distributed network file systems
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Brian Picciano
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The Web
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description: >-
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What is it good for?
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series: nebula
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tags: tech
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---
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339
src/_posts/2021-04-06-evaluation-of-network-filesystems.md
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title: >-
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Evaluation of Network Filesystems
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description: >-
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There can only be one.
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series: nebula
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tags: tech
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---
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It's been a bit since updating my progress on what I've been lately calling the
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"cryptic nebula" project. When I last left off I was working on building the
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[mobile nebula][mobile_nebula] using [nix][nix]. For the moment I gave up on
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that dream, as flutter and nix just _really_ don't get along and I don't want to
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get to distracted on problems that aren't critical to the actual goal.
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Instead I'd like to pursue the next critical component of the system, and
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that's a shared filesystem. The use-case I'm ultimately trying to achieve is:
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* All hosts communicate with each other via the nebula network.
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* All hosts are personal machines owned by individuals, _not_ cloud VMs.
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* A handful of hosts are always-on, or at least as always-on as can be achieved
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in a home environment.
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* All hosts are able to read/write to a shared filesystem, which is mounted via
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FUSE (or some other mechanism, though I can't imagine what) on their computer.
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* Top-level directories within the shared filesystem can be restricted, so
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that only a certain person (or host) can read/write to them.
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What I'm looking for is some kind of network filesystem, of which there are
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_many_. This document will attempt to evaluate all relevant projects and come up
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with the next steps. It may be that no project fits the bill perfectly, and that
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I'm stuck either modifying an existing project to my needs or, if things are
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looking really dire, starting a new project.
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The ultimate use-case here is something like a self-hosted, distributed [keybase
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filesystem](https://book.keybase.io/docs/files); somewhere where individuals in
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the cluster can back up their personal projects, share files with each other,
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and possibly even be used as the base layer for more complex applications on
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top.
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The individuals involved shouldn't have to deal with configuring their
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distributed FS, either to read from it or add storage resources to it. Ideally
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the FS process can be bundled together with the nebula process and run opaquely;
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the user is just running their "cryptic nebula" process and everything else is
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handled in the background.
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## Low Pass Filter
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There are some criteria for these projects that I'm not willing to compromise
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on; these criteria will form a low pass filter which, hopefully, will narrow our
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search appreciably.
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The network filesystem used by the cryptic nebula must:
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* Be able to operate over a nebula network (obviously).
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* Be open-source. The license doesn't matter, as long as the code is available.
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* Run on both Mac and Linux.
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* Not require a third-party to function.
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* Allows for a replication factor of 3.
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* Supports sharding of data (ie each host need not have the entire dataset).
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* Allow for mounting a FUSE filesystem in any hosts' machine to interact with
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the network filesystem.
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* Not run in the JVM, or any other VM which is memory-greedy.
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The last may come across as mean, but the reason for it is that I forsee the
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network filesystem client running on users' personal laptops, which cannot be
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assumed to have resources to spare.
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## Rubric
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Each criteria in the next set lies along a spectrum. Any project may meet one of
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thses criteria fully, partially, or not at all. For each criteria I assign a
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point value according to how fully a project meets the criteria, and then sum up
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the points to give the project a final score. The project with the highest final
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score is not necessarily the winner, but this system should at least give some
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good candidates for final consideration.
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The criteria, and their associated points values, are:
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* **Hackability**: is the source-code of the project approachable?
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- 0: No
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- 1: Kind of, and there's not much of a community.
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- 2: Kind of, but there is an active community.
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- 3: Yes
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* **Documentation**: is the project well documented?
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- 0: No docs.
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- 1: Incomplete or out-of-date docs.
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- 2: Very well documented.
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* **Transience**: how does the system handle hosts appearing or disappearing?
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- 0: Requires an automated system to be built to handle adding/removing
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hosts.
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- 1: Gracefully handled.
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* **Priority**: is it possible to give certain hosts priority when choosing
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which will host/replicate some piece of data?
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- 0: No.
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- 1: Yes.
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* **Caching**: will hosts reading a file have that file cached locally for the
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next reading (until the file is modified)?
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- 0: No.
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- 1: Yes.
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* **Conflicts**: if two hosts updated the same file at the same time, how is
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that handled?
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- 0: The file can no longer be updated.
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- 1: One update clobbers the other, or both go through in an undefined
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order.
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- 2: One update is disallowed.
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- 3: A copy of the file containing the "losing" update is created (ie: how
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dropbox does it).
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- 4: Strategy can be configured on the file/directory level.
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* **Consistency**: how does the system handle a file being changed frequently?
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- 0: File changes must be propagated before subsequent updates are allowed (fully consistent).
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- 1: Files are snapshotted at some large-ish interval (eventually consistent).
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- 2: File state (ie content hash, last modifid, etc) is propagated
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frequently but contents are only fully propagated once the file has
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"settled" (eventually consistent with debounce).
