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On 09/16/2017 07:06 AM, Kai Krakow wrote: |
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> Am Fri, 15 Sep 2017 14:28:49 -0400 |
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> schrieb Rich Freeman <rich0@g.o>: |
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> |
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>> On Fri, Sep 8, 2017 at 3:16 PM, Kai Krakow <hurikhan77@×××××.com> |
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>> wrote: |
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>>> |
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>>> At least in btrfs there's also a caveat that the original extents |
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>>> may 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 |
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>>> copy will occupy more space than expected until you defragment the |
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>>> file: |
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>> |
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>> True, but keep in mind that this applies in general in btrfs to any |
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>> kind of modification to a file. If you modify 1MB in the middle of a |
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>> 10GB file on ext4 you end up it taking up 10GB of space. If you do |
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>> the same thing in btrfs you'll probably end up with the file taking up |
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>> 10.001GB. Since btrfs doesn't overwrite files in-place it will |
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>> typically allocate a new extent for the additional 1MB, and the |
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>> original content at that position within the file is still on disk in |
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>> the original extent. It works a bit like a log-based filesystem in |
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>> this regard (which is also effectively copy on write). |
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> |
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> Good point, this makes sense. I never thought about that. |
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> |
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> But I guess that btrfs doesn't use 10G sized extents? And I also guess, |
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> this is where autodefrag jumps in. |
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|
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According to btrfs-filesystem(8), defragmentation breaks reflinks, in |
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all but a few old kernel versions where I guess they tried to fix the |
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problem and apparently failed. This really makes much of what btrfs does |
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altogether pointless if you ever defragment manually or have autodefrag |
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enabled. Deduplication is broken for the same reason. |
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