Imagine you have a file named /some/dir/file.jpg.
In a filesystem, there’s an inode for /some. It contains an entry for /some/dir, which is also an inode, and then in the very deepest level, there is an inode for /some/dir/file.jpg. You can rename /some to /something_else if you want. Think of it kind of like a table:
+-------+--------+----------+-------+
| inode | parent | name | data |
+-------+--------+----------+-------+
| 1 | (null) | some | (dir) |
| 2 | 1 | dir | (dir) |
| 3 | 2 | file.jpg | jpeg |
+-------+--------+----------+-------+
+-------------------+------+
| key | data |
+-------------------+------+
| some/dir/file.jpg | jpeg |
+-------------------+------+
You can, if you want, use a completely different character to separate “components” in S3, rather than using /, because / is not special. If you want something like “some:dir:file.jpg” or “some.dir.file.jpg” you can do that. Again, because / is not special.
S3 is not a filesystem because the semantics are different from the kind of semantics we expect from filesystems. You can’t take the high-level API provided by a filesystem, use S3 as the backing storage, and expect to get good performance out of it unless you use a ton of translation.
Stuff like NFS or CIFS are filesystems. They behave like filesystems, in practice. You can rename files. You can modify files. You can create directories.
For example, maybe you have a database mapping file paths to S3 objects. This gives you a separate metadata layer, with S3 as the storage layer for large blocks of data.
Fair enough, basing folders on object names split by / is pretty inefficient. I wonder why they didn't go with a solution like git's trees.
What, exactly, is inefficient about it?
Think for a moment about the data structures you would use to represent a directory structure in a filesystem, and the data structures you would use to represent a key/value store.
With a filesystem, if you split a string /some/dir/file.jpg into three parts, “some”, “dir”, “file.jpg”, then you are actually making a decision about the tree structure. And here’s a question—is that a balanced tree you got there? Maybe it’s completely unbalanced! That’s actually inefficient.
Let’s suppose, instead, you treat the key as a plain string and stick it in a tree. You have a lot of freedom now, in how you balance the tree, since you are not forced to stick nodes in the tree at every / character.
It’s just a different efficiency tradeoff. Certain operations are now much less efficient (like “rename a directory” which, on S3, is actually “copy a zillion objects). Some operations are more efficient, like “store a file” or “retrieve a file”.
This does not make S3 a bad storage, just a bad filesystem, not everything needs to be a filesystem.
Arguably is it good that S3 is not a filesystem, as it can be a leaky abstraction eg in git you cannot have two tags name "v2" and "v2/feature-1" as you cannot have both a file and a folder with the same name.
For something more closely related to URLs than filenames forcing a filesystem abstraction is a limitation as "/some/url", "/some/url/", and "/some/url/some-default-name-decided-by-the-webserver" can be different.[1]
[0] where a different tradeoff is that searching a file by name is slower but reading many small files can be faster.
[1] maybe they should be the same, but enforcing it is a bad idea
Alas, no. It represents a tag, e.g. «folder/», that points to a zero byte object.
You can then upload two files, e.g. «folder/file1.txt» and «folder/file2.txt», delete the «folder/», being a tag, and still have the «folder/file1.txt» and «folder/file2.txt» file intact in the S3 bucket.
Deleting «folder/» in a traditional file system, on the other hand, will also delete «file1.txt» and «file2.txt» in it.
However, there are file managers, FTP clients, and S3 clients that will do that for you by deleting individual files.
So essentially an S3 file manager must be able to handle the situation where there are files without a "directory"—and that I assume is also the most common case as well for S3. Might just not have the "directories" in the first place.
They appear to exist because the key is split on the slash character for navigation in the web front-end. This gives the familiar appearance of a filesystem, but the implementation is at a much higher level.
> the API implies that common prefixes indicate related objects.
That’s something users do. The API doesn’t imply anything is related.
And prefixes can be anything, not just directories. If you have /some/dir/file.jpg, then you can query using /some/dir/ as a prefix (like a directory!) or you can query using /so as a prefix, or /some/dir/fil as a prefix. It’s just a string. It only looks like a directory when you, the user, decide to interpret the / in the file key as a directory separator. You could just as easily use any other character.
If you have something which is dir/file, then NORMALLY “dir” does not exist at all. Only dir/file exists. There is nothing to rename.
If you happen to have something which is named “dir”, then it’s just another file (a.k.a. object). In that scenario, you have two files (objects) named “dir” and “dir/file”. Weird, but nothing stopping you from doing that. You can also have another object named “dir///../file” or something, although that can be inconvenient, for various reasons.
More like O(max(number of files, total file size)). You can’t rename objects in S3. To simulate a rename, you have to copy an object and then delete the old one.
