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Am Thu, 07 Sep 2017 17:46:27 +0200 |
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schrieb Helmut Jarausch <jarausch@××××××.be>: |
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> Hi, |
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> |
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> sorry, this question is not Gentoo specific - but I know there are |
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> many very knowledgeable people on this list. |
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> |
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> I'd like to "hard-link" a file X to Y - i.e. there is no additional |
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> space on disk for Y. |
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> |
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> But, contrary to the "standard" hard-link (ln), file Y should be |
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> stored in a different place (inode) IF it gets modified. |
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> With the standard hard-link, file X is the same as Y, so any changes |
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> to Y are seen in X by definition. |
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> |
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> Is this possible |
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> - with an ext4 FS |
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> - or only with a different (which) FS |
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|
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You can do this with "cp --reflink=always" if the filesystem supports |
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it. |
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|
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To my current knowledge, only btrfs (since a long time) and xfs (in |
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newer kernel versions) support it. Not sure if ext4 supports it or |
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plans support for it. |
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|
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It is different to hard linking as the new file is linked by a new |
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inode, thus it has it's own time stamp and permissions unlike hard |
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links. Just contents are initially shared until you modify them. Also |
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keep in mind that this increases fragmentation especially when there |
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are a lot of small modifications. |
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|
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At least in btrfs there's also a caveat that the original extents may |
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not actually be split and the split extents share parts of the |
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original extent. That means, if you delete the original later, the copy |
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will occupy more space than expected until you defragment the file: |
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|
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File A extent map: [1111][22 2 2][3333] |
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File B extent map: [1111][22 2 2][3333] |
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Modify b: [1111][22][4][2][3333] <- one block modified |
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|
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Delete file a: [----][22 2 2][----] <- extent 2 still mapped |
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File b extent map: [1111][22][4][2][3333] |
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|
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So extent 2 is still on disk in its original state [2222]. |
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Defragment file b: [1111][2242][3333] |
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File a: [----][----][----] <- completely gone now |
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-- |
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Regards, |
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Kai |
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|
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Replies to list-only preferred. |
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