|By Brian Carrier||
|August 12, 2005 03:00 PM EDT||
We have all done it before. You accidentally type in the wrong argument to rm or select the wrong file for deletion. As you hit enter, you notice your mistake and your stomach drops. You reach for the backup of the system and realize that there isn't one.
There are many undelete tools for FAT and NTFS file systems, but there are few for Ext3, which is currently the default file system for most Linux distributions. This is because of the way that Ext3 files are deleted. Crucial information that stores where the file content is located is cleared during the deletion process.
In this article, we take a low-level look at why recovery is difficult and look at some approaches that are sometimes effective. We will use some open source tools for the recovery, but the techniques are not completely automated.
What Is a File?
Before we can see how to recover files, we need to look at how files are stored. Typically, file systems are located inside of a disk partition. The partition is usually organized into 512-byte sectors. When the partition is formatted as Ext3, consecutive sectors will be grouped into blocks, whose size can range from 1,024 to 4,096 bytes. The blocks are grouped together into block groups, whose size will be tens of thousands of blocks. Each file has data stored in three major locations: blocks, inodes, and directory entries. The file content is stored in blocks, which are allocated for the exclusive use of the file. A file is allocated as many blocks as it needs. Ideally, the file will be allocated consecutive blocks, but this is not always possible.
The metadata for the file is stored in an inode structure, which is located in an inode table at the beginning of a block group. There are a finite number of inodes and each is assigned to a block group. File metadata includes the temporal data such as the last modified, last accessed, last changed, and deleted times. Metadata also includes the file size, user ID, group ID, permissions, and block addresses where the file content is stored.
The addresses of the first 12 blocks are saved in the inode and additional addresses are stored externally in blocks, called indirect blocks. If the file requires many blocks and not all of the addresses can fit into one indirect block, a double indirect block is used whose address is given in the inode. The double indirect block contains addresses of single indirect blocks, which contain addresses of blocks with file content. There is also a triple indirect address in the inode that adds one more layer of pointers.
Last, the file's name is stored in a directory entry structure, which is located in a block allocated to the file's parent directory. An Ext3 directory is similar to a file and its blocks contain a list of directory entry structures, each containing the name of a file and the inode address where the file metadata is stored. When you use the ls -i command, you can see the inode address that corresponds to each file name. We can see the relationship between the directory entry, the inode, and the blocks in Figure 1.
When a new file is created, the operating system (OS) gets to choose which blocks and inode it will allocate for the file. Linux will try to allocate the blocks and inode in the same block group as its parent directory. This causes files in the same directory to be close together. Later we'll use this fact to restrict where we search for deleted data.
The Ext3 file system has a journal that records updates to the file system metadata before the update occurs. In case of a system crash, the OS reads the journal and will either reprocess or roll back the transactions in the journal so that recovery will be faster then examining each metadata structure, which is the old and slow way. Example metadata structures include the directory entries that store file names and inodes that store file metadata. The journal contains the full block that is being updated, not just the value being changed. When a new file is created, the journal should contain the updated version of the blocks containing the directory entry and the inode.
Several things occur when an Ext3 file is deleted from Linux. Keep in mind that the OS gets to choose exactly what occurs when a file is deleted and this article assumes a general Linux system.
At a minimum, the OS must mark each of the blocks, the inode, and the directory entry as unallocated so that later files can use them. This minimal approach is what occurred several years ago with the Ext2 file system. In this case, the recovery process was relatively simple because the inode still contained the block addresses for the file content and tools such as debugfs and e2undel could easily re-create the file. This worked as long as the blocks had not been allocated to a new file and the original content was not overwritten.
With Ext3, there is an additional step that makes recovery much more difficult. When the blocks are unallocated, the file size and block addresses in the inode are cleared; therefore we can no longer determine where the file content was located. We can see the relationship between the directory entry, the inode, and the blocks of an unallocated file in Figure 2.
Now that we know the components involved with files and which ones are cleared during deletion, we can examine two approaches to file recovery (besides using a backup). The first approach uses the application type of the deleted file and the second approach uses data in the journal. Regardless of the approach, you should stop using the file system because you could create a file that overwrites the data you are trying to recover. You can power the system off and put the drive in another Linux computer as a slave drive or boot from a Linux CD.
The first step for both techniques is to determine the deleted file's inode address. This can be determined from debugfs or The Sleuth Kit (TSK). I'll give the debugfs method here. debugfs comes with most Linux distributions and is a file system debugger. To start debugfs, you'll need to know the device name for the partition that contains the deleted file. In my example, I have booted from a CD and the file is located on /dev/hda5:
# debugfs /dev/hda5
debugfs 1.37 (21-Mar-2005)
We can then use the cd command to change to the directory of the deleted file:
debugfs: cd /home/carrier/
The ls -d command will list the allocated and deleted files in the directory. Remember that the directory entry structure stores the name and the inode of the file and this listing will give us both values because neither is cleared during the deletion process. The deleted files have their inode address surrounded by "<" and ">":
debugfs: ls -d
415848 (12) . 376097 (12) .. 415864 (16) .bashrc
<415926> (28) oops.dat
|theusr 07/09/09 09:29:00 AM EDT|
The figure 2 maybe misleading: the links between the address blocks and the file content are still there (though the address blocks are unallocated), that what's make the recovery possible.
|Mike Kay 01/15/08 03:57:07 PM EST|
Excellent article. Followed it step by step and successfully recovered a .XLS spreadsheet that had been deleted from the /tmp folder on Ubuntu Gutsy. It also found an associated .jpg that I wasn't looking for!
