Title is clear, allocating 2GB files takes no time / no space on disk, and thus disk fragmentation can occur.
# mkdir /tmp/x
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# mongod --dbpath /tmp/x --port 27111 --replSet rs1 --oplogSize 10000
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# mongo --port 27111
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> rs.initiate()
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Sat Nov 30 08:02:11.712 [FileAllocator] allocating new datafile x/local.1, filling with zeroes...
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Sat Nov 30 08:02:11.796 [FileAllocator] done allocating datafile x/local.1, size: 2047MB, took 0.084 secs
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Sat Nov 30 08:02:11.797 [FileAllocator] allocating new datafile x/local.2, filling with zeroes...
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Sat Nov 30 08:02:11.801 [FileAllocator] done allocating datafile x/local.2, size: 2047MB, took 0.004 secs
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Sat Nov 30 08:02:11.801 [FileAllocator] allocating new datafile x/local.3, filling with zeroes...
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Sat Nov 30 08:02:11.805 [FileAllocator] done allocating datafile x/local.3, size: 2047MB, took 0.003 secs
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Sat Nov 30 08:02:11.805 [FileAllocator] allocating new datafile x/local.4, filling with zeroes...
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Sat Nov 30 08:02:11.810 [FileAllocator] done allocating datafile x/local.4, size: 2047MB, took 0.005 secs
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Sat Nov 30 08:02:11.811 [FileAllocator] allocating new datafile x/local.5, filling with zeroes...
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Sat Nov 30 08:02:11.815 [FileAllocator] done allocating datafile x/local.5, size: 2047MB, took 0.004 secs
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# ll -h
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total 1032
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drwxr-xr-x 2 root wheel 512B Nov 30 08:02 _tmp/
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drwxr-xr-x 2 root wheel 512B Nov 30 08:04 journal/
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-rw------- 1 root wheel 64M Nov 30 08:02 local.0
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-rw------- 1 root wheel 2G Nov 30 08:02 local.1
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-rw------- 1 root wheel 2G Nov 30 08:02 local.2
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-rw------- 1 root wheel 2G Nov 30 08:02 local.3
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-rw------- 1 root wheel 2G Nov 30 08:02 local.4
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-rw------- 1 root wheel 2G Nov 30 08:02 local.5
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-rw------- 1 root wheel 16M Nov 30 08:02 local.ns
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-rwxr-xr-x 1 root wheel 0B Nov 30 08:04 mongod.lock*
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# du -sh
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1.0M .
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