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Re: Performance issues with /gnu/store in virtual machines

From: Christopher Baines
Subject: Re: Performance issues with /gnu/store in virtual machines
Date: Sun, 20 Aug 2017 11:22:09 +0100

On Sun, 20 Aug 2017 11:02:16 +0200
Ricardo Wurmus <address@hidden> wrote:

> Hi Chris,
> > So I was trying out using VMs created through Guix for more things,
> > particularly as they can be run as part of derivations, e.g. the
> > system tests for Guix.
> >
> > However, I'm seeing anything that does I/O performing very poorly
> > compared with the host system.  
> (I’m not using VMs as KVM doesn’t work on my laptop.)
> Have you tried specifying “virtio” drivers for the virtual disks?  I
> don’t think they are the default and I’m hopeful that with virtio
> drivers I/O performance will be comparable.

I think I may already be using virtio, at least that is what is
specified as the trans option for the mount in the VM? I think this is
the relevant bit from the vm start script:

  -virtfs local,path="/gnu/store",security_model=none,mount_tag="TAG_gnu_store"

This is how the mount shows up in the vm:

  TAG_gnu_store on /gnu/store type 9p (ro,relatime,sync,dirsync,trans=virtio)

One thing I have just tried, is adding the cache=loose option in (by
directly editing the mapping->file-system procedure in (gnu system
vm)). For my really quick test [1], this did increase the initial
runtime from ~8 to ~11 seconds, but subsequent runs were down to ~5
seconds from the original ~8.

5 seconds though, is still 16 times longer than the ~0.3 seconds this
command takes to run on the host, so it's still slow by comparison.

1: time find /gnu/store/*-guile* -name guile | wc -l

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