Home > Articles > Operating Systems, Server

Journaling Filesystems for Linux

  • Print
  • + Share This
Minimizing system restart time is the primary advantage of using a journaling filesystem, but there are many others. As "newer" filesystems, journaling filesystems can take advantage of newer techniques for enhancing filesystem performance.

The previous article in this series provided background information on how data storage is organized and allocated on Linux and Unix systems, highlighting some of the more modern approaches used to improve performance, deal with larger files, and so on. One constant among all classic Linux and Unix filesystems is the general approach to the way the disk is updated when writing to a disk. Writing to a disk drive or other long-term storage is one of the slowest operations performed by computers, simply because it requires physical rather than electronic motion. For this reason, writing to a filesystem is usually done asynchronously, so that other processes on the system can continue to execute while data is being written to disk. Many filesystems cache data in memory until sufficient processor time is available, or a specific amount of data needs to be written to disk.

The problem with standard caching and asynchronous disk updates is that if the system goes down in the middle of an update, the filesystem is usually left in an inconsistent state. File information may not have been updated to reflect blocks that have been added to or deallocated those files, and directories may not have been correctly updated to reflect files that have been created or deleted. Similarly, the free list or filesystem bitmap may not have been correctly updated to reflect blocks that have been allocated or deallocated from files and directories.

To verify the consistency of a filesystem before attempting to mount and use it, Linux systems run a program called fsck, which stands for "file system check." If a filesystem isn't marked as being clean (by a bit in the filesystem superblock), the filesystem must be exhaustively checked for consistency before it can be mounted. Among other things, the fsck program for the ext2 filesystem verifies the consistency of all of the inodes, files, and directories in the filesystem, checks that all blocks marked as allocated are actually owned by some file or directory, and verifies that all blocks owned by files and directories are marked as allocated in the filesystem bitmap. As you can imagine, this can take quite a while to do on huge filesystems, and could therefore substantially delay making your system available to users.

Journaling filesystems keep a journal (or log) of the changes that are to be made to the filesystem, and then asynchronously apply those changes to the filesystem. Sets of related changes in the log are marked as being completed when they have been successfully written to the filesystem, and are then deleted from the log. If a computer crashes during the middle of these updates, the operating system need only replay the pending transactions in the log to restore the filesystem to a consistent state, rather than having to check the entire filesystem. Journaling filesystems therefore minimize system downtime due to filesystem corruption—by replacing the need to check the consistency of an entire filesystem with the requirement of replaying a fairly small log of changes, systems that use journaling filesystems can be made available to the user much more quickly after a system crash or any other type of downtime.

Minimizing system restart time is the primary advantage of using a journaling filesystem, but there are many others. As "newer" filesystems, journaling filesystems can take advantage of newer techniques for enhancing filesystem performance. Many journaling filesystems create and allocate inodes as they are needed, rather than preallocating a specific number of inodes when the filesystem is created. This removes limitations on the number of files and directories that can be created on that partition, increases performance, and reduces the overhead involved if you subsequently want to change the size of a journaling filesystem. Journaling filesystems also typically incorporate enhanced algorithms for storing and locating file and directory data, such as B-Trees, B+Trees, or B*Trees.

Nowadays, the terms "logging" and "journaling" are usually used interchangeably when referring to filesystems that record changes to filesystem structures and data to minimize restart time and maximize consistency. Classically, log-based filesystems are actually a distinct type of filesystem that uses a log-oriented representation for the filesystem itself, and also usually require a garbage collection process to reclaim space internally. Journaling filesystems use a log, which is simply a distinct portion of a filesystem or disk. Where and how logs are stored and used differs with each type of journaling filesystem. I tend to use the term "journaling filesystem" so as not to anger any of my old Computer Science professors who may still be living.

More Filesystems than You Can Shake a Memory Stick at

One of the biggest features of Linux as an Open Source endeavor is that the availability of the source code for the operating system makes it easy to understand and extend the operating system itself. All operating systems provide APIs for integrating low-level services, but having the source code is like the difference between reading the blueprints for a house and being allowed inside it with a toolbelt. Having the source code also eliminates the chance of undocumented APIs, which you might only be familiar with if your mailing address is in Redmond.

The availability of kernel source code and decent APIs for integrating low-level operating system services has resulted in some excellent extensions to the core capabilities of Linux, especially including support for new and existing filesystems. The best-known journaling filesystem for Linux, the Reiser File System, is an excellent example of this. The ReiserFS was born on Linux, and was the first journaling filesystem whose source code was integrated into the standard Linux kernel development tree. More recently (later versions of the 2.4 kernel family), the source code for the ext3 and JFS journaling filesystems have been integrated into the core Linux kernel source tree. As you see later in this article, the ext3 filesystem is a truly impressive effort—a logical follow-on to the ext2 filesystem that is completely compatible with existing ext2 filesystems and data structures. However, Linux has also benefited from some excellent journaling filesystems (such as JFS) with surprising roots—proprietary Unix vendors.

To a large extent, Linux is ringing the death knell for proprietary versions of Unix. Why spend a zillion dollars for hardware and a proprietary version of Unix when Linux is freely available and will run on everything from a sexy SMP machine to the PDA in your pocket? Most of the standard Unix vendors have seen the light to some extent, and understand the importance of embracing (or at least playing nicely) with Linux. To this end, existing Unix vendors—such as IBM and Silicon Graphics—have contributed the source code for some of their most exciting research efforts, the journaling filesystems that these proprietary vendors use on some or all of their hardware. IBM released the source code for its Journal File System, JFS, as Open Source in 2000. Similarly, Silicon Graphics released the source code for its XFS (eXtended File System) as Open Source at the same time. Regardless of the PR value inherent in releasing projects on which they've spent millions of research dollars, the bottom line of these contributions is the tremendous benefit that the capability to understand and use these filesystems brings to Linux systems.

The next few sections highlight the most popular journaling filesystems that are available for Linux and discuss some of the things that make each of them unique. As you'd expect, there are plenty of other journaling filesystems that are under development for Linux, as both research and open source projects. This article focuses on the ones that are actively used on Linux systems today, and which you may therefore actually encounter in the near future.

  • + Share This
  • 🔖 Save To Your Account

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