounted:
tmpfs on /usr/local/var/varnish type tmpfs (rw,size=256M)

A small one will be enough:

Filesystem      Size  Used Avail Use% Mounted on
tmpfs           256M   82M  175M  32% /usr/local/var/varnish

Search for a file named "_.vsm".

If this file is on a "normal" filesystem (disk) linux will periodically
flush the contents to disk, which depending on your traffic can be some
tens of megabytes every some seconds.

-- 
Jorge Nerín
<jnerin at gmail.com>
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