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MRAMFS: A compressing file system for non-volatile RAM

MRAMFS: A compressing file system for non-volatile RAM
Nathan K. Edel, Deepa Tuteja, Ethan L. Miller, and Scott A. Brandt in Proceedings of the 12th IEEE/ACM International Symposium on Modeling, Analysis, and Simulation of Computer and Telecommunication Systems (MASCOTS 2004), Volendam, Netherlands, October 2004.

This paper allows me to provide both a file systems paper and look at an interesting approach to byte-addressable non-volatile memory (NVM).

We have developed a prototype in-memory file system which utilizes data compression on inodes, and which has preliminary support for compression of file blocks.  Our file system, mramfs, is also based on data structures tuned for storage efficiency in non-volatile memory.

One of the interesting aspects of NVM is that it has characteristics of storage (persistence) and memory (byte-addressability).  Storage people are used to having vast amounts of time to do things: it is quite difficult, though not impossible, to do anything computationally with data that will be an important factor when it is combined with the overhead of I/O latency to disk drives.  In-memory algorithms worry about optimal cache line usage and efficient usage of the processor, but they don’t need to worry about what happens when the power goes off.

Bringing these two things together requires re-thinking things.  NVM isn’t as fast as DRAM.  Storage people aren’t used to worrying about CPU cache effects on data resilience.

So mramfs looks at this from a very file systems centric perspective: how do we exploit this nifty new memory to build a new kind of RAM disk: it’s still RAM but now it’s persistent.   NVRAM is slower than DIMM and hence it makes sense to compress it to increase the effective data transfer rate (though it is not clear if that really will be the case.)

I didn’t find a strong motivation for compression, though I can see the viability of it now, in a world in which we want to pack as much as we can into a 64 byte cache line.  The authors point out that one of the previous systems (Conquest) settled on a 53 byte inode size. The authors studied existing systems and found they could actually compress down to 20 bytes (or less) for a single inode.   They achieved this using a combination of gamma compression and compressing common file patterns (mode, uid, and gid).  Another reason for this approach is they did not wish to burden their file system with a computationally expensive compression scheme.

MRAMFS Figure 1In Figure 1 (from the paper) the authors provide a graphic description of their data structures.  This depicts a fairly traditional UNIX style file system, with an inode table, name space (directories), references from directory entries to the inodes.  Inodes then point to control structures that eventually map to the actual data blocks.

The actual memory is managed by the file system from a single chunk of non-volatile memory; the memory is virtually addressed and the paper points out that they don’t actually care how that mapping is achieved.

Multiple inodes are allocated together in inode blocks with each block consisting of 16 (variable length) inodes.  The minimum size of a block is 256 bytes. inodes are rewritten in place whenever possible, which can lead to slack space.  If an inode doesn’t fit within its existing space, the entire block is reconstructed and then written to a new block.  Aftewards, the block pointer is changed to point to the new block.  Then the old block is freed.

One thing that is missing from this is much reasoning about crash consistency, which surprised me.

The authors have an extensive evaluation section, comparing to ext2fs, ramfs, and jffs2 (all over RAM disk).  Their test was a create/unlink micro-benchmark, thus optimizing the meta-data insertion/deletion case.  They then questioned their entire testing mechanism by pointing out that the time was also comparable to what they achieved using tmpfs building the openssl package from source.  Their final evaluation was done without the compression code enabled (“[U]nfortuantely, the data compression code is not yet reliable enough to complete significant runs of Postmark or of large builds…”).  They said they were getting about 20-25% of the speed without compression.

Despite this finding, their conclusion was “We have shown that both metadata and file data blocks are highly compressisble with little increase in code complexity.  By using tuned compression techniques, we can save more than 60% of the inode space required by previous NVRAM file systems, and with little impact on performance.”

My take-away?  This was an early implementation of a file system on NVM.  It demonstrates one of the risks of thinking too much in file systems terms.  We’ll definitely have to do better.

The DEMOS File System

The DEMOS File System
Michael L. Powell, In Proceedings of the sixth ACM symposium on Operating systems principles, pp. 33-42.

This paper delves into the nitty gritty details of constructing physical file systems.  I was surprised that it had relatively few citations (61 according to Google Scholar when I checked) because, having read it, I would hand this paper to someone asking me “what are file systems?”  I suspect that the more frequently cited paper in this area will be “A Fast File System for UNIX,” which cites to this paper.

