1. File System Topics
Lei Xu

2. Agenda
Introduction
VFS
Optimizations
Examples
F&Q

3. Introduction
“A file system is a means to organize data expected to be retained after a program terminates by providing procedures to store, retrieve and update data, as well as manage the available space on the device(s) which contain it.” – from Wikipedia
Store data
Organize data
Access data
Manage storage resources (e.g. hard drive)

4. Relationship to Architecture Course
Acknowledge to the slides from 830 course

5. Relationship to Architecture Course
File system is designed between memory and secondary storage (or remote servers)
One of the most complex part in an operating system
Main R&D focuses:
Performance: throughput, latency, scalability
Reliability and availability
Management: snapshot and etc.
Acknowledge to the slides from 830 course

6. Different types of file systems
Local file systems
Stored data on local hard drives, SSDs, floppy drives, optical disks or etc.
Examples: NTFS, EXT4, HFS+, ZFS
Network/distributed file systems
Stored data on remote file server(s)
Example: NFS, CIFS/Samba, AFP, Hadoop DFS, Ceph
Pseudo file systems
Example: procfs, devfs, tmpfs
“List of file systems”

7. Agenda
Introduction
VFS
Optimizations
Examples
F&Q

8. Overall Architecture of Linux file system components
Acknowledgement: “Anatomy of the Linux file system”, IBM developerWorks.

9. Virtual File System (VFS)
VFS is the essential concept in UNIX-like FS
Specify an interface between the kernel and a concrete file system
Introduced by SUN in 1985
Pass system calls to the underlying file systems
E.g. pass sys_write() to Ext4 (i.e. ext4_write())
Three major metadata in VFS
Metadata: the data about data (wikipedia)
Super block, dentry and inode
OO design
Each component defines a set of data members and the functions to access them

10. Super block A segment of metadata that describes a file system
Is constructed when mount a file system
Usually, a persistent copy of super block is stored in the beginning of a storage device
Describes:
File system type, size, status (e.g. dirty bit, read only bit)
Block size, max file bytes, device size..
How to find other metadata and data.
How to manipulates these data (i.e. sb_ops)

11. Inode “Index-node” in Unix-style file system
All information about one file (or directory)
Except its name
In UNIX-like system, file names are stored in the directory file: the content of it is an “array” of file names
E.g. owner, access rights, mode, size, time and etc.
Pointers to data

12. Directory Entry (dentry)
Dentry conceptually points a file name to its corresponding Inode
Each file/directory has a dentry presenting it
File systems use dentry to lookup a file in the hierarchical namespace
Each dentry has a pointer to the dentry of its parent directory
Each dentry of a directory has a list of dentries of its sub- directories and sub-files

13. Agenda
Introduction
VFS
Optimizations
Examples
F&Q

14. Optimizations
Most of file system optimizations are designed based on the characteristics of the memory hierarchy and storage devices.
Recall:
RAM ns
Disks: 5-10 ms
2-3 orders of magnitude difference
Almost all widely used local file systems are designed for hard disk drives, which have their unique characteristics

15. Hard Disk Drive (HDD)
Stores data on one or more rotating disks, coated with magnetic material
Introduce by IBM in 1956
Use magnetic head to read data

16. The very early HDD…..
Acknowledge to:

17. HDD (Cont’d) The essential structure of HDD has not changed too much…
Constitute with several disks
Each disk is divided to tracks, each of which then is divided to sectors
The single most significant factor:
Seek time

18. Why seek time matters
When access a data (sector), the HDD head must first move to the track (seek time), then rotates the disk to the sector (rotational time)
Seek time: 3 ms on high-end server disks, 12 ms on desktop-level disks [1]
Rotational time: 5.56ms on 5400 RPM HDD, 4.17ms on RPM HDD [1]
As a result, sequential IO is much faster than random IO, because there is no seek /rotational time
[1],

19. General Optimizations
Based on two principles:
RAM access is much faster than the access on disk
Sequential IOs is much faster than random IOs on disk
So we design file systems that
Largely utilizes CPU/RAM to reduce IO to disks (various caches/write buffers)
Prefers sequential IOs
Computes disk layout to arrange related data sequentially located on disks

20. Dcache Dentry cache (dcache) Directories are stored as files on disks.
For each file lookup, we want obtain the inode from the given full file path
OS looks the dentries from the root to all parent directories in the path.
E.g. for looking up file “/Users/john/Documents/course.pdf”, OS needs traverse the dentries that presents “/”, “Users”, “john”, “Documents”, and “course.pdf”
To accelerate this:
We use a global hash table (dcache) to map “file path” -> dentry
A two-list solution: one for active dentries, and one for “recent unused dentries” (LRU).

21. Inode cache
Similar to the dcache, OS maintains a cache for inode objects.
Each inode object has 1-to-1 relation to a dentry
If the dentry object is evicted, this inode is evicted

22. Page Cache
…a “transparent” buffer for disk-backed pages kept in RAM for fast access… [wikipedia]
A write-back cache
Main purpose: reducing the # of IOs to disks
Access based on page (usually 4KB).
Page cache is per-file based.
A Redix-tree in inode object.
Prefetch pages to serve future read
Absorb writes to reduce # of IOs
The dirty pages (modified) are flushed to disks for : 1) each 30s or 5s, or 2) OS wants to reclaim RAMs
Also can be forced to flush by calling “fsync()” system call

23. Agenda
Introduction
VFS
Optimizations
Examples
F&Q

24. Examples Several concrete file system designs
Ext4, classic UNIX-like file system concepts
NTFS, advanced Windows file system
ZFS, “the last word of file system”
NFS, a standard network file system
Google File System, a special distributed file system for special requirements

25. Ext4 The latest version of the “extended file system” (Ext2/3/4)
The standard Linux file system for a long time
Inspired from UFS from BSD/Solaris
Group files to block groups
Keep file data near to inodes
Ack:

26. NTFS “New Technology File System” (NTFS)
The standard file system in Windows world.
A Master File Table (MFT) contains all metadata.
Directory is also a file

27. ZFS ZFS: “the last word of file system”
The most advanced local file system in production
128 bits space (2128 bytes in theory)
larger the # of sand in the earth…
A lot of advanced features:
E.g. transactional commits, end-to-end integration, snapshot, volume management and much more…
Will never lose data and always be consistent.
Every OS community wants to clone or copy its features…
Btrfs on Linux, ReFS on Windows, ZFS on FreeBSD

28. NFS “Network File System (NFS)” A protocol developed by SUN in 1984
A set of RPC calls
IETF standard
Supported by all major OSs
Simple and efficient

29. Google File System (GFS)
A large distributed file system specially designed for MapReduce framework
High throughput
High availability
Special designed. Not compatible to VFS/POSIX API.
Requires clients linked to the GFS library.
Hadoop DFS clones the concepts of GFS

30. More File Systems Interesting file systems that are worth to explore
Btrfs (B-tree FS) from oracle, expected to be the next standard Linux file system. Many concepts are shared with ZFS.
ReFS: The file system for Windows 8 (from Microsoft). Many concepts are shared with ZFS (too!).
WAFL (Write Anywhere File Layout) file system from NetApp.
FUSE (Filesystem in Userspace): a cross-platform library that allows developers to write file system running in user mode

31. FAQ?
Thanks