Unlike renames in typical file systems, that isn’t atomic (there will be a time period in which both the old and the new object exist), and it becomes slower the larger the file.
Querying ids by prefix doesn’t make any sense for a normal ID type. Just making this operation available and part of your public API indicates that prefixes are semantically relevant to your API’s ID type.
I can look up names with the prefix “B” and get Bart, Bella, Brooke, Blake, etc. That doesn’t imply that there’s some kind of semantics associated with prefixes. It’s just a feature of your system that you may find useful. The fact that these names have a common prefix, “B”, is not a particularly interesting thing to me. Just like if I had a list of files, 1.jpg, 10.jpg, 100.jpg, it’s probably not significant that they’re being returned sequentially (because I probably want 2.jpg after 1.jpg).
It's an incorrect take that a system to manage files must follow a set of patterns like the ones you mentioned to be called "file system".
I would argue that not supporting folders or many other file operations make something not a filesystem today.
I could create a system not supporting folders because it relies on tags or something else. Or I could create a system which is write-only and doesn't support rename or delete.
These systems would be file systems according to how the term has been used for 40 (?) years at least. Just don't see any point in restricting the term to exclude random variants.
Many tools end up handling this by showing a folder named "a" containing a folder named "" (empty string). This confuses users quite a bit. It's more than the inodes, it's how the tooling handles the abstraction.
This created a prefix named “/“ which confused the hell out of the application.
Not only you cannot rename a single file, but you also cannot rename a "folder" (because that would imply a bulk rename on a large number of children of that "folder")
This is the fundamental difference between a first class folder and just a convention on prefixes of full path names.
If you don't allow renames, it doesn't really make sense to have each "folder" store the list of the children.
You can instead have a giant ordered map (some kind of b-tree) that allows you for efficient lookup and scanning neighbouring nodes.
https://docs.aws.amazon.com/AmazonS3/latest/API/API_ListObje...
The console UI shows folders but they don’t actually exist in S3. They’re made up by the UI.
S3 doesn't have folders. The UI fakes them by creating a 0-byte object (or file, if you will). It's a kludge.
Of course it can still be useful to group objects in the S3 UI, but it would probably be better to use some kind of prefix-centric UI rather than reusing the folder metaphor when it doesn't match the paradigm people are used to.
You can’t examine when a folder was created (it doesn’t exist in the first place), you can’t rename a folder (it doesn’t exist), you can’t delete a folder (again, it doesn’t exist).
If you truly believe S3 has absolutely no connection to folders, you would answer Yes and No.
“Any hierarchical path structure is a folder” is maybe your definition of “folder”, from what I can tell. I would say that S3 lets you treat paths as hierarchical, but that S3 does not have folders—obviously I have a different definition of “folder” than you do.
We’ve discovered that we have different definitions of “folder”, and therefore, we are not going to agree about whether it is true that “S3 does not have folders” unless we have an argument about what the correct definition of “folder” is. I’m not really interested in that discussion—it’s enough to understand what somebody means when they say “S3 does not have folders” even if you think their definitions are wrong.
Folders are an important part of the way most people use filesystems.
On the Mac, the Finder lets you have files with slashes in their names, even though it's a Unix file system underneath. Don't believe me? Go try to use the Finder to make a directory whose name is "Reports from 2024/03/10". See?
But as everyone knows, slash is the ONLY character you're not allowed to have in a file or directory name under Unix. It's enforced in the kernel at the system call inteface. There is absolutely no way to make a file with a slash in it. Yet there it is!
The original MacOS operating system used the ":" character to delimit directory names, instead of "/", so you could have files and directories with slashes in their names, justs not with colons in their names.
When Apple transitioned from MacOS to Unix, they did not want to freak out their users by reaming all their files.
So now try to use the Finder (or any app that uses the standard file dialog) to make a folder or file with a ":" in its name on a modern Mac. You still can't!
So now go into the shell and list out the parent directory containing the directory you made with a slash in its name. It's actually called "Reports from 2024:03:10"!
The Mac Finder and system file dialog user interfaces actually switche "/" and ":" when they show paths on the screen!
Try making a file in the shell with colons in it, then look at it in the finder to see the slashes.
However, back in the days of the old MacOS that permitted slashes in file names, there was a handy network gateway box called the "Gatorbox" that was a Localtalk-to-Ethernet AFP/NFS bridge, which took a subtly different approach.
https://en.wikipedia.org/wiki/GatorBox
It took advantage of the fact (or rather it triggered the bug) that the Unix NFS implementation boldly made an end-run around the kernel's safe system call interface that disallowed slashes in file names. So any NFS client could actually trick Unix into putting slashes into file names via the NFS protocol!
It appeared to work just fine, but then down the line the Unix "restore" command would totally shit itself! Of course "dump" worked just fine, never raising an error that it was writing corrupted dumps that you would not be able to read back in your time of need, so you'd only learn that you'd been screwed by the bug and lost all your files months or years later!
