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On Sat, Nov 26, 2011 at 9:38 PM, Michał Górny <mgorny@g.o> wrote: |
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> On Sat, 26 Nov 2011 21:28:51 +0530 |
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> Nirbheek Chauhan <nirbheek@g.o> wrote: |
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>> There are still a few specific cases in which CoW would indeed be |
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>> useful. IIRC, reflinking of files works across btrfs *subvolumes*, and |
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>> such a copy would normally be detected as a cross-device move. |
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> |
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> For such a thing, shouldn't rename() work neat anyway? |
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> |
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No, because reflink is an ioctl that works directly on the FS level by |
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sharing data blocks, and should theoretically not bother about the |
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file hierarchy. On the other hand, rename() is a userland API and must |
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behave itself. |
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>> Another use would be an patch-merge which makes use of *ranged |
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>> reflinks* to only CoW copy those parts of the file that were |
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>> changed[1]. rsync has support for this, but only while appending to |
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>> files (--append-verify --no-whole-file). |
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> |
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> So, it'd be like: |
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> 1) CoW-dup old file, |
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> 2) patch-merge into the duped old file, |
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> 3) replace. |
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> |
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> Am I understanding correctly? |
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> |
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You can do that, or perhaps you can just do the patch-merge in-place. |
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Not sure about the crash guarantees in the latter case. The former |
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(rename) is documented here: |
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https://btrfs.wiki.kernel.org/articles/f/a/q/FAQ_1fe9.html#What_are_the_crash_guarantees_of_overwrite-by-rename.3F |
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But in all this, the hard part is really the "patch-merge" for |
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anything except appends. ;) |
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-- |
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~Nirbheek Chauhan |
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|
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Gentoo GNOME+Mozilla Team |
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