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I'm trying to optimize a severely memory constrained system (DNS-323 NAS, 64MB) and wonder if you know or know how to find out the memory usage for different file systems?

Naturally, the memory used will be affected by the size of the fs tree, but relatively speaking. For example, will ext4 use up 2 times more memory than ext2 and btrfs 3 times? My google-foo hasn't turned up anything, so how could one find out?

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Not really. For most major filesystems, filesystem only plays a major effect with how the data is stored on disk. When it's in memory, it's a part of the kernel's Virtual File System. Some filesystem features, such as on-the-fly encryption and data deduplication do require additional memory, but those are easily turned off.

Different file systems might have different CPU requirements though, in terms of reading/writing data. Using a journaled file system like Ext3/4 vs. a non-journaled filesystem like Ext2 will require marginally more processing power to save a file.

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Thank you for the info, Darth Droid. But, just for academics, would it be an accurate view to sum the module + the module dependencies (from lsmod) and the cache size (from slabtop)? On my system that is 225320B for ext3, 448107B for ext4 and 725859B for btrfs modules. Slabtops shows ext4_inode_cache having a cache size of 1612K and ext4_groupinfo_4k exactly the same (only have ext4 fs mounted). I'm guessing this a overly naive approach? – Anders Olsson Aug 28 '13 at 19:08
I'm afraid I'm not that familiar with the details to answer that. – Darth Android Aug 28 '13 at 19:12

I just want to add that ZFS (not technically a Linux FS but still usable) has very high memory requirements.

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