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sync writes any data buffered in memory out to disk.
This can include (but is not limited to) modified
superblocks, modified inodes, and delayed reads and writes.
This must be implemented by the kernel; The sync program
does nothing but exercise the sync system call.
The kernel keeps data in memory to avoid doing (relatively
slow) disk reads and writes. This improves performance, but
if the computer crashes, data may be lost or the filesystem
corrupted as a result. sync ensures everything in memory is
written to disk.
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