Home > Articles > Operating Systems, Server

How It Works: Filesystems

David Chisnall takes a look at one of the least glamorous parts of an operating system - the filesystem - to see what it needs to do, and how a good or bad design can affect user experience.
Like this article? We recommend

When you use a computer to store a document, you typically put it in a file. This isn’t a very good abstraction, but it’s familiar. You give this file a name (and maybe some other metadata) and put it into a directory. You might set some permissions on it, too.

Your operating system sees a disk as a block device, a linear array of 512-byte blocks that must be read or written in one go. Unless you’re a FORTH programmer, this doesn’t look at all like what you expect from persistent storage. The filesystem sits between you and the disk and translates between the two models.

Storing Files

In the UNIX sense of the word, a file is an array of bytes. For most filesystems, it’s an array of disk blocks with some associated metadata. The main job of any filesystem is finding which blocks belong to a given file and which belong to no files (and so can be used for new files or appended to an existing file).

MS-DOS used a filesystem called FAT, for the file allocation tables that were at its heart. In spite of the many serious design flaws with FAT, it has steadfastly refused to die. The file allocation tables themselves are a simple array containing the index of the next block in the file. Once you have the first block for any file, you find the next one by looking at the index in the FAT.

One obvious flaw in this approach is that it gives O(n) behavior when seeking to a specific location in a file. If you have 4KB clusters on disk, and want to seek to the end of a 200MB file, you’ll need more than 50,000 lookups in the FAT. If every one of these lookups requires a disk access, this process will take around seven minutes. Even with one disk access per thousand lookups, ait can take half a second. For this reason, good performance requires keeping the FAT in memory.

In the UNIX world, things are done somewhat differently. Every file has a master inode. This is a data structure containing a load of metadata such as the permissions, creation time, and so on, and a short list of blocks. For small files, this is all that’s needed. For larger files, there’s an overflow capability in which a second disk block is used just to store a list of blocks. For really huge files, two or three layers of indirection can be used, so the inode contains an address of a block containing addresses of blocks containing addresses of blocks containing the file. At most, three disk reads are needed to find any address in a file stored like this.

The BeOS file system (BFS) used a slight variation of this system. Instead of storing a set of blocks, it stored a set of block ranges. If the file was stored contiguously on disk, only a single range was required, no matter how big the file became. In the worst case, this technique took twice as much space as storing individual blocks, since two integers had to be stored for a single block.

The inverse part of this is storing the free space. With FAT, files and free space were stored in the same way—free blocks were flagged in the FAT with a special identifier. Finding a free block meant performing a linear search through the table until this identifier was encountered. Since there was no efficient way of finding a group of blocks of a specific size, this led to the problem of fragmentation, the word used to describe what happens when files are not stored contiguously on disk, but rather are stored in fragments. Fragmentation is a problem for mechanical disks, since seeking is quite slow. A mechanical disk might have a seek time of 5 ms. If you need to seek every 512 bytes you read, even if reading an individual block takes no time, the maximum transfer rate you can get is 100 KB/s. In linear reads, the same disk can probably get around 50 MB/s. For this reason, fragmentation is a major problem for filesystems.

While the design of FAT seems silly here, it’s worth noting that it predates MS-DOS and actually was designed for Microsoft BASIC. For storing BASIC programs (rarely more than a few KB each) on low-density floppy disks, FAT isn’t such a bad design.

Storing free space efficiently is a difficult problem, because you need to be able to update the free space map quickly and also find a block of space of a specific efficiently. ZFS has quite a neat solution to this puzzle. A ZFS storage volume is split into chunks, within which free space is tracked independently. Whenever a space is allocated or freed, the range of space and the operation are written to a log. This is a very fast operation, since it’s just writing three values to the disk. Periodically, the log is read, and an AVL tree (a self-balancing tree structure) is created in memory. The tree can be searched easily for free space of a specific size, and can be written out to disk in the same form as the log, with all of the duplicate entries removed. For example, if you allocate block 12 and then free block 12, this pair of operations will cancel each other out in the tree and not be recorded when the tree is converted to log form.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020