The target for DEMOS is the CRAY-1 supercomputer, at the time the fastest computer in the world.  As a matter of comparison, modern mobile devices have more computational power (and often more I/O bandwidth) than the CRAY-1 did.

DEMOS Figures 1 and 2The author discusses the design of a new file system for use with a custom operating system for the Los Alamos National Laboratory’s CRAY-1 computer system.  A key for this project was that it seeks to improve performance as much as possible.  After all, why build a super-computer if you then cripple it with features that don’t enhance its performance?

What I find delightful about this paper is that it describes the basic constituent parts of a file system, as well as strategies for optimizing performance.  It does so in a clear and understandable fashion.DEMOS Figures 3 and 4

DEMOS utilizes a UNIX-like hierarchical file system model.  It has directories and files. It does not have the link model from Multics so paths to files are unique in DEMOS.  Files are managed in units of blocks (4096 bytes) but I/O is specified as bytes (interestingly, they specify eight bit bytes as nine bit machines were still in use.)

The authors discuss file sizes.  To the best of my knowledge this is one of the earliest papers covering this common subject  (which is revisited periodically because workloads change and file sizes also change).  One of the common themes I have seen in other work is mirrored here: most files are small.  Figure 1 shows a CDF for file sizes.  We note that the majority of files in their system are small, with approximately 75% being less than 1KB; this is consistent with later work as well.  Their second figure (Figure 2) describes the proportion of transfer sizes and their source.   We see a spike in the 100, perhaps 256 or 512 being “natural block sizes” that applications would use.

demos figure 5They establish lofty performance requirements: “[T]he file system will have to support a bandwidth of 20-60 megabits/second or higher”. Our performance requirements today would be much higher, but this recognizes the reality that then (as now) the I/O bandwidth of storage is often the rate limiting factor.

DEMOS is paired with a centralized storage facility (“Common File System” or CFS) that is to provide the function of what we would now think of as a centralized file server.  While not yet implemented by the time of the paper, their plan was to introduce automatic file migration and staging.

The central bit of the paper then describes the constituent parts of the file system.  This maps rather well onto what I have seen in the typical file system: a “request interpreter” that handles requests from applications.  Even their description is appropriate: “parameter validation and request translation”; a “buffer manager” that handles the allocation of buffer cache space (often virtual cache these days); and a “disk driver” that handles low level data operations, such as filling or storing the contents of buffers.

Figures 3 and 4 capture their insight into the disk manager.  This dovetails with their discussion about efficiency of I/O, including observations about queue management (“shortest seek time first” order for requests, and then sub-sorted by “shortest latency time first”).  This is a clear “hat tip” to the impact that rotational latency and track seek time has on performance.

Speaking of performance, the authors discuss this.  It leads to their observations on improving I/O performance: “I/O operations out to proceed in parallel with computation”.  Their point is that serializing these things decreases overall performance.  Their second observation: “[T]he length of time an I/O operation takes should be reduced as much as possible.”  This seems logical and is one reason why they use their optimized strategy.

There is a section on “file system buffering” that touches on the tradeoffs between using memory for buffer caching versus other possible uses.  Thus, the authors evaluate how increased buffering impacts their CPU utilization – this is in keeping with their goal of parallelizing I/O and computation.  Their observation?  The greatest benefit comes from a small number of buffers, in their analysis eight buffers provides most of the benefit. Building on that Figures 6 and 7, they observe there is a clear limit to the benefit of further buffering.  These days we do not think too much about this because we tend to use virtual caches, so the amount of physical memory is really managed by the virtual memory management code, yet the observation would likely still apply.  There is a limit to the benefit of buffering.

The authors also point out that disk allocation is a challenging.  They employ allocation bit maps, cluster allocations, over-allocate, and even use simplistic predictive read-ahead.  They refer to these as “strategy” routines.

In general, this is a great introduction to the basic structure of a media file system.  There are plenty of details that will be refined in later work.

 

 

 

TENEX

Tenex, a Paged Time Sharing System for the PDP-10
Communications of the ACM, March 1972, Volume 15, Number 3
Daniel G. Bobrow, Jerry D. Burchfiel, Daniel L. Murphy, and Raymond S. Tomlinson, Bolt Beranek and Newman Inc.

