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diff --git a/src/_posts/2021-01-14-the-web.md b/src/_posts/2021-01-14-the-web.md index d80caaf..cae564a 100644 --- a/src/_posts/2021-01-14-the-web.md +++ b/src/_posts/2021-01-14-the-web.md @@ -3,6 +3,7 @@ title: >- The Web description: >- What is it good for? +series: nebula tags: tech --- diff --git a/src/_posts/2021-04-06-evaluation-of-network-filesystems.md b/src/_posts/2021-04-06-evaluation-of-network-filesystems.md new file mode 100644 index 0000000..b80eb8d --- /dev/null +++ b/src/_posts/2021-04-06-evaluation-of-network-filesystems.md @@ -0,0 +1,339 @@ +--- +title: >- + Evaluation of Network Filesystems +description: >- + There can only be one. +series: nebula +tags: tech +--- + +It's been a bit since updating my progress on what I've been lately calling the +"cryptic nebula" project. When I last left off I was working on building the +[mobile nebula][mobile_nebula] using [nix][nix]. For the moment I gave up on +that dream, as flutter and nix just _really_ don't get along and I don't want to +get to distracted on problems that aren't critical to the actual goal. + +Instead I'd like to pursue the next critical component of the system, and +that's a shared filesystem. The use-case I'm ultimately trying to achieve is: + +* All hosts communicate with each other via the nebula network. +* All hosts are personal machines owned by individuals, _not_ cloud VMs. +* A handful of hosts are always-on, or at least as always-on as can be achieved + in a home environment. +* All hosts are able to read/write to a shared filesystem, which is mounted via + FUSE (or some other mechanism, though I can't imagine what) on their computer. +* Top-level directories within the shared filesystem can be restricted, so + that only a certain person (or host) can read/write to them. + +What I'm looking for is some kind of network filesystem, of which there are +_many_. This document will attempt to evaluate all relevant projects and come up +with the next steps. It may be that no project fits the bill perfectly, and that +I'm stuck either modifying an existing project to my needs or, if things are +looking really dire, starting a new project. + +The ultimate use-case here is something like a self-hosted, distributed [keybase +filesystem](https://book.keybase.io/docs/files); somewhere where individuals in +the cluster can back up their personal projects, share files with each other, +and possibly even be used as the base layer for more complex applications on +top. + +The individuals involved shouldn't have to deal with configuring their +distributed FS, either to read from it or add storage resources to it. Ideally +the FS process can be bundled together with the nebula process and run opaquely; +the user is just running their "cryptic nebula" process and everything else is +handled in the background. + +## Low Pass Filter + +There are some criteria for these projects that I'm not willing to compromise +on; these criteria will form a low pass filter which, hopefully, will narrow our +search appreciably. + +The network filesystem used by the cryptic nebula must: + +* Be able to operate over a nebula network (obviously). +* Be open-source. The license doesn't matter, as long as the code is available. +* Run on both Mac and Linux. +* Not require a third-party to function. +* Allows for a replication factor of 3. +* Supports sharding of data (ie each host need not have the entire dataset). +* Allow for mounting a FUSE filesystem in any hosts' machine to interact with + the network filesystem. +* Not run in the JVM, or any other VM which is memory-greedy. + +The last may come across as mean, but the reason for it is that I forsee the +network filesystem client running on users' personal laptops, which cannot be +assumed to have resources to spare. + +## Rubric + +Each criteria in the next set lies along a spectrum. Any project may meet one of +thses criteria fully, partially, or not at all. For each criteria I assign a +point value according to how fully a project meets the criteria, and then sum up +the points to give the project a final score. The project with the highest final +score is not necessarily the winner, but this system should at least give some +good candidates for final consideration. + +The criteria, and their associated points values, are: + +* **Hackability**: is the source-code of the project approachable? + - 0: No + - 1: Kind of, and there's not much of a community. + - 2: Kind of, but there is an active community. + - 3: Yes + +* **Documentation**: is the project well documented? + - 0: No docs. + - 1: Incomplete or out-of-date docs. + - 2: Very well documented. + +* **Transience**: how does the system handle hosts appearing or disappearing? + - 0: Requires an automated system to be built to handle adding/removing + hosts. + - 1: Gracefully handled. + +* **Priority**: is it possible to give certain hosts priority when choosing + which will host/replicate some piece of data? + - 0: No. + - 1: Yes. + +* **Caching**: will hosts reading a file have that file cached locally for the + next reading (until the file is modified)? + - 0: No. + - 1: Yes. + +* **Conflicts**: if two hosts updated the same file at the same time, how is + that handled? + - 0: The file can no longer be updated. + - 1: One update clobbers