Ask Slashdot: Distributed Filesystems for Linux?

Ledge Kindred asks: "I am looking for a distributed filesystem to run on my Linux boxes at home. I have several and most of the "extra" space on each one is "going to waste" - I'd like to be able to combine it all into a single network-able filesystem. How?" Click below for more.

"So far the two (three?) solutions that had the most promise are: AFS or Arla, and Coda.

The reasons against: AFS is commercial and I don't want to pay $15,000 in licenses just for a convenience to me. Arla still appears to be extremely alpha quality, even for a Linux hacker used to seeing major parts of his kernel labeled "alpha" or "beta". I had Coda up and running for a couple of days before I ran into a fairly severe flaw in the fundamental design that showed it to be inappropriate for what I want it to do. (But Coda is still the coolest thing since individually-wrapped cheese slices, and if you don't need to worry about that little problem, it's cooler than sex.)

I've found lots of references to the "GFS" project which is not at all what I want, and here and there mentions of other projects such as "DFS", "xFS" and a distributed filesystem for Beowulf clusters but no further details, URLs or most importantly - code - could I dig up.

I don't need RAID, redundancy, failover, or anything like that. I only need to take these extra machines on my home network and make all their extra disk space look like a single volume on the network. Support for Linux as a client is, obviously, essential, but I also have Windows, BeOS, *BSD and Solaris machines on my network, so clients for those would be appreciated but not necessary. Since this is just for me at home, (yes, I've got all that crap on my network at home - so I'm a little crazy) I'd rather stick with free software. Is there anything that can do this? "

If not, then it sounds like it would be an interesting project to work on. The ability to be able to harness the spare disk space across a private network can only be a good thing.

Comments

For your setup

[aheitner]

Coda would be overkill -- the depot-style requirements are intended for a distributed environment like a university, in which all the clients constantly accessing the servers to do anything would kill the system. Actually, afs (which is currently used at CMU) is intended to work similarly -- clients build updated caches of appropriate application directories for their architectures, with the result that machines running a constantly out-of-date minimal core OS are served a centralized set of applications ...

For home network purposes, where a few users are unlikely to overwhelm the server, use NFS. It's easy, it's well supported across OSs, its performance may not be incredible but nothing you're likely to do will strain it. Even if you're moving huge files around, you're not going to have 10 people moving huge files around simultaneously.

Actually, there's one more fun option to consider: Inter-Mezzo, a distributed fs written in PERL in a few weeks by the creator of CODA, Peter Braam. It's small, it's pretty quick (the speed-critical parts are in C :), it's cool ... it can do a lot of the things CODA can, but it's much lighterweight, doesn't require its own fstype. I don't have the link handy, and I don't know if it has the same caching requirements as CODA ...

My kludgy solution

[dizco]

I have the same problem. I've got 5 boxes running linux & os/2, and want all the "spare" space to transparently appear as a single volume on all boxes. I couldn't find anything that would do this effectivly for me, so i brewed my own. Unfortunatly (for most of you) this requires an os/2 box.
Here's the details:
1) all the boxes export their spare space as nfs mounts.
2) a nifty IFS (installable file system) from IBM's EWS (employee-written software) program called Toronto Virtual File System is installed on one of the os2 boxes (we'll call this box os2tvfs)
3) os2tvfs mounts all the exported drives
4) with tvfs, all the mounted NFS drives are mounted into a tvfs drive (z: in my setup)
5) os2tvfs exports z:
6) any box that wants to access the big-virtual-volume mounts os2tvfs:/z:/

So how's it work? Lets go through an example:

box1 exports d:\, a 10 gig ide drive on an os2 system
d:\ contains a bunch of stuff, for this example we'll focus on "d:\mp3s\foo.mp3"

box2 exports /s1/ a 6.4gig scsi drive on a linux system
box2 has a file on it located at "/s1/mp3s/bar.mp3"


box1 then mounts os2tvfs:/z:/ as v:\
on box1, a directory listing of v:\mp3s\ contains both foo.mp3 and bar.mp3. if i copy baz.mp3 to v:\mp3s, it ends up as box2:/s1/mp3s/baz.mp3, as long as their is enough free space on box2:/bfi1/ for it, because i assigned a higher write priority to that volume when i mounted it with TVFS (it's a scsi drive- might as well use it up first). It shows up as os2tvfs:/z:/mp3s/baz.mp3.