TENEX is a new time sharing system implemented on a DEC PDP-10 augmented by special paging hardware developed at BBN. This report specifies a set of goals which are important for any time sharing system. It describes how the TENEX design and implementation achieve these goals. These include specifications for a powerful multiprocess large memory virtual machine, intimate terminal interaction, comprehensive uniform file and I/O capabilities, and clean flexible system structure. Although the implementation described here required some compromise to achieve a system operational within six months of hardware checkout, TENEX has met its major goals and provided reliable service at several sites and through the ARPA network.

Storage organization and management in TENEX
Daniel L. Murphy
AFIPS ’72 (Fall, part I) Proceedings of the December 5-7, 1972, fall joint computer conference, part I
Pages 23-32, Anaheim, California — December 05 – 07, 1972

The first of these two papers discusses TENEX; much of the paper is not about file systems, but there are about a page and a half about the TENEX file system.  The second paper goes into greater detail about storage – including the file system – for TENEX.  I have picked these papers for several reasons:

  • They demonstrate the impact of the MULTICS work on the systems that follow (certainly beyond the obvious UNIX work).
  • They introduce the concept of virtual integration with the file system
  • They introduce the concept of copy-on-write
  • They show the fundamental drive to maintain backwards compatibility
  • They introduce the concept of a suffix (or extension) as a means of identifying the purpose of a file
  • They delve into the details of open file state management

A TENEX file has a compound name structure:

A powerful and versatile directory and file naming facility is provided in which a particular file is identified by a fixed-depth path which includes device, directory name, file name, extension, and version.

The fixed-depth path is a limitation the TENEX developers chose to implement for backwards compatibility with existing PDP-10 programs, an early example of how application compatibility is often a critical concern for operating systems development.  The authors do note they are considering expanding upon this to make it arbitrary depth – a feature of MULTICS.

Both papers also discuss the Job concept, the idea of a set of related processes.  The implication is that processes within a single job can share resources, thus providing more of that “balance between sharing and isolation” that operating systems have to handle.  When a file is opened successfully, a Job File Number is created in a table.  That encapsulates the information about how to find the given file and instead uses an index value – in other words, a file descriptor or file handle.  “Once the initial association of JFN and file has been established, the JFN is used for all ensuing operations on the file, including sequential reading and writing, opening, closing, etc.

TENEX then allows random access to the file by combining the JFN with an index identifying the desired element.  The authors point out that this is more flexible than previous systems in which the file was not random access.

TENEX File to Process MappingThis becomes flexible when describing the page map for a given process.  The Process Map points from an entry in the virtual address space to a corresponding JFN and index (offset).  Thus, the contents of that page can be retrieved on demand from the underlying file system.

None of this should look particularly surprising to anyone familiar with modern operating systems, of course.  This just happens to be part of the path to get to where we are today.  The papers actually go into greater detail about the details here, including access control, but that isn’t germane to my file systems focus.

Since the file path names identify files over the domain of all jobs in the system, it is evident that our naming and mapping procedures readily provide a means for sharing storage. Using the appropriate path names (including legality checks), processes in two or more different jobs can identify the same file, and each can obtain a JFN for it. Nothing in the mapping procedures specified above requires that either process be aware of the other’s access, and so each process constructs an identifier and places it in its process map (Figure 4).

In other words, the contents of regions of a file can be shared across processes.  This is, in fact, transparent to the processes.

Sharing at this level would be particularly important because of the limited address space and desire to share code – the papers discuss this, and point out the benefits of this form of sharing.

This leads to their inclusion of copy-on-write.  “One other important TENEX feature which facilitates sharing is a type of page access called copy-on-write. To our knowledge, this facility was first developed and used on the BBN-LISP system for the
XDS-9407.”  Thus, while not original to TENEX, this is a logical extension beyond what MULTICS had described.  Copy-on-write is a mainstay of both modern operating systems and some file systems.

Interestingly, TENEX seems to implement a rudimentary page cache as well:

To implement the file sequential monitor calls (e.g., byte-in, byte-out) the monitor maintains a number of “window” pages in a separate map invisible to the user process. For each file with sequential operations in progress, the monitor maps the file page which is to receive or provide the next byte. Each call from the user causes one or more bytes to be loaded from or stored into this page, and a count updated to determine if a new page should be mapped. Movement through the file is accomplished by mapping successive pages, and the sequential access module does not have to be aware of the physical device on which the page resides nor interface with I/O driver modules to read or write it. This modularity is very satisfying from an operating system design point of view.

