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Take initramdisk of rhel 6.2, which is cpio archive packed by XZ. And run it through file: $ file /tmp/initrd.img /tmp/initrd.img: data It happens on RHEL6 and even on Fedora 16. And despite the fact that /usr/share/misc/magic contains: # http://tukaani.org/xz/xz-file-format.txt 0 ustring \xFD7zXZ\x00 XZ compressed data !:mime application/x-xz Additional info: $ hexdump /tmp/initrd.img|head -n5 0000000 005d 0000 ff04 ffff ffff ffff 00ff 0d18 0000010 04dd 8363 7421 c707 f6d2 d69f 1091 7626 0000020 0ade 0f1f 48db 24ef 121f a4ba 5962 b486 0000030 2e9d 164c 96ae 9776 d317 8071 1d28 5c89 0000040 43d7 26d6 370f 1349 3018 8c78 a86b 874d
This is LZMA compression. There's already bug for RHEL6 mentioning initrd.img. Closing as DUPLICATE. *** This bug has been marked as a duplicate of bug 747999 ***