Of course, this solution is kinda bad because it creates a ton of extra network traffic, but it was the only one i could find that did what i wanted.


--sean

Re:The Charon Filesystem

[Salamander]

>but is already faster than Ext2fs, and way ahead of XFS and NTFS.

By what measures and for what workloads? Such claims are meaningless without describing the environment, and are the realm of marketroids (particularly the MS kind) not scientists or engineers.

I find it most odd that you would tout the system's distributed nature and then compare it only against local FSes. How well does it perform in sharing situations, either locally or through slow WAN links? What level of coherency does it guarantee? How is failure recovery (a very tricky issue for a DFS) handled? How about disconnected operation?

To be perfectly blunt, the lack of even an attempt to address these sorts of crucial issues makes me wonder whether the part about Charon being distributed is "part of the plan" that hasn't actually been implemented (or even designed) yet. The DFS literature is littered with papers about systems that would supposedly blow everything else away, but that never actually got implemented. I've been there, I've done that, and the sad truth is that the realities of implementing a usable DFS - i.e. one that isn't pathologically ill-behaved in at least one of the areas alluded to above - generally shred naive ideals of superfast coolness.

>Only changed disk blocks and metadata are replicated, as opposed to entire files (and only on close)

If this is really what you meant to say, it's great performance but has dire implications for recoverability. This only strengthens my suspicion that you haven't really climbed into the mud pit in earnest yet.

As with everything, "it depends"

[Salamander]

Many people seem to be jumping in with suggested solutions, but it seems to me that the problem has not yet been adequately described. For example:

1. What kind of sharing do you anticipate? Some systems that handle read-only sharing well fall down when even a single writer enters the picture.
   
2. Aside from efficiency, what kind of coherency guarantees are you comfortable with? Some systems will behave exactly like a local filesystem in terms of what happens when one client writes and another reads, and can be used for "network-unaware" applications. Others, notably NFS, play "fast and loose" so you have to do explicit performance-robbing flushes to have any sort of guarantees. If you put build trees on a distributed/network filesystem you have to worry about when file modification times get updated, as well as about the file contents themselves; having "make" mistakenly tell you nothing needs to be done can be more than a little annoying.
   
3. What kind of failure-recovery guarantees do you need? Is it OK to lose the odd unflushed write every now and then after a failure?
   
4. Do you need support for either advisory or mandatory byte-range locks?
   

I'm also curious what you found lacking in GFS. I have my own different ideas about "how things should be done" but perhaps explaining why you consider it inappropriate will shed some light on your needs.

As far as practical advice goes, I think most of the relevant products and approaches have been mentioned; I don't promise to have secret knowledge of any "magic bullets". DFS technology is an area where I feel we're still looking for the right answers (sometimes even the right questions). That's why I enjoy working on DFSes, but it does mean that there's a large element of "choose your poison" in evaluating current offerings.

The Charon Filesystem

[1010011010]

I dislike letting the cat out of the bag this early, but those of you who pay attention to linux kernel-development lists already know some of this.

We're writing a new distributed filesystem called Charon. It will be patented ("patent pending"), copyrighted, and GPLed. It's a true 64-bit, journaled filesystem that supports exabyte-plus file and volume sizes, sophisticated access control lists, per-directory quotas, distributed zero-knowledge protocol authentication, encryption, replication, named streams and indices (see BeFS, ReiserFS -- although we don't use B-trees of any type). It's in alpha stage right now, and full of debug code, but is already faster than Ext2fs, and way ahead of XFS and NTFS. We will be porting it to Solaris and NT after development on Linux is complete.

Unlike Coda, AFS, DFS, etc. replication, every Charon server is a full read-write replicant. Only changed disk blocks and metadata are replicated, as opposed to entire files (and only on close) as in Coda. Charon clients are partial replicants -- they use the local file system as cache and rely on their home server(s) for token management and authentication. The system also supports heirachical failover and replication.

Because of the way it is designed, it also supports a very nice feature for GUIs and web servers -- a very fast built-in file types database that provides a single repository for mime type, friendly name, icon(s), description, extension, and other information. Sort of alike the Windows registry, but much less stupid and much higher-performance.

Stay tuned! This isn't vaporware.