Thus, byte level access to block level devices is managed via this window page mechanism.  The files are not strictly memory mapped, though, so this is more like a buffer cache than a page cache.

They also use the file system to implement inter-process communications – a form of file-backed shared memory.

Page management is tightly tied to this implementation as well, though the description involves what we would likely consider the memory management unit and page fault handling logic as well as the page to file/offset mapping necessary to provide the system’s demand paging.

Two other interesting aspects of their file systems model includes a pair of extra mapping layers: one for mapping from logical storage address to physical storage location, and the other mapping from multiple distinct page references to a single storage block.

The underlying rationale here is that this permits relocating the storage to different locations, typically from higher speed storage (when warm/hot) to slower speed storage (when cold).

This mechanism doesn’t involve changing the actual description of the storage and instead moves to a logical storage addressing model.  It was interesting to me to see this level of indirection added in such an early system, but clearly the mismatch in speeds between various types of storage dictated the importance of this scheme.  Once again, it is interesting to see how little the problems we face have actually changed.

The data sharing model also uses an extra level of indirection.  I’m familiar with this model from my own work in Windows, where shared memory is indirectly mapped in a similar fashion.  That this mechanism was around in the early 1970s is once again a reminder of how little operating systems have fundamentally changed.

There are many aspects of this paper that I have glossed over, in no small part because they don’t really apply to modern systems – we don’t have to worry about drum memories, for example, no more than we need to worry about punch card readers.  These two papers, however, clearly lay out a deeper realization of the file system than I have seen in prior work.

TENEX differed from MULTICS in a number of ways and the two systems remained competitors for many years.  TENEX ultimately would become TOPS-20 and in turn be supported by Digital Equipment Corporation.  It was an important part of the early (pre-VAX) ARPANET and survived for many years as a viable system.

If you would like to read more about this, I’d recommend Dan Murphy’s Origins and Development of TOPS-20 post.  It provides further fascinating background on how TENEX evolved and how systems evolved.  I leave you with the final words from that post:

Although this book is about DEC’s 36-bit architecture, it is clear now that hardware CPU architectures are of declining importance in shaping software. For a long time, instruction set architectures drove the creation of new software. A new architecture would be introduced, and new software systems would be written for it. The 36-bit architecture was large in comparison to most other systems which permitted interactive use. It was also lower in cost than most other systems of that size. These factors were important in the creation of the kind of software for which the architecture is known.

Now, new architectures are coming along with increasing frequency, but they are simply “slid in” under the software. The software systems are far too large to be rewritten each time, and a system which cannot adapt to new architectures will eventually suffer declining interest and loss of competitive hardware platforms. TOPS-20 didn’t pass that test, although it did contribute a number of ideas to the technology of interactive systems. How far these ideas ultimately spread is a story yet to be told.

There is considerable insight in this for me, particularly the admonishment “software systems are far too large to be rewritten each time” as it resonates with (one of) my own research directions.

 

 

MULTICS

A General-Purpose File System For Secondary Storage
R. C. Daley and P.G. Neumann
Published in the Proceedings of the American Federation of Information Processing Societies 1965, Fall Joint Computer Conference, vol. 27, pp. 213-229.

This is the seminal paper discussing how file systems were envisioned within the MULTICS operating system. While you can still run MULTICS, it is a curiosity at this point.  However, virtually all operating systems we now use today descended from MULTICS and thus, its design profoundly influenced their development.

This paper is a delightfully easy read, written at the dawn of this new field of multi-programming. Prior to this time computers were essentially single user.  The introduction of the idea of sharing a computer with other users was nascent.  Thus, the experts working in the field at the time had to begin thinking about things like organization, security, and sharing.

Indeed, a common tension in operating systems literature in general is between isolation and sharing.  Isolation is great from a security perspective, but is inefficient.  Each user of the system often uses the same programs, for example, but we do not want to keep a distinct copy of the same thing for each user as that would be wasteful.   This profoundly impacts the file systems work because the file system is the point of persistence, the level at which shared resources become manifest.

But I’m jumping ahead at this point.  Let’s start with the simple question: What is a file system?  As we will find during this journey, its meaning and usage is far richer than one might think upon first reading.  While this paper is not the first paper to discuss storage and file systems, it is a good example of the state of the art in 1965.

