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Ext4 data loss; explanations and workarounds

Mar 20, 2009, 01:32 (0 Talkback[s])

"What had happened? When applications want to overwrite an existing file with new or changed data (for example, a configuration file after the user has changed a setting) frequently they first create a temporary file for the new data and then rename it with the system call - rename(). The logic behind this is that, if something goes wrong during the write process, say the computer crashes or there's a power failure, at least the old version of the file will be retained.

"The process involves two things. First, metadata in the file system is changed. An inode is created for the new file that references the data, and a new index entry is generated that points to the new inode. After a rename(), the index entry of the old file is changed so that it points to the new inode. Second, the data itself is written. To do this, the filing system must first allocate a sufficient number of data blocks on the disk and then write the data to those blocks."

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