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I'd like to know if there's some rule (or formula) I can apply to find out how much of disk space will be used by the filesystem in an ext4 partition. for example, in a partition of 100 GB, how much can I actually use? does it depend on other parameters like inode size, etc?

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I don't know if it applies to you, but just in case: most of the time the huge majority of the apparent overhead of ext2/3/4 will be due to the default significant amount of reserved blocks for root. Make sure you specify -m 0 (or 1) to mkfs.ext4 when creating the filesystem or to tune2fs if adjusting it afterwards. The default is 5, which stands for 5% of total blocks and can be excessive in most cases.

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The reserved space serves two purposes: to lower the fragmentation chances and to have some space for fsck. I do not recommend to make it 0. –  Mircea Vutcovici Sep 12 '11 at 20:00
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Difficult question to answer this as the amount of DATA space would vary with the number of files, their size, ACLs, Root reserved space, blocksize etc. There is not really a easily fixed answer to this question.

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The most significant overhead comes from the inode tables, so it obviously depends on how many inodes you allocate. With the default options, every 128mb of disk gets 2 mb ( 8192 x 256 bytes each ) of inodes, or 1.6% overhead.

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