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* **POSIX**: how POSIX compliant is the mounted fileystem?
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- 0: Only the most basic features are implemented.
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- 1: Some extra features are implemented.
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- 2: Fully POSIX compliant.
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* **Scale**: how many hosts can be a part of the cluster?
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- 0: A finite number.
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- 1: A finite number of dedicated hosts, infinite ephemeral.
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- 2: Infinite hosts.
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* **Failure**: how does the system handle failures (network partitions, hosts
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hanging, buggy client versions)?
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- 0: Data loss.
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- 1: Reads and writes are halted.
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- 2: Reads are allowed but writes are halted.
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- 3: System is partially read/write, except effected parts.
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* **Limitations**: are there limits on how big files can be, or how big
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directories can be?
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- 0: Files are limited to below 1TB in size.
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- 1: Directories are limited to below 100,000 files.
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- 2: No limits.
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* **Encryption**: how is data encrypted?
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- 0: Not at all, DIY.
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- 1: Encrypted at rest.
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- 2: Per-user encryption.
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* **Permissions**: how are modifications to data restricted?
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- 0: Not at all.
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- 1: Permissions are only superifically enforced.
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- 2: Fully enforced user/group restrictions, complex patterns, and/or POSIX ACLs.
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* **Administration**: how much administration is required for the system to
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function?
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- 0: Frequent.
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- 1: Infrequent.
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- 2: Essentially none.
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* **Simplicity**: how understandable is the system as a whole?
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- 0: Very complex.
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- 1: Understandable with some study.
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- 2: Very simple, easy to predict.
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* **Visibility**: how much visibility is available into processes within the
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system?
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- 0: Total black box.
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- 1: Basic logging.
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- 2: CLI tooling.
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- 3: Exportable metrics (e.g. prometheus).
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## Evaluations
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With the rubric defined, let's start actually working through our options! There
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are many, many different possibilities, so this may not be an exhaustive list.
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### [Ceph](https://docs.ceph.com/en/latest/cephfs/index.html)
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> The Ceph File System, or CephFS, is a POSIX-compliant file system built on
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> top of Ceph’s distributed object store, RADOS. CephFS endeavors to provide a
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> state-of-the-art, multi-use, highly available, and performant file store for
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> a variety of applications, including traditional use-cases like shared home
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> directories, HPC scratch space, and distributed workflow shared storage.
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- Hackability: 2. Very active community, but it's C++.
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- Documentation: 2. Hella docs, very daunting.
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- Transience: 0. Adding hosts seems to require multiple configuration steps.
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- Priority: 1. There is fine-tuning on a per-host basis.
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- Caching: 1. Clients can cache both metadata and block data.
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- Conflicts: 1. The FS behaves as much like a real FS as possible.
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- Consistency: 0. System is CP.
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- POSIX: 2. Fully POSIX compliant.
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- Scale: 2. Cluster can grow without any real bounds.
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- Failure: 3. There's no indication anywhere that Ceph goes into any kind of cluster-wide failure mode.
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- Limitations: 2. There are performance considerations with large directories, but no hard limits.
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- Encryption: 0. None to speak of.
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- Permissions: 2. POSIX ACLs supported.
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- Administration: 1. This is a guess, but Ceph seems to be self-healing in general, but still needs hand-holding in certain situations (adding/removing nodes, etc...)
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- Simplicity: 0. There are many moving pieces, as well as many different kinds of processes and entities.
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- Visibility: 3. Lots of tooling to dig into the state of the cluster, as well as a prometheus module.
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TOTAL: 22
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#### Comments
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Ceph has been recommended to me by a few people. It is clearly a very mature
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project, though that maturity has brought with it a lot of complexity. A lot of
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the complexity of Ceph seems to be rooted in its strong consistency guarantees,
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which I'm confident it fulfills well, but are not really needed for the
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use-case I'm interested in. I'd prefer a simpler, eventually consistent,
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system. It's also not clear to me that Ceph would even perform very well in my
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use-case as it seems to want an actual datacenter deployment, with beefy
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hardware and hosts which are generally close together.
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### [GlusterFS](https://docs.gluster.org/en/latest/)
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> GlusterFS is a scalable network filesystem suitable for data-intensive tasks
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> such as cloud storage and media streaming. GlusterFS is free and open source
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> software and can utilize common off-the-shelf hardware.
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- Hackability: 2. Mostly C code, but there is an active community.
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- Documentation: 2. Good docs.
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- Transience: 0. New nodes cannot add themselves to the pool.
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- Priority: 0. Data is distributed based on consistent hashing algo, nothing else.
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- Caching: 1. Docs mention client-side caching layer.
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- Conflicts: 0. File becomes frozen, manual intervention is needed to save it.
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- Consistency: 0. Gluster aims to be fully consistent.