This paper offers us some useful definitions:

file is simply an ordered sequence of elements, where an element could be a machine word, a character, or a bit, depending upon the implementation.

That seems to be a rather broad definition, but it is a reasonable place for us to start.  This does not impose structure on the content itself, which proves to be one reason why this abstraction ultimately turns out to be a powerful one.  “At the level of the file system, a file is formatless.

This paper also establishes the name space abstractions as well:

As far as a particular user is concerned, a file has one name, and that name is symbolic.  (Symbolic names may be arbitrarily long, and may have syntax of their own.  For example, they may consist of several parts, some of which are relevant to the nature of the file, e.g., ALPHA FAP DEBUG.)

This again paints a rather broad abstraction.  The name has meaning to the user, but otherwise is just symbolic data for the file system.  The paper goes on to define the now classic name space specific abstraction:

directory is a special file which is maintained by the file system, and which contains a list of entries.  To a user, an entry appears to be a file and is accessed in terms of its symbolic entry name, which is the user’s file name.  An entry name need be unique only within the directory in which it occurs.

 

Thus, this paper lays out the quintessential aspect of modern file system name spaces: they are hierarchically organized.  The paper describes this in greater detail and refers to links and branches.

The authors describe how users might work in different parts of the hierarchical name space.  They observe that this then creates a situation in which sharing of files might be an issue, and thus they introduce the concept of links to resolve this.


link in this context is an entry in a directory that refers to an existing file within the file system.  Thus, we see the genesis of links, though the paper does not clearly delineate symbolic links from hard links.  This does help motivate why these features show up in UNIX a number of years later, however.

The paper goes into greater detail about how they envision this hierarchical name space functioning, including traversal, working directory, and links.

From there the authors then turn their attention to a problem inherent in having a single shared file system name space with data contents belonging to different users, namely managing access to the individual files.  Thus, they introduce access controls.  They note that a file system could default to either permissive or restrictive access within this model.  From this point they incorporate the access control list, the access mode for a given file, and the concept of access attributes.  Of the five attributes listed, four of them are familiar: read, execute, write, and append access have analogs in modern file systems.  The fifth, trap is interesting, in that it defines an explicit exception mechanism for access control that requires external validation for access – an interesting generality that is not typically present in modern file systems.

They also describe file sharing, introducing the concept of explicit operations to lock access to a given file, or to unlock the file.  They suggest that a locked file would require the user provide a designated key to permit accessing the file; I have seen this approach in some file systems, actually, though it is fairly uncommon.

The paper describes access control at length with no real surprises otherwise, other than perhaps the fact that many of these features disappeared from later operating systems, only to be resurrected and added many years later.

There is quite a bit in the paper about backup and restore processing.  Much of the detail here is interesting historically but does not really add much to my exploration of file systems.  If you are looking for more information about the history of backup, I do encourage you to read those sections.  Having done magnetic tape backups in the past, I’m content with leaving them be.

There is one observation I will point out from this section, however.  The authors actually discuss a recurring theme in file systems – the fact that storage itself ends up being a multi-level media management challenge.

In most cases a user does not need to know how or where a file is stored by the file system.  A user’s primary concern is that the file be readily available to him when he needs it.  In general, only the file system knows on which device a file resides.

The file system is designed to accommodate any configuration of secondary storage devices.  These devices may cover a wide range of speeds and capacities.  All considerations of speed and efficiency of storage devices are left to the file system.  Thus all user programs and all other system programs are independent of the particular configuration of secondary storage.

They go on to describe migration of data from hot to cold storage as needed and point out these are functions of the file system.  I found this an interesting insight since even today we routinely deal with these sorts of situations, such as the Strata paper from SOSP 2017 (“Strata: a Cross Media File System“).

The remainder of the paper focuses on how the file system is implemented in MULTICS

I must admit, I found this section of the paper both detailed and yet fascinating because of the broad sweeping nature in which the authors lay down fundamental ideas that we see in modern operating systems.  Some of it is not really in the scope of file systems (“segment management” which appears to equate to shared executables, such as programs and shared libraries in modern systems,) the concept of demand segment loading (which presages demand paging,) the concept of file system search, mechanisms for managing file systems, memory recycling (reminiscent of page reclamation in modern virtual memory systems based upon its description,) device management, and I/O queueing.  They finish up by describing their “multilevel storage management module”, backup system, and utility functions.  The latter has (now) amusing functions like “file to cards” and “tape to file”.

