Redundancy in various filesystems

This is a quick summary of redundant elements purposedly maintained in filesystems.

FAT16 and FAT32 filesystems typically have two copies of file allocation table (FAT). It is possible because the table is relatively small and the resulting overhead is not significant. Despite this some devices (e.g. mobile phone Sony Ericsson W580i) do not update the second copy of the table.

As for NTFS filesystem, the full copy of Master File Table (MFT) doesn't exist because it would be too large and too expensive to update. However, NTFS stores a copy of the beginning of the MFT. This copy has variable size depending on a cluster size. Only the records describing the system files are copied. There is no copy of the user file records.

ExFAT filesystem, which one might come across during a pen drive recovery or an SD card recovery, only stores a single copy of the file allocation table, most likely for performance reasons.

HFS and HFS+ do not have a copy of the Catalog File, although it might be theoretically possible, because the copy size wouldn't be too large. However, the designers opted not to do it.

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