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- POSIX: 2. Fully POSIX compliant.
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- Scale: 2. No apparent limits.
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- Failure: 3. Clients determine on their own whether or not they have a quorum for a particular sub-volume.
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- Limitations: 2. Limited by the file system underlying each volume, I think.
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- Encryption: 2. Encryption can be done on the volume level, each user could have a private volume.
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- Permissions: 2. ACL checking is enforced on the server-side, but requires syncing of users and group membership across servers.
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- Administration: 1. Beyond adding/removing nodes the system is fairly self-healing.
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- Simplicity: 1. There's only one kind of server process, and the configuration of volumes is is well documented and straightforward.
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- Visibility: 3. Prometheus exporter available.
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TOTAL: 23
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#### Comments
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GlusterFS was my initial choice when I did a brief survey of DFSs for this
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use-case. However, after further digging into it I think it will suffer the
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same ultimate problem as CephFS: too much consistency for a wide-area
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application like I'm envisioning. The need for syncing user/groups across
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machines as actual system users is also cumbersome enough to make it not a
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great choice.
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### [MooseFS](https://moosefs.com/)
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> MooseFS is a Petabyte Open Source Network Distributed File System. It is easy
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> to deploy and maintain, highly reliable, fault tolerant, highly performing,
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> easily scalable and POSIX compliant.
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>
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> MooseFS spreads data over a number of commodity servers, which are visible to
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> the user as one resource. For standard file operations MooseFS acts like
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> ordinary Unix-like file system.
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- Hackability: 2. All C code, pretty dense, but backed by a company.
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- Documentation: 2. There's a giant PDF you can read through like a book. I
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guess that's.... good?
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- Transience: 0. Nodes must be added manually.
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- Priority: 1. There's "Storage Classes".
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- Caching: 1. Caching is done on the client, and there's some synchronization
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with the master server around it.
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- Conflicts: 1. Both update operations will go through.
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- Consistency: 0. Afaict it's a fully consistent system, with a master server
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being used to synchronize changes.
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- POSIX: 2. Fully POSIX compliant.
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- Scale: 2. Cluster can grow without any real bounds.
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- Failure: 1. If the master server is unreachable then the client can't
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function.
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- Limitations: 2. Limits are very large, effectively no limit.
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- Encryption: 0. Docs make no mention of encryption.
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- Permissions: 1. Afaict permissions are done by the OS on the fuse mount.
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- Administration: 1. It seems that if the topology is stable there shouldn't be
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much going on.
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- Simplicity: 0. There are many moving pieces, as well as many different kinds of processes and entities.
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- Visibility: 2. Lots of cli tooling, no prometheus metrics that I could find.
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TOTAL: 17
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Overall MooseFS seems to me like a poor-developer's Ceph. It can do exactly the
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same things, but with less of a community around it. The sale's pitch and
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feature-gating also don't ingratiate it to me. The most damning "feature" is the
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master metadata server, which acts as a SPOF and only sort of supports
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replication (but not failover, unless you get Pro).
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## Cutting Room Floor
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The following projects were intended to be reviewed, but didn't make the cut for
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various reasons.
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* Tahoe-LAFS: The FUSE mount (which is actually an SFTP mount) doesn't support
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mutable files.
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* HekaFS: Doesn't appear to exist anymore(?)
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* IPFS-cluster: Doesn't support sharding.
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* MinFS: Seems to only work off S3, no longer maintained anyway.
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* DRDB: Linux specific, no mac support.
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* BeeGFS: No mac support (I don't think? I couldn't find any indication it
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supports macs at any rate).
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* NFS: No support for sharding the dataset.
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## Conclusions
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Going through the featuresets of all these different projects really helped me
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focus in on how I actually expect this system to function, and a few things
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stood out to me:
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* Perfect consistency is not a goal, and is ultimately harmful for this
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use-case. The FS needs to propagate changes relatively quickly, but if two
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different hosts are updating the same file it's not necessary to synchronize
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those updates like a local filesystem would; just let one changeset clobber
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the other and let the outer application deal with coordination.
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* Permissions are extremely important, and yet for all these projects are
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generally an afterthought. In a distributed setting we can't rely on the OS
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user/groups of a host to permission read/write access. Instead that must be
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done primarily via e2e encryption.
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* Transience is not something most of these project expect, but is a hard
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requirement of this use-case. In the long run we need something which can be
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run on home hardware on home ISPs, which is not reliable at all. Hosts need to
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be able to flit in and out of existence, and the cluster as a whole needs to
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self-heal through that process.
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In the end, it may be necessary to roll our own project for this, as I don't
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think any of the existing distributed file systems are suitable for what's
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needed.
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[mobile_nebula]: https://github.com/cryptic-io/mobile_nebula
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[nix]: https://nixos.org/manual/nix/stable/
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