Re: [Virtio-fs] [qemu-web PATCH v2] Add virtio-fs in OSv overview blog post

[Fotis Xenakis]

From: Dr. David Alan Gilbert <dgilbert@redhat.com>

* Fotis Xenakis (foxen@windowslive.com) wrote:

> This post briefly goes over the main points of virtio-fs and OSv, a
> unikernel running under QEMU/KVM and taking advantage of its virtio-fs
> implementation.
>
> Changes since v1:
> - Fixed wording and links, as suggested by Thomas Huth.
> - Added a short example of virtio-fs usage in OSv.
>
> Signed-off-by: Fotis Xenakis <foxen@windowslive.com>

> +One central point is OSv's support for booting from virtio-fs: this enables
> +deploying a modified version or a whole new application **without rebuilding**
> +the image, just by adjusting its root file system contents on the host. Last,
> +owing to the DAX window practically providing low-overhead access to the host's
> +page cache, scalability is also expected to excel, with it being a common
> +concern due to the potentially high density of unikernels per host.

Hi Fotis,

Hello Dave,

  Since I'm not used to unikernels, I'm a little confused by this; I'd
appreciate some explanation.

  In your unikernel, does the root filesystem just contain data?  I mean
being a 'unikernel' aren't all the binaries and support all linked into

the kernel itself?

Short answer: the root file system doesn't contain only data, the

executable can also be loaded from it. Although a unikernel, it

supports both the embedded-in-kernel and separate-filesystem

approaches.

OSv is more heavy-weight than most unikernels, in the sense that it

supports a lot of features found in general-purpose OSs. One could

just describe it as a specialized, light-weight OS. On the filesystem

side, it's actually pretty close e.g. to Linux: it has a VFS and multiple

filesystems:

• pseudo-fs (devfs and others)
• ramfs, which is indeed embedded into the kernel
• ZFS and ROFS, on regular block devices
• NFS, virtio-fs
  

Also like Linux, it initially boots into its initramfs (embedded in the

kernel) and then typically mounts a root file system from a device

(ZFS, ROFS or virtio-fs), before executing the application code.

In case this is not clear, please feel free to ask further!

Fotis

Dave
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK