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[qemu-web PATCH v2] Add virtio-fs in OSv overview blog post
|
From: |
Fotis Xenakis |
|
Subject: |
[qemu-web PATCH v2] Add virtio-fs in OSv overview blog post |
|
Date: |
Wed, 4 Nov 2020 18:46:16 +0200 |
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>
---
_posts/2020-11-04-osv-virtio-fs.md | 143 +++++++++++++++++++
screenshots/2020-11-04-unikernel-vs-gpos.svg | 1 +
2 files changed, 144 insertions(+)
create mode 100644 _posts/2020-11-04-osv-virtio-fs.md
create mode 100644 screenshots/2020-11-04-unikernel-vs-gpos.svg
diff --git a/_posts/2020-11-04-osv-virtio-fs.md
b/_posts/2020-11-04-osv-virtio-fs.md
new file mode 100644
index 0000000..78ccab1
--- /dev/null
+++ b/_posts/2020-11-04-osv-virtio-fs.md
@@ -0,0 +1,143 @@
+---
+layout: post
+title: "Using virtio-fs on a unikernel"
+author: Fotis Xenakis
+date: 2020-11-04 02:00:00 +0200
+categories: [storage, virtio-fs, unikernel, OSv]
+---
+
+This article provides an overview of [virtio-fs](https://virtio-fs.gitlab.io/),
+a novel way for sharing the host file system with guests and
+[OSv](https://github.com/cloudius-systems/osv), a specialized, lightweight
+operating system (unikernel) for the cloud, as well as how these two fit
+together.
+
+## virtio-fs
+
+Virtio-fs is a new host-guest shared filesystem, purpose-built for local file
+system semantics and performance. To that end, it takes full advantage of the
+host's and the guest's colocation on the same physical machine, unlike
+network-based efforts, like virtio-9p.
+
+As the name suggests, virtio-fs builds on virtio for providing an efficient
+transport: it is included in the (currently draft, to become v1.2) virtio
+[specification](https://github.com/oasis-tcs/virtio-spec) as a new device. The
+protocol used by the device is a slightly extended version of
+[FUSE](https://github.com/libfuse/libfuse), providing a solid foundation for
+all file system operations native on Linux. Implementation-wise, on the QEMU
+side, it takes the approach of splitting between the guest interface (handled
+by QEMU) and the host file system interface (the device "backend"). The latter
+is handled by virtiofsd ("virtio-fs daemon"), running as a separate process,
+utilizing the
+[vhost-user](https://www.qemu.org/docs/master/interop/vhost-user.html) protocol
+to communicate with QEMU.
+
+One prominent performance feature of virtio-fs is the DAX (Direct Access)
+window. It's a shared memory window between the host and the guest, exposed as
+device memory (a PCI BAR) to the second. Upon request, the host (QEMU) maps
file contents to the window for the guest to access directly. This bears
performance
+gains due to taking VMEXITs out of the read/write data path and bypassing the
+guest page cache on Linux, while not counting against the VM's memory (since
+it's just device memory, managed on the host).
+
+
+
+Virtio-fs is under active development, with its community focussing on a pair
of
+device implementation in QEMU and device driver in Linux. Both components are
+already available upstream in their initial iterations, while upstreaming
+continues further e.g. with DAX window support.
+
+## OSv
+
+OSv is a [unikernel](https://en.wikipedia.org/wiki/Unikernel) (framework). The
+two defining characteristics of a unikernel are:
+
+- **Application-specialized**: a unikernel is an executable machine image,
+ consisting of an application and supporting code (drivers, memory management,
+ runtime etc.) linked together, running in a single address space (typically
+ in guest "kernel mode").
+- **Library OS**: each unikernel only contains the functionality mandated by
its
+ application in terms of non-application code, i.e. no unused drivers, or even
+ whole subsystems (e.g. networking, if the application doesn't use the
+ network).
+
+OSv in particular strives for binary compatibility with Linux, using a [dynamic
+linker](https://github.com/cloudius-systems/osv/wiki/Dynamic-Linker). This
means
+that applications built for Linux should run as OSv unikernels without
requiring
+modifications or even rebuilding, at least most of the time. Of course, not the
+whole Linux ABI is supported, with system calls like `fork()` and relatives
+missing by design in all unikernels, which lack the notion of a process.
Despite
+this limitation, OSv is quite full featured, with full SMP support, virtual
+memory, a virtual file system (and many filesystem implementations, including
+ZFS) as well as a mature networking stack, based on the FreeBSD sources.
+
+At this point, one is sure to wonder "Why bother with unikernels?". The problem
+they were originally
+[introduced](http://unikernel.org/files/2013-asplos-mirage.pdf) to solve is the
+bloated software stack in modern cloud computing. Running general-purpose
+operating systems as guests, typically for a single application/service, on top
+of a hypervisor which already takes care of isolation and provides a standard
+device model means duplication, as well as loss of efficiency. This is were
+unikernels come in, trying to be just enough to support a single application
+and as light-weight as possible, based on the assumption that they are
executing
+inside a VM. Below is an illustration of the comparison between
+general-purpose OS, unikernels and containers (as another approach to the same
+problem, for completeness).
+
+
+
+## OSv, meet virtio-fs
+
+As is apparent e.g. from the container world, it is very common for
applications
+running in isolated environments (such as containers, or unikernels even more
+so) to require host file system access. Whereas containers sharing the host
+kernel thus have an obvious, controlled path to the host file system, with
+unikernels this has been more complex: all solutions were somewhat heavyweight,
+requiring a network link or indirection through network protocols. Virtio-fs
+then provided a significantly more attractive route: straight-forward mapping
of
+fs operations (via FUSE), reusing the existing virtio transport and decent
+performance without high memory overhead.
+
+The OSv community quickly identified the opportunity and came up with a
+read-only implementation on its side, when executing under QEMU. This
emphasized
+being lightweight complexity-wise, while catering to many of its applications'
+requirements (they are stateless, think e.g. serverless). Notably, it includes
+support for the DAX window (even before that's merged in upstream QEMU),
+providing [excellent performance](https://github.com/foxeng/diploma), directly
+rivalling that of its local (non-shared) counterparts such as ZFS and ROFS (an
+OSv-specific read-only file system).
+
+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.
+
+For example, to build the `cli` OSv image, bootable from virtio-fs, using the
+core OSv [build
+system](https://github.com/cloudius-systems/osv#building-osv-kernel-and-creating-images):
+```
+scripts/build fs=virtiofs export=all image=cli
+```
+This results in a minimal image (just the initramfs), while the root fs
contents
+are placed in a directory on the host (`build/export` here, by default).
+
+[Running](https://github.com/cloudius-systems/osv#running-osv) the above image
+is just a step away (may want to use the virtio-fs development version of
+[QEMU](https://gitlab.com/virtio-fs/qemu/-/tree/virtio-fs-dev), e.g. for DAX
+window support):
+```
+scripts/run.py --virtio-fs-tag=myfs --virtio-fs-dir=$(pwd)/build/export
+```
+This orchestrates running both virtiofsd and QEMU, using the contents of
+`build/export` as the root file system. Any changes to this directory, directly
+from the host will be visible in the guest without re-running the previous
build
+step.
+
+## Conclusion
+
+OSv has gained a prominent new feature, powered by virtio-fs and its QEMU
+implementation. This allows efficient, lightweight and performant access to the
+host's file system, thanks to the native virtio transport, usage of the FUSE
+protocol and the DAX window architecture. In turn, it enables use cases like
+rapid unikernel reconfiguration.
diff --git a/screenshots/2020-11-04-unikernel-vs-gpos.svg
b/screenshots/2020-11-04-unikernel-vs-gpos.svg
new file mode 100644
index 0000000..010db7e
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\ No newline at end of file
--
2.29.2
- [qemu-web PATCH v2] Add virtio-fs in OSv overview blog post,
Fotis Xenakis <=