Hacking Gmail

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218 Part III — Conquering Gmail How GmailFS Works GmailFS works with four parts: FUSE, which provides the interface to the Linux kernel that allows additional file systems to be created by programs (in what is technically known as userspace); Libgmail, which provides the interface to Gmail; Gmail, which provides the storage; and GmailFS itself, which links the three others together. The part of the system where FUSE talks with the Linux kernel is beyond the scope of this book, and is well documented on the web. And by now you should be confident with sending and receiving mail with Libgmail. So all you need to understand is how the files are stored on Gmail. What Makes Up a File? To really understand the system, you need to know how a general UNIX file system identifies a file. Under an ordinary Unix file system, a file consists of three things: the content of the file itself; a file called an inode, which contains the details of the file; and a pointer to the inode inside another file that represents a directory. This is perhaps a little confusing, so consider an example. Say you want to access a file called supersecretpasswords.txt. To display the contents of the file you would ordinarily use the command cat supersecretpasswords.txt. You can see this in Figure 16-2. FIGURE 16-2: Displaying the contents of a file
Chapter 16 — Using GmailFS 219 For the cat command to access the file, the operating system first opens the file that represents the directory. Within that is a list of filenames, each with a corresponding name of an inode file. The operating system then opens the inode, and it is the inode that tells the operating system where the file is on the disk and all of the rest of the data about the file. This metadata is quite extensive and contains the following information: The location of the item’s contents on the disk What the item is (such as a file, directory, or symbolic link) The item’s size, in bytes The time the file’s inode was last modified — also called the ctime The time the file’s contents were last modified — the mtime The time the file was last accessed — the atime The number of names the file has — hard links The file’s owner — the UID The file’s group — the GID The file’s permissions — for example, 755 Because the file’s contents, the inode, and the pointer to it from the directory are all separate, a single file can have many names. Each name is called a hard link. Deleting a link doesn’t delete the file or the inode itself, only the link, as there may be other links pointing to the inode, and hence to the contents of the file. When a file has no hard links left, the kernel will count it as deleted and allow it to be physically overwritten on the disk itself. So, so far you have two types of file that dictate the file system: the directory file, which matches the filename to the inode, and the inode, which matches lots of metadata to a block of data on the disk. The third part of the file system, then, is the physical block on the disk. For most file systems, this is indeed a physical address, but as different types of storage have different ways of addressing their own bits (pun intended), this section, too, can be abstracted away into a file. So, you have the directory pointing to the inode, the inode pointing to the datablock, and the datablock pointing to the actual data — and then, as shown in Figure 16-2, the data pointing to world domination. Excellent.
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Hacking Gmail Ben Hammersley
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Hacking Gmail Ben Hammersley
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Meanwhile, back in Florence, Anna,
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Credits Executive Editor Chris Webb
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Contents Acknowledgments . . . . .
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Contents xiii Chapter 7: Gmail Libr
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Contents xv How GmailFS Works . . .
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Introduction Welcome to Hacking Gma
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Hacking Gmail
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Desktop Integration chapter The fir
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Chapter 1 — Desktop Integration 5
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12 Part I — Starting to Use Gmail
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Skinning Gmail chapter Being a web-
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How Gmail Works chapter By now you
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Gmail Libraries chapter In the prev
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Checking for Mail chapter Now that
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Sending Mail chapter Now that you k
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Index SYMBOLS backslash (/) keyboar
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Index ■ F-G 277 fstab, mounting G
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Index ■ H-L 279 overview, 186-188
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Index ■ L-M 281 Mail::Webmail::Gm
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Index ■ P-S 283 j key, 18 k key,
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