SpaceFish: A Userspace File System in Shared Memory

SpaceFish is an experimental userspace file system in shared memory, implemented from scratch, tested successfully so far on Ubuntu and Fedora Linux. It uses the LD_PRELOAD mechanism to intercept system call wrappers and other filesystem-related libc functions, in order to sandbox all file operations, in a scalable manner, at the user level.

For the duration of a spacefish session, all operations related to files are executed on in-memory copies of the files, with the actual filesystem left intact. The operating-system-maintained file system is not touched, and the OS is not even aware that its file operations are being re-directed.

SpaceFish is specifically designed to be highly performant on multi-core architecture.

Authors

Zhuangdi Xu; Alexis Champsaur

Bucket allocation

File data is allocated through "buckets." Buckets can be re-used, and there are several algorithms for how free data buckets are allocated. Exactly one bucket allocation algorithm must be selected through CPPFLAGS in the Makefile.

This can be:

• ALLOC_STEPS: Shared memory for file data is allocated from the OS as 1 GB steps, as needed by SpaceFish. A SpaceFish-wide "rough index" keeps track of approximately where the last bucket was allocated, and any process that needs a new buckets starts at the point and walks forward until it finds one. When it does, the process sets the rough index to the index where it founds its free bucket.
• ALLOC_ALL_RANDOM: "Random-and-walk" algorithm. All "steps" are allocated at once. That is, shared memory does not grow as needed by UFS. A process that needs a free bucket starts at a random position in the logical array of all data buckets, and walks forward until it finds a free one.
• ALLOC_ALL_ROUGH_INDEX: All steps are allocated at once. Then, the "rough index" mechanism described in ALLOC_STEPS is used to obtain free buckets.
• ALLOC_ALL_PER_CORE_RANDOM: All steps are allocated at once. Before looking for a free bucket, a process checks its CPU number and from that determines a slice of the logical array of all buckets from which to allocate. In this "slice", the random-and-walk algorithm described in ALLOC_ALL_RANDOM is used to obtain a free data bucket.
• ALLOC_ALL_PER_CORE_ROUGH_INDEX: Each core only has a slice, as in above case; however, inside its slice, the core uses the "rough index" method as a starting point to looking for a free bucket.

Building

1. Select one bucket allocation algorithm listed above in the Makefile. If unsure, use ALLOC_ALL_RANDOM.

2. Select a shared memory size. (TOTAL_STEPS_ALLOWED in system/include/config.h).

3. make

Running

1. Launch the daemon first: ./bin/daemon

2. Wait for "allocated steps" message

3. To run an application over SpaceFish (all file-related operations for this application will be re-directed to SpaceFish), set the LD_PRELOAD environment variable to bin/libufs.so.