CS110: Principles of Computer Systems

Spring 2021
Instructors Roz Cyrus and Jerry Cain
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Lecture 03: Layering, Naming, and Filesystem Design

  • The diagram below shows how raw hardware could be leveraged to support filesystems as we're familiar with them. There's a lot going on in the diagram below, so we'll use the next several slides to dissect it and let you know what's going on.
    • I will provide a high level explanation of how the physical hardware of the drive is accessed and otherwise manipulated.
    • This is where we stopped at the end of today's pre-lecture.

Lecture 03: Layering, Naming, and Filesystem Design

  • Filesystem metadata
    • The first block is the boot block, which typically contains information about the hard drive itself. It's so named because its contents are generally tapped when booting—i.e. restarting—the operating system.
    • The second block is the superblock, which contains information about the filesystem superimposed onto the hardware. 

Lecture 03: Layering, Naming, and Filesystem Design

  • Filesystem metadata, continued
    • The rest of the metadata region stores the inode table, which at the highest level stores information about each file stored somewhere within the filesystem.
    • The diagram below makes the metadata region look much larger than it really is. In practice, at most 10% of the entire drive is set aside for metadata storage. The rest is used to store file payload.

Lecture 03: Layering, Naming, and Filesystem Design

  • File contents
    • File payloads are stored in quantums of 512 bytes (or whatever the block size is).
    • When a file isn't a multiple of 512 bytes, then its final block is a partial. The portion of that final block that contains meaningful payload is easily determined from the file size.
    • The diagram below includes illustrations for a 32 byte and a 1028 (i.e. 2 * 512 + 4) byte file, so each enlists some block to store a partial.

Lecture 03: Layering, Naming, and Filesystem Design

  • The inode
    • We need to track which blocks are used to store the payload of a file.
      • Blocks 1025, 1027, and 1028 are part of the same file, and you know because they're the same color in the diagram.
      • inodes are 32-byte data structures that store metainfo about a single file. Stored within an inode are items like file owner, file permissions, creation times, file type, file size, and the sequence of blocks enlisted to store payload.

Lecture 03: Layering, Naming, and Filesystem Design

  • The inode, continued
    • Look at the contents of inode 2, outlined in green.
      • The file size is 1028 bytes, so three blocks are needed to store everything. The first two are saturated, but the third stores 1028 % 512, or 4, meaningful bytes.
      • The block nums are listed as 1027, 1028, and 1025, in that order. Bytes 0-511 reside within block 1027, bytes 512-1023 within block 1028, bytes 1024-1027 at the front of block 1025.

Lecture 03: Layering, Naming, and Filesystem Design

  • The inode, continued
    • A file's inodes tell us where we'll find its payload, but the inode itself must reside on the the drive. (Where else could it persist between computer power cycles?)
    • A series of blocks comprise the inode table, which in our diagram stretches from block 2 through block 1023.
    • Because inodes are small—only 32 bytes—each block within the inode table can store 16 inodes side by side, like the books of a 16-volume encyclopedia in a single cubbyhole.

Lecture 03: Layering, Naming, and Filesystem Design

  • The inode, continued
    • We rely on filenames and a hierarchy of named directories to organize our files, and we prefer those names—e.g. /usr/class/cs110/WWW/index.html—to seemingly magic numbers that incidentally identify where the corresponding inodes sit in the inode table.
    • If we needed to remember the inode number of every file on our system, we'd be sad.

Lecture 03: Layering, Naming, and Filesystem Design

  • The inode, continued
    • We could wedge a filename field inside each inode. But that a bad idea, because:
      • Inodes are small, but filenames are long. My assign1 solution resides in a file named /usr/class/cs110/staff/master_repos/assign1/imdb.cc. At 51 characters, the name wouldn't fit in an inode.
      • Linearly searching an inode table for a named file would be slow. My own laptop has about two million files, so the inode table is at least that big.

Lecture 03: Layering, Naming, and Filesystem Design

  • Introducing the directory file type
    • The solution is to introduce the directory as a new file type. You may be surprised to find that this requires almost no changes to our existing scheme, as we can layer directories atop the file abstraction we already have. In almost all filesystems, directories are just files, (with the exception that they are marked as directories by the file type field in the inode). The file payload is the accumulation of 16-byte slivers that form a table mapping names to inode numbers.

Lecture 03: Layering, Naming, and Filesystem Design

  • Introducing the directory file type
    • Have a look at the contents of block 1024, i.e. the contents of file with inumber 1, in the diagram below. This directory contains two files, so its total file size is 32; the first 16 bytes form the first row of the table (14 bytes for the filename, 2 for the inumber), and the second 16 bytes form the second row of the table. When looking for a file in a directory, we’re searching for that name and the inumber right next to it.

Lecture 03: Layering, Naming, and Filesystem Design

  • Introducing the directory file type
    • What does the file lookup process look like, then? Consider a file at /usr/class/cs110/example.txt. First, we find the inode for the file / (which by design is always associated with inumber 1). We search inode 1's payload for the token usr and its companion inumber. Let's say it's at inode 5. Then, we get inode 5's contents and search for the token class in the same way. From there, we look up the token cs110 and then example.txt.

Lecture 03: Layering, Naming, and Filesystem Design

  • What about large files?
    • In the Unix V6 filesystem (the one that’s described as a case study in your textbook), inodes store a maximum of 8 block numbers. This presumably limits the total file size to 8 * 512 = 4096 bytes, but fortunately that’s not really the case.

Lecture 03: Layering, Naming, and Filesystem Design

  • What about large files? We have a solution!
    • To resolve this problem, we use a scheme called indirect addressing. Normally, the inode stores block numbers that directly identify payload blocks.
      • As an example, let's say the file is stored across blocks 2001-2008. The inode will store the numbers 2001-2008. We want to append to the file, but the inode can't store any more block numbers.
      • Instead, let's allocate a single block—let's say this is block 2050—and let's store the numbers 2001-2009 in that block. Then update the inode to store only block number 2050.
      • When we want to get the contents of the file, we check the inode and see this flag is set. We get the first block number, read that block, and then read the direct block numbers—ones storing true user payload—from that block.
        • This is known as singly-indirect addressing.
        • We can store up to 8 singly indirect block numbers in an inode, and each can store 512 / 2 = 256 block numbers. This increases the maximum file size to 8 * 256 * 512 = 1,048,576 bytes = 1 MB.
    • How do we know when an inode relies on indirect addressing?
      • Simply examine the file size.  If it's "big", then assume indirect addressing.
      • Optionally, include an extra bool (or even a single bitflag) in the inode.

Lecture 03: Layering, Naming, and Filesystem Design

  • What about large files? We have a solution!
    • What about large files? We have a better solution!
      • That's still not that big. To make the max file size even bigger, Unix V6 uses the 8th block number of the inode to store a doubly indirect block number.
        • In the inode, the first 7 block numbers store to singly indirect block numbers, but the last block number identifies to a block which itself stores singly-indirect block numbers.
        • The total number of singly indirect block numbers we can have is 7 + 256 = 263, so the maximum file size is 263 * 256 * 512 = 34,471,936 bytes = 34MB.
        • That's still not very large by today's standards, but remember we're referring to a file system design from 1975, when file system demands were lighter than they are today.
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