40 lines
2.0 KiB
Plaintext
40 lines
2.0 KiB
Plaintext
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fs/unionfs/README.txt
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=====================
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This directory contains the NuttX Union File System. The Union file
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system is provides a mechanism to overlay two different, mounted file
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systems so that they appear as one. In general this works like this:
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1) Mount file system 1 at some location, say /mnt/file1
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2) Mount file system 2 at some location, say /mnt/file2
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3) Call unionfs_mount() to combine and overly /mnt/file1 and mnt/file2
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as a new mount point, say /mnt/unionfs.
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/mnt/file1 and /mnt/file2 will disappear and be replaced by the single
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mountpoint /mnut/unionfs. The previous contents under /mnt/file1 and
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/mnt/file2 will appear merged under /mnt/unionfs. Files at the same
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relative path in file system1 will take presence. If another file of the
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same name and same relative location exists in file system 2, it will
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not be visible because it will be occluded by the file in file system1.
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See include/nutts/unionfs.h for additional information.
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The Union File Sysem is enabled by selecting the CONFIG_FS_UNIONFS option
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in the NuttX configruation file.
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The original motivation for this file was for the use of the built-in
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function file system (BINFS) with a web server. In that case, the built
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in functions provide CGI programs. But the BINFS file system cannot hold
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content. Fixed content would need to be retained in a more standard file
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system such as ROMFS. With this Union File System, you can overly the
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BINFS mountpoint on the the ROMFS mountpoint, providing a single directory
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that appears to contain the executables from the BINFS file system along
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with the web content from the ROMFS file system.
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Another possible use for the Union File System could be to augment or
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replace files in a FLASH file system. For example, suppose that you have
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a product that ships with content in a ROMFS file system provided by the
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on-board FLASH. Later, you overlay that ROMFS file system with additional
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files from an SD card by using the Union File System to overlay, and
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perhaps replace, the ROMFS files.
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