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Recently I installed Debian Squeeze, first using ext3 and then again using ext4 on the same machine. The automatic fsck done after a certain number of mounts is much faster using ext4 (about 1 min) than ext3 (about 5 min).

What are the reasons for this significant difference in speed? If ext4 is much faster why does the Debian installer default to using ext3?

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The last part of your question should be addressed to the Debian dev team, really. – rozcietrzewiacz Nov 16 '11 at 13:14
@rozcietrzewiacz: there may be someone here who has a link to a public discussion that resulted in the decision. – Tshepang Nov 17 '11 at 14:48
up vote 7 down vote accepted

That's one of the most advertised benefits of ext4 (see it mentioned in the Features on Wikipedia).

The reason? Filesystem developers worked hard to achieve this.

Here's a short summary quoted from Wikipedia:

Faster file system checking

In ext4, unallocated block groups and sections of the inode table are marked as such. This enables e2fsck to skip them entirely on a check and greatly reduces the time it takes to check a file system of the size ext4 is built to support.

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