Perdida de datos en Ext4 con Kubuntu version Jaunty

He de decir que me paso y herramientas con testdisk y photorec no me pudieron ayudar asi que el bug es un poco grande sigo intentando recuperar os mantendre informados.

Un reporte publicado en el registro de errores para el próximo Ubuntu 9.04 "Jaunty Jackalope" describe una masiva pérdida de datos cuando se usa el nuevo sistema de archivos Ext4, el próximo estándar para práctiamente todas las principales distribuciones de GNU/Linux modernas.

Este gran problema se reporta en la edición Kubuntu de Jaunty apenas después de que los archivos del escritorio de KDE4 han sido cargados, resultado en la pérdida de todos los datos que fueron creados, incluyendo muchos de los propios archivos de configuración de KDE (!).

Rápidamente el desarrollador de Ext4 Ted Ts'o se apresuró a responder que el problema de fondo es la implementación de Ext4 de la "asignación diferida" (delayed allocation), también presente en otros sistemas de archivos como XFS y Btrfs, por la cual los nuevos datos puden demorar hasta 60 segundos en escribirse físicamente.

Ts'o explica que esto es "realmente más un problema de diseño de la aplicación" y que los programadores se acostumbraron al comportamiento del anterior Ext3, donde las escrituras se realizan cada 5 segundos. De cualquier forma, los parches no serían incluídos en el próximo Kernel 2.6.29, sino que fueron programados para el siguiente Kernel 2.6.30.

El soporte para Ext4 se incorporó a "Jaunty Jackalope" en su anterior versión Alpha 3.

 

Noticia original.

 

A bug report posted in the bug tracker for the next version of Ubuntu 9.04 (Jaunty Jackalope) describes a massive data loss problem when using Ext4, the future standard file system for Linux, available as an option when installing Ubuntu 9.04. The report describes a crash occurring shortly after the KDE 4 desktop files had been loaded, resulting in the loss of all of the data that had been created, including many KDE configuration files.

In a reply, Ext4 Developer Ted Ts'o explains the background of the problem. Like other modern file systems, such as XFS, Ext4 implements delayed allocation – writing new data can take up to 60 seconds. This increases the performance and allows for optimisation of the data organisation on the hard drive platter.

The KDE and GNOME desktop applications often read and write a large number of small files (for example, the configuration files for your personal settings). If the system crashes there may not be enough time for the data to be allocated and written to the hard drive – under ext4, the files may be truncated. This is because of delayed allocation. When a new file is created, the change is noted in the journal, but the data isn't written to the disk for a new file for anything between 45 and 150 seconds. The file system then catches up, allocating space for the file and writing the data. The exact technical details (which are critical system calls ftruncate() and rename()) can be found in the Ext4 developer's answer to the bug report.

Ts'o describes a workaround that tries to accurately identify this case and avoid the delayed allocation, but points out that other modern file systems, such as XFS and the new Btrfs, are also affected by this problem. The patches will not be included in the coming release of 2.6.29, but are queued for the 2.6.30 kernel.

Ts'o says that the application should be fixed so it does not write and rewrite small files. He advises that "this is really more of an application design problem more than anything else." Programmers had become accustomed to and dependent on, the behaviour of Ext3, which has a commit interval of 5 seconds and a default journalling mode of "data=ordered." Ext3's default journalling mode means that metadata is written to the journal in ordered mode, so any associated data changes would be forced to be written to the disk first. When Ext3 became the default file system developers came to rely on its behaviour.

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