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SizeFS is a mock filesystem for creating files of particular sizes with specified contents.

Project Description

A mock Filesystem that exists in memory only and allows for the creation of files of a size specified by the filename.

For example, reading a file named 128M+1B will return a file of 128 Megabytes plus 1 byte, reading a file named 128M-1B will return a file of 128 Megabytes minus 1 byte

Within the filesystem one level of folders may be created. Each of these folders can have its extended attributes set to determine the default contents of each file within the folder. The attributes of individual files may be overridden, and, when mounted as a filesystem using fuse, should be set using ‘xattr’ for OS X, or ‘attr’ for Linux. The attributes are described below in the ‘Extended Usage’ section.

Files may only be created within the folders and can only be named with a valid size descriptor. The names of the files should be a number followed by one of the letters B, K, M, G, T, P or E (to mean bytes, kilobytes, megabytes …). Optionally an addition or subtraction may be specified to modify the base size of the file.

Examples of valid filenames:

100K     - A 100 kilobyte file.
4M       - A 4 megabyte file.
2G-1B    - A file 1 byte smaller than 2 gigabytes.
100K+10K - A file 10 kilobytes larger than 100 kilobytes.
10E      - A ten exabyte file (yes really!)

File contents are generated as they are read, so it is entirely possible to ‘create’ files that are larger than any available RAM or HD storage. This can be very useful for testing large external storage systems, and the +/- operations are useful for exploring file size limitations without having to specify a file size as a huge number of bytes. The contents of each file are specified by a set of regular expressions that are initially inherited from the containing folder.

Example Usage

Create Size File objects in memory:

from sizefs import SizeFS
sfs = SizeFS()'/1B', 1, 0, None)'/2B', 2, 0, None)'/2K', 1024, 0, None)'/128K', 1024*128, 0, None)'/4G', 4*1024*1024, 0, None)

The folder structure can be used to determine the content of the files:'/zeros/5B', 5, 0, None)
out> 00000'/ones/5B', 5, 0, None)
out> 11111'/alpha_num/5B', 5, 0, None)
out> TMdEv

The folders ‘ones’, ‘zeros’ and ‘alpha_num’ are always present, but new folders can also be created. When files are created in a folder, the xattrs of the folder determine that file’s content until the file’s xattrs are updated:

sfs.mkdir('/regex1', None)
sfs.setxattr('/regex1', 'generator', 'regex', None)
sfs.setxattr('/regex1', 'filler', 'regex', None)
print'/regex1/5B', 5, 0, None)

out> regex

sfs.setxattr('/regex1/5B', 'filler', 'string', None)
print'/regex1/5B', 5, 0, None)

out> string

sfs.setxattr('/regex1/5B', 'filler', 'a{2}b{2}c', None)
print'/regex1/5B', 5, 0, None)

out> aabbc

Files can also be added to SizeFS without reading their contents using sfs.create():

sfs.mkdir('/folder', None)
sfs.create('/folder/5B', None)
print'/folder/5B', 5, 0, None)

out> 11111

And as discussed above, the name of the file determines its size:

# Try to read more contents than the files contains
print len('/regex3/128K', 256*1000, 0, None))

out> 128000

# Try to read more contents than the files contains
print len('/regex3/128K-1B', 256*1000, 0, None))

out> 127999

# Try to read more contents than the files contains
print len('/alphanum/128K+1B', 256*1000, 0, None))

out> 128001

Extended Usage

The ‘generator’ xattr property defines the file content and can be set to one of:

ones       - files are filled with ones
zeros      - files are filled with zeros
alpha_num  - files are filled with alpha numeric characters
regex      - files are filled according to a collection of regular expression patterns

We can set up to 5 properties to control the regular expression patterns:

prefix     - defined pattern for the start of a file (default = "")
suffix     - defined pattern for the end of a file (default = "")
filler     - repeating pattern to fill file content (default = 0)
padder     - single character to fill between content and footer (default = 0)
max_random - the largest number a + or * will resolve to

Where ‘prefix’, ‘suffix’, ‘filler’, and ‘padder’ conform to the following grammar:

<Regex> ::= <Pattern>

<Pattern> ::= <Expression>
        | <Expression> <Pattern>

<Expression> ::= <Char> [<Multiplier>]
           | "(" <Pattern> ")" [<Multiplier>]
           | "[" <Set> "]" [<Multiplier>]

<Multiplier> ::= "*"
           | "+"
           | "?"
           | '{' <Num> '}'

<Set> ::= <Char>
      | <Char> "-" <Char>
      | <Set> <Set>

If the requested file sizes are too small for the combination of header, footer and some padding, then a warning will be logged, but the file will still return as much content as possible to fill the exact file size requested.

The file contents will always match the following pattern:


The generator will always produce a string containing the prefix and suffix if a file of sufficient size is requested. Following that, the generator will fill the remaining space with ‘filler’ generated as many times as can be contained. If a filler pattern is generated that does not fit within the remaining space the remainder is filled using the (possibly incomplete) padder pattern. The padder pattern will only be used if a complete filler pattern will not fit in the space remaining.

‘max_random’ is used to define the largest random repeat factor of any + or * operators.

Random seeks within a file may produce inconsistent results for general file contents, however prefix and suffix will always be consistent with the requested pattern.


Requires nose

From the command line:


Mounting as a filesystem

Mac Mounting -

Usage: [--debug] <mount_pount> --version

    --debug           Debug
    -h --help         Show this screen.
    --version         Show version.

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