So not only does NFS stand for "No File Security", it also stands for "Nasty Forbidden Slashes"!
https://news.ycombinator.com/item?id=31820504
>NFS originally stood for "No File Security".
>The NFS protocol wasn't just stateless, but also securityless!
>Stewart, remember the open secret that almost everybody at Sun knew about, in which you could tftp a host's /etc/exports (because tftp was set up by default in a way that left it wide open to anyone from anywhere reading files in /etc) to learn the name of all the servers a host allowed to mount its file system, and then in a root shell simply go "hostname foo ; mount remote:/dir /mnt ; hostname `hostname`" to temporarily change the CLIENT's hostname to the name of a host that the SERVER allowed to mount the directory, then mount it (claiming to be an allowed client), then switch it back?
>That's right, the server didn't bother checking the client's IP address against the host name it claimed to be in the NFS mountd request. That's right: the protocol itself let the client tell the server what its host name was, and the server implementation didn't check that against the client's ip address. Nice professional protocol design and implementation, huh?
>Yes, that actually worked, because the NFS protocol laughably trusted the CLIENT to identify its host name for security purposes. That level of "trust" was built into the original NFS protocol and implementation from day one, by the geniuses at Sun who originally designed it. The network is the computer is insecure, indeed.
[...]
From the Unix-Haters Handbook:
https://archive.org/stream/TheUnixHatersHandbook/ugh_djvu.tx...
Don't Touch That Slash!
UFS allows any character in a filename except for the slash (/) and the ASCII NUL character. (Some versions of Unix allow ASCII characters with the high-bit, bit 8, set. Others don't.)
This feature is great — especially in versions of Unix based on Berkeley's Fast File System, which allows filenames longer than 14 characters. It means that you are free to construct informative, easy-to-understand filenames like these:
1992 Sales Report
Personnel File: Verne, Jules
rt005mfkbgkw0 . cp
Unfortunately, the rest of Unix isn't as tolerant. Of the filenames shown above, only rt005mfkbgkw0.cp will work with the majority of Unix utilities (which generally can't tolerate spaces in filenames).
However, don't fret: Unix will let you construct filenames that have control characters or graphics symbols in them. (Some versions will even let you build files that have no name at all.) This can be a great security feature — especially if you have control keys on your keyboard that other people don't have on theirs. That's right: you can literally create files with names that other people can't access. It sort of makes up for the lack of serious security access controls in the rest of Unix.
Recall that Unix does place one hard-and-fast restriction on filenames: they may never, ever contain the magic slash character (/), since the Unix kernel uses the slash to denote subdirectories. To enforce this requirement, the Unix kernel simply will never let you create a filename that has a slash in it. (However, you can have a filename with the 0200 bit set, which does list on some versions of Unix as a slash character.)
Never? Well, hardly ever.
Date: Mon, 8 Jan 90 18:41:57 PST
From: [email protected] (Steve Sekiguchi)
Subject: Info-Mac Digest V8 #3 5
I've got a rather difficult problem here. We've got a Gator Box run-
ning the NFS/AFP conversion. We use this to hook up Macs and
Suns. With the Sun as a AppleShare File server. All of this works
great!
Now here is the problem, Macs are allowed to create files on the Sun/
Unix fileserver with a "/" in the filename. This is great until you try
to restore one of these files from your "dump" tapes, "restore" core
dumps when it runs into a file with a "/" in the filename. As far as I
can tell the "dump" tape is fine.
Does anyone have a suggestion for getting the files off the backup
tape?
Thanks in Advance,
Steven Sekiguchi Wind River Systems
sun!wrs!steve, [email protected] Emeryville CA, 94608
I’m fine with it, I actually appreciate the logic and simplicity behind it, but the amount of times I’ve tried to explain why “folders” on S3 keep disappearing while people stare at me like I’m an idiot is really frustrating.
(When you remove the last file in a “folder” on S3, the “folder” disappears, because that pattern no longer appears in the bucket k/v dictionary so there’s no reason to show it as it never existed in the first place).
I use S3 just as a web bucket of files (I know it's not the best way to do that but it's what I could easily obtain through our company's processes). But in this case it makes a lot of sense though I try to avoid making folders. But other people using the same hosting do use them.
(also don't get me started on the whole s3api thing)
It is also a fiction! There are no folders in S3.
> When you create a folder in Amazon S3, S3 creates a 0-byte object with a key that's set to the folder name that you provided. For example, if you create a folder named photos in your bucket, the Amazon S3 console creates a 0-byte object with the key photos/. The console creates this object to support the idea of folders.
https://docs.aws.amazon.com/AmazonS3/latest/userguide/using-...