Saved me hours of retyping. Thanks a lot.
|Jahangir 10/22/07 05:26:36 PM EDT|
This was really the best article i could find inspite of 3 hrs of googling.
But what if you are trying to recover a 6GB VM.
|ruintower 04/23/06 09:07:29 PM EDT|
Trackback Added: ext3 undelete; I “mis-deleted” a big file several days ago. So I umount the the partition immediately and searched the recovery method because I knew (but forgot) some methods to recovery file in Linux. However, the result is disappointed. Alt...
|marco 03/13/06 08:04:20 AM EST|
U have saved my life.
U are a GURU,
|marco 03/13/06 08:04:04 AM EST|
U have saved my life.
U are a GURU,
The cloud market growth today is largely in public clouds. While there is a lot of spend in IT departments in virtualization, these aren’t yet translating into a true “cloud” experience within the enterprise. What is stopping the growth of the “private cloud” market? In his general session at 18th Cloud Expo, Nara Rajagopalan, CEO of Accelerite, explored the challenges in deploying, managing, and getting adoption for a private cloud within an enterprise. What are the key differences between wh...
Jan. 17, 2017 12:45 AM EST Reads: 6,005
Internet of @ThingsExpo, taking place June 6-8, 2017 at the Javits Center in New York City, New York, is co-located with the 20th International Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry players in the world. @ThingsExpo New York Call for Papers is now open.
Jan. 16, 2017 11:30 PM EST Reads: 3,516
WebRTC has had a real tough three or four years, and so have those working with it. Only a few short years ago, the development world were excited about WebRTC and proclaiming how awesome it was. You might have played with the technology a couple of years ago, only to find the extra infrastructure requirements were painful to implement and poorly documented. This probably left a bitter taste in your mouth, especially when things went wrong.
Jan. 16, 2017 09:00 PM EST Reads: 7,441
IoT is at the core or many Digital Transformation initiatives with the goal of re-inventing a company's business model. We all agree that collecting relevant IoT data will result in massive amounts of data needing to be stored. However, with the rapid development of IoT devices and ongoing business model transformation, we are not able to predict the volume and growth of IoT data. And with the lack of IoT history, traditional methods of IT and infrastructure planning based on the past do not app...
Jan. 16, 2017 03:15 PM EST Reads: 365
"LinearHub provides smart video conferencing, which is the Roundee service, and we archive all the video conferences and we also provide the transcript," stated Sunghyuk Kim, CEO of LinearHub, in this SYS-CON.tv interview at @ThingsExpo, held November 1-3, 2016, at the Santa Clara Convention Center in Santa Clara, CA.
Jan. 16, 2017 02:30 PM EST Reads: 1,551
With major technology companies and startups seriously embracing IoT strategies, now is the perfect time to attend @ThingsExpo 2016 in New York. Learn what is going on, contribute to the discussions, and ensure that your enterprise is as "IoT-Ready" as it can be! Internet of @ThingsExpo, taking place June 6-8, 2017, at the Javits Center in New York City, New York, is co-located with 20th Cloud Expo and will feature technical sessions from a rock star conference faculty and the leading industry p...
Jan. 16, 2017 01:45 PM EST Reads: 3,599
"There's a growing demand from users for things to be faster. When you think about all the transactions or interactions users will have with your product and everything that is between those transactions and interactions - what drives us at Catchpoint Systems is the idea to measure that and to analyze it," explained Leo Vasiliou, Director of Web Performance Engineering at Catchpoint Systems, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York Ci...
Jan. 16, 2017 12:30 PM EST Reads: 5,482
The 20th International Cloud Expo has announced that its Call for Papers is open. Cloud Expo, to be held June 6-8, 2017, at the Javits Center in New York City, brings together Cloud Computing, Big Data, Internet of Things, DevOps, Containers, Microservices and WebRTC to one location. With cloud computing driving a higher percentage of enterprise IT budgets every year, it becomes increasingly important to plant your flag in this fast-expanding business opportunity. Submit your speaking proposal ...
Jan. 16, 2017 12:30 PM EST Reads: 5,021
Discover top technologies and tools all under one roof at April 24–28, 2017, at the Westin San Diego in San Diego, CA. Explore the Mobile Dev + Test and IoT Dev + Test Expo and enjoy all of these unique opportunities: The latest solutions, technologies, and tools in mobile or IoT software development and testing. Meet one-on-one with representatives from some of today's most innovative organizations
Jan. 16, 2017 12:00 PM EST Reads: 1,401
20th Cloud Expo, taking place June 6-8, 2017, at the Javits Center in New York City, NY, will feature technical sessions from a rock star conference faculty and the leading industry players in the world. Cloud computing is now being embraced by a majority of enterprises of all sizes. Yesterday's debate about public vs. private has transformed into the reality of hybrid cloud: a recent survey shows that 74% of enterprises have a hybrid cloud strategy.
