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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: Tue, 22 Aug 2017 12:20:34 +0100

On Tue, 22 Aug 2017 12:11:43 +0200
address@hidden (Ludovic Courtès) wrote:

> Hi Chris,
> Christopher Baines <address@hidden> skribis:
> >   1.13    0.000019           0      3853           openat  
> [...]
> >  13.56    0.193014          50      3853           openat  
> So openat(2) is 4 orders of magnitude slower in the VM, which is a
> problem.
> I guess a large part of that is due to the fact that VM created with
> ‘guix system vm’ use unionfs as their root file system, which is a
> FUSE thing (see the “volatile” options in (gnu system vm), (gnu system
> linux-initrd), and (gnu build linux-boot).)  I’m not sure how we could
> avoid it.

I'm unsure if the unionfs will be involved though, as find is searching
in the /gnu/store directory, which is shared with the host:

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

This might be misunderstanding how mounts work, but this could be
either layered on top, or below the unionfs, and I'm guessing its on
top, which in my mind would mean that the unionfs isn't involved?

> Can you confirm that the same ‘find’ command is faster in a full-blown
> “guix system disk-image” VM?

Yep, I see 8 usecs/call (compared to 50).

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