So these give us asynchronous operation, paging, backup, hierarchical storage management, security, file sharing, and directory sharing.  Most of these concepts survive to this day.  Indeed, what I found most surprising after reading this paper is how few of these ideas disappeared: traps and file system search are the two that spring to my mind.

Thus the lesson of today’s paper: in 1965 the MULTICS team more or less laid down a model for how file systems were to work in virtually all modern operating systems.   The subsequent work will provide us with greater insight into the details, but the basic shape of our file systems has not strayed far from this early vision.

 

On the history of File Systems

I have made it a goal for 2018 to answer a question several people have asked me: what papers should I read to learn more about file systems. So I’ve decided to attempt to copy a format that I’ve found useful – The Morning Paper. I admit, I am not sure I’ll be able to keep up the frenetic pace of a paper each day that he maintains, but I do know there are plenty of papers to read.

My motivation for doing this is simple: there are quite a few papers on this topic, I certainly haven’t read them all (or even the majority of them) and reading through them, along with my own interpretation of why the paper matters to File Systems will be useful for me.  It also gives me a set of information that I can point out to people that ask me “so what should I read to learn more about this topic.”

Why File Systems?  For me it’s been largely a quirk of fate.  I’ve been working on operating systems for many years now, and file systems happens to be the area in which I’ve spent more time than any other.  For something that is conceptually so simple – after all, it just maps files to blocks on your disk, it has considerable complexity.  File systems are also the gateway to one of the more challenging parts of the operating system: the bit that stores persistent state.  Errors in file systems are often not transient.  That makes them challenging.  The bar is set high because we don’t want to lose anyone’s data.

File systems are part of the plumbing of the operating system.  They are essential to proper operation but when everything is working properly, nobody really notices they are there.  Only when things go wrong does anyone notice.  So it is within this world that I will delve.

The place to start is to even attempt to define file systems.  While you might think this is simple, it turns out to be surprisingly challenging.  So I’ll approach it by providing some examples and then delving into what means to be a file system.

Media File Systems

The easiest place to start is in the concept of the media file system.  “Media” in this context means any tangible medium on which we can systematically record and/or read persistent information.  Examples would include disks, tapes, non-volatile memory, or optical media. Whether or not RAM file systems fall into this category is an interesting question – and it helps demonstrate my point that pinning down this concept is more challenging than one might otherwise think.

Disk drives are typically what most people think of first for this category, though we now often view solid-state disk drives in the same category.  File systems reside on top of media devices and keep track of the organization of the data itself.  Some file systems can span multiple disk, or use transactional journals, or provide resilience against errors in the media itself.  The continuum here is surprisingly broad, but in the end the purpose of the file system is to map from the vagaries of the media to a (mostly) uniform model of behavior so application programs “just work”.

Network File Systems

As we added computers to networks, we found it useful to be able to transfer information stored on one computer to another.  This permitted applications to access data, regardless of where it was actually stored.  By constructing a file system that uses a remote access protocol, we can present the remote storage device to the local system as if it were just a local file system – or rather, almost the same.

Good examples of this include the Network File System (NFS) originally developed by SUN Microsystems in the 1980s and the Andrew File System (AFS) originally developed by Carnegie-Mellon University in the same time period.  These days many people also use the Server Message Block (SMB) based network file systems; the roots of this are also back in the 1980s.  All three of these remain in use today, in fact, though they have evolved from the earliest versions.

Key-Value Stores

There is considerable overlap between the world of file systems and databases.  Several early file systems were really just single level lookup mechanisms, rather than the hierarchical name space that is so common these days.  It is actually common to implement file systems so that hierarchy is added to a flat indexed name space (a “flat” file system).  These are in essence one form of key-value store.  Some file systems permit addressing by using keys, whether explicitly assigned or implicitly generated.  A subsection of the literature focus more on this type of file system and I will make sure to cover several of these.

Name Spaces

Sometimes a file system is more about the namespace than anything else.  For example the proc file system does not actually reside on top of any sort of storage.  The name space provides a convenient way to find information that is generated on demand.  In these cases, it is the name space that really matters, not how the information is stored.

This conflation of storage, presentation, and name-space add to the richness and complexity of work being done in file systems.  My goal is to explore these issues, by reviewing the literature.