Jan. 16, 2017 11:30 AM EST Reads: 4,145
SYS-CON Events announced today that Super Micro Computer, Inc., a global leader in Embedded and IoT solutions, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 7-9, 2017, at the Javits Center in New York City, NY. Supermicro (NASDAQ: SMCI), the leading innovator in high-performance, high-efficiency server technology, is a premier provider of advanced server Building Block Solutions® for Data Center, Cloud Computing, Enterprise IT, Hadoop/Big Data, HPC and E...
Jan. 16, 2017 11:30 AM EST Reads: 5,682
SYS-CON Events announced today that Linux Academy, the foremost online Linux and cloud training platform and community, will exhibit at SYS-CON's 20th International Cloud Expo®, which will take place on June 6-8, 2017, at the Javits Center in New York City, NY. Linux Academy was founded on the belief that providing high-quality, in-depth training should be available at an affordable price. Industry leaders in quality training, provided services, and student certification passes, its goal is to c...
Jan. 16, 2017 11:30 AM EST Reads: 1,882
WebRTC is the future of browser-to-browser communications, and continues to make inroads into the traditional, difficult, plug-in web communications world. The 6th WebRTC Summit continues our tradition of delivering the latest and greatest presentations within the world of WebRTC. Topics include voice calling, video chat, P2P file sharing, and use cases that have already leveraged the power and convenience of WebRTC.
Jan. 16, 2017 08:30 AM EST Reads: 3,041
"A lot of times people will come to us and have a very diverse set of requirements or very customized need and we'll help them to implement it in a fashion that you can't just buy off of the shelf," explained Nick Rose, CTO of Enzu, in this SYS-CON.tv interview at 18th Cloud Expo, held June 7-9, 2016, at the Javits Center in New York City, NY.
Jan. 16, 2017 06:30 AM EST Reads: 4,516
WebRTC sits at the intersection between VoIP and the Web. As such, it poses some interesting challenges for those developing services on top of it, but also for those who need to test and monitor these services. In his session at WebRTC Summit, Tsahi Levent-Levi, co-founder of testRTC, reviewed the various challenges posed by WebRTC when it comes to testing and monitoring and on ways to overcome them.
Jan. 16, 2017 06:30 AM EST Reads: 5,867
Every successful software product evolves from an idea to an enterprise system. Notably, the same way is passed by the product owner's company. In his session at 20th Cloud Expo, Oleg Lola, CEO of MobiDev, will provide a generalized overview of the evolution of a software product, the product owner, the needs that arise at various stages of this process, and the value brought by a software development partner to the product owner as a response to these needs.
Jan. 16, 2017 05:30 AM EST Reads: 1,105
WebRTC services have already permeated corporate communications in the form of videoconferencing solutions. However, WebRTC has the potential of going beyond and catalyzing a new class of services providing more than calls with capabilities such as mass-scale real-time media broadcasting, enriched and augmented video, person-to-machine and machine-to-machine communications. In his session at @ThingsExpo, Luis Lopez, CEO of Kurento, introduced the technologies required for implementing these idea...
Jan. 16, 2017 04:30 AM EST Reads: 4,536
The WebRTC Summit New York, to be held June 6-8, 2017, at the Javits Center in New York City, NY, announces that its Call for Papers is now open. Topics include all aspects of improving IT delivery by eliminating waste through automated business models leveraging cloud technologies. WebRTC Summit is co-located with 20th International Cloud Expo and @ThingsExpo. WebRTC is the future of browser-to-browser communications, and continues to make inroads into the traditional, difficult, plug-in web co...
Jan. 16, 2017 03:30 AM EST Reads: 2,855
The Internet of Things will challenge the status quo of how IT and development organizations operate. Or will it? Certainly the fog layer of IoT requires special insights about data ontology, security and transactional integrity. But the developmental challenges are the same: People, Process and Platform. In his session at @ThingsExpo, Craig Sproule, CEO of Metavine, demonstrated how to move beyond today's coding paradigm and shared the must-have mindsets for removing complexity from the develop...
Jan. 16, 2017 03:30 AM EST Reads: 5,245
While not quite mainstream yet, WebRTC is starting to gain ground with Carriers, Enterprises and Independent Software Vendors (ISV’s) alike. WebRTC makes it easy for developers to add audio and video communications into their applications by using Web browsers as their platform. But like any market, every customer engagement has unique requirements, as well as constraints. And of course, one size does not fit all. In her session at WebRTC Summit, Dr. Natasha Tamaskar, Vice President, Head of C...
Jan. 16, 2017 03:15 AM EST Reads: 5,756