the other, or both go through in an undefined + order. + - 2: One update is disallowed. + - 3: A copy of the file containing the "losing" update is created (ie: how + dropbox does it). + - 4: Strategy can be configured on the file/directory level. + +* **Consistency**: how does the system handle a file being changed frequently? + - 0: File changes must be propagated before subsequent updates are allowed (fully consistent). + - 1: Files are snapshotted at some large-ish interval (eventually consistent). + - 2: File state (ie content hash, last modifid, etc) is propagated + frequently but contents are only fully propagated once the file has + "settled" (eventually consistent with debounce). + +* **POSIX**: how POSIX compliant is the mounted fileystem? + - 0: Only the most basic features are implemented. + - 1: Some extra features are implemented. + - 2: Fully POSIX compliant. + +* **Scale**: how many hosts can be a part of the cluster? + - 0: A finite number. + - 1: A finite number of dedicated hosts, infinite ephemeral. + - 2: Infinite hosts. + +* **Failure**: how does the system handle failures (network partitions, hosts + hanging, buggy client versions)? + - 0: Data loss. + - 1: Reads and writes are halted. + - 2: Reads are allowed but writes are halted. + - 3: System is partially read/write, except effected parts. + +* **Limitations**: are there limits on how big files can be, or how big + directories can be? + - 0: Files are limited to below 1TB in size. + - 1: Directories are limited to below 100,000 files. + - 2: No limits. + +* **Encryption**: how is data encrypted? + - 0: Not at all, DIY. + - 1: Encrypted at rest. + - 2: Per-user encryption. + +* **Permissions**: how are modifications to data restricted? + - 0: Not at all. + - 1: Permissions are only superifically enforced. + - 2: Fully enforced user/group restrictions, complex patterns, and/or POSIX ACLs. + +* **Administration**: how much administration is required for the system to + function? + - 0: Frequent. + - 1: Infrequent. + - 2: Essentially none. + +* **Simplicity**: how understandable is the system as a whole? + - 0: Very complex. + - 1: Understandable with some study. + - 2: Very simple, easy to predict. + +* **Visibility**: how much visibility is available into processes within the + system? + - 0: Total black box. + - 1: Basic logging. + - 2: CLI tooling. + - 3: Exportable metrics (e.g. prometheus). + +## Evaluations + +With the rubric defined, let's start actually working through our options! There +are many, many different possibilities, so this may not be an exhaustive list. + +### [Ceph](https://docs.ceph.com/en/latest/cephfs/index.html) + +> The Ceph File System, or CephFS, is a POSIX-compliant file system built on +> top of Ceph’s distributed object store, RADOS. CephFS endeavors to provide a +> state-of-the-art, multi-use, highly available, and performant file store for +> a variety of applications, including traditional use-cases like shared home +> directories, HPC scratch space, and distributed workflow shared storage. + +- Hackability: 2. Very active community, but it's C++. +- Documentation: 2. Hella docs, very daunting. +- Transience: 0. Adding hosts seems to require multiple configuration steps. +- Priority: 1. There is fine-tuning on a per-host basis. +- Caching: 1. Clients can cache both metadata and block data. +- Conflicts: 1. The FS behaves as much like a real FS as possible. +- Consistency: 0. System is CP. +- POSIX: 2. Fully POSIX compliant. +- Scale: 2. Cluster can grow without any real bounds. +- Failure: 3. There's no indication anywhere that Ceph goes into any kind of cluster-wide failure mode. +- Limitations: 2. There are performance considerations with large directories, but no hard limits. +- Encryption: 0. None to speak of. +- Permissions: 2. POSIX ACLs supported. +- 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...) +- Simplicity: 0. There are many moving pieces, as well as many different kinds of processes and entities. +- Visibility: 3. Lots of tooling to dig into the state of the cluster, as well as a prometheus module. + +TOTAL: 22 + +#### Comments + +Ceph has been recommended to me by a few people. It is clearly a very mature +project, though that maturity has brought with it a lot of complexity. A lot of +the complexity of Ceph seems to be rooted in its strong consistency guarantees, +which I'm confident it fulfills well, but are not really needed for the +use-case I'm interested in. I'd prefer a simpler, eventually consistent, +system. It's also not clear to me that Ceph would even perform very well in my +use-case as it seems to want an actual datacenter deployment, with beefy +hardware and hosts which are generally close together. + +### [GlusterFS](https://docs.gluster.org/en/latest/) + +> GlusterFS is a scalable network filesystem suitable for data-intensive tasks +> such as cloud storage and media streaming. GlusterFS is free and open source +> software and can utilize common off-the-shelf hardware. + +- Hackability: 2. Mostly C code, but there is an active community. +- Documentation: 2. Good docs. +- Transience: 0. New nodes cannot add themselves to the pool. +- Priority: 0. Data is distributed based on consistent hashing algo, nothing else. +- Caching: 1. Docs mention client-side caching layer. +- Conflicts: 0. File becomes frozen, manual