File System Topics Lei Xu

Agenda Introduction VFS Optimizations Examples F&Q

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)

Relationship to Architecture Course Acknowledge to the slides from 830 course

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

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” http://en.wikipedia.org/wiki/List_of_file_systems

Agenda Introduction VFS Optimizations Examples F&Q

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

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

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)

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

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

Agenda Introduction VFS Optimizations Examples F&Q

Optimizations Most of file system optimizations are designed based on the characteristics of the memory hierarchy and storage devices. Recall: RAM 50-100 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

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

The very early HDD….. Acknowledge to:

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

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 7200 RPM HDD [1] As a result, sequential IO is much faster than random IO, because there is no seek /rotational time [1], http://en.wikipedia.org/wiki/Disk-drive_performance_characteristics

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

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).

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

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

Agenda Introduction VFS Optimizations Examples F&Q

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

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: http://bit.ly/tjipWY

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

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

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

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

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

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