intervention is needed to save it. +- Consistency: 0. Gluster aims to be fully consistent. +- POSIX: 2. Fully POSIX compliant. +- Scale: 2. No apparent limits. +- Failure: 3. Clients determine on their own whether or not they have a quorum for a particular sub-volume. +- Limitations: 2. Limited by the file system underlying each volume, I think. +- Encryption: 2. Encryption can be done on the volume level, each user could have a private volume. +- Permissions: 2. ACL checking is enforced on the server-side, but requires syncing of users and group membership across servers. +- Administration: 1. Beyond adding/removing nodes the system is fairly self-healing. +- Simplicity: 1. There's only one kind of server process, and the configuration of volumes is is well documented and straightforward. +- Visibility: 3. Prometheus exporter available. + +TOTAL: 23 + +#### Comments + +GlusterFS was my initial choice when I did a brief survey of DFSs for this +use-case. However, after further digging into it I think it will suffer the +same ultimate problem as CephFS: too much consistency for a wide-area +application like I'm envisioning. The need for syncing user/groups across +machines as actual system users is also cumbersome enough to make it not a +great choice. + +### [MooseFS](https://moosefs.com/) + +> MooseFS is a Petabyte Open Source Network Distributed File System. It is easy +> to deploy and maintain, highly reliable, fault tolerant, highly performing, +> easily scalable and POSIX compliant. +> +> MooseFS spreads data over a number of commodity servers, which are visible to +> the user as one resource. For standard file operations MooseFS acts like +> ordinary Unix-like file system. + +- Hackability: 2. All C code, pretty dense, but backed by a company. +- Documentation: 2. There's a giant PDF you can read through like a book. I + guess that's.... good? +- Transience: 0. Nodes must be added manually. +- Priority: 1. There's "Storage Classes". +- Caching: 1. Caching is done on the client, and there's some synchronization + with the master server around it. +- Conflicts: 1. Both update operations will go through. +- Consistency: 0. Afaict it's a fully consistent system, with a master server + being used to synchronize changes. +- POSIX: 2. Fully POSIX compliant. +- Scale: 2. Cluster can grow without any real bounds. +- Failure: 1. If the master server is unreachable then the client can't + function. +- Limitations: 2. Limits are very large, effectively no limit. +- Encryption: 0. Docs make no mention of encryption. +- Permissions: 1. Afaict permissions are done by the OS on the fuse mount. +- Administration: 1. It seems that if the topology is stable there shouldn't be + much going on. +- Simplicity: 0. There are many moving pieces, as well as many different kinds of processes and entities. +- Visibility: 2. Lots of cli tooling, no prometheus metrics that I could find. + +TOTAL: 17 + +Overall MooseFS seems to me like a poor-developer's Ceph. It can do exactly the +same things, but with less of a community around it. The sale's pitch and +feature-gating also don't ingratiate it to me. The most damning "feature" is the +master metadata server, which acts as a SPOF and only sort of supports +replication (but not failover, unless you get Pro). + +## Cutting Room Floor + +The following projects were intended to be reviewed, but didn't make the cut for +various reasons. + +* Tahoe-LAFS: The FUSE mount (which is actually an SFTP mount) doesn't support + mutable files. + +* HekaFS: Doesn't appear to exist anymore(?) + +* IPFS-cluster: Doesn't support sharding. + +* MinFS: Seems to only work off S3, no longer maintained anyway. + +* DRDB: Linux specific, no mac support. + +* BeeGFS: No mac support (I don't think? I couldn't find any indication it + supports macs at any rate). + +* NFS: No support for sharding the dataset. + +## Conclusions + +Going through the featuresets of all these different projects really helped me +focus in on how I actually expect this system to function, and a few things +stood out to me: + +* Perfect consistency is not a goal, and is ultimately harmful for this + use-case. The FS needs to propagate changes relatively quickly, but if two + different hosts are updating the same file it's not necessary to synchronize + those updates like a local filesystem would; just let one changeset clobber + the other and let the outer application deal with coordination. + +* Permissions are extremely important, and yet for all these projects are + generally an afterthought. In a distributed setting we can't rely on the OS + user/groups of a host to permission read/write access. Instead that must be + done primarily via e2e encryption. + +* Transience is not something most of these project expect, but is a hard + requirement of this use-case. In the long run we need something which can be + run on home hardware on home ISPs, which is not reliable at all. Hosts need to + be able to flit in and out of existence, and the cluster as a whole needs to + self-heal through that process. + +In the end, it may be necessary to roll our own project for this, as I don't +think any of the existing distributed file systems are suitable for what's +needed. + +[mobile_nebula]: https://github.com/cryptic-io/mobile_nebula +[nix]: https://nixos.org/manual/nix/stable/ |