Il 16/01/20 02:21, Gordon Messmer ha scritto:
On 1/15/20 8:18 AM, Alessandro Baggi wrote:
Then I sync src/ to dest/ using "rsync -avS src/ dest/", all ok but
when I run "du -h dest/testfile" I get 0 and if I run "du -b
dest/testfile" I get the correct size in bytes.
That's not a bug, that's what sparse files are.
In POSIX systems, it's possible to treat a regular file like memory,
and one of the things you might do with such a feature is use a file
to keep track of the last time a user logged in. The simplest way to
so that is to save the time value at the offset of the user's UID. My
uid is 556600005, so if the file weren't sparse, that one entry would
create an enormous file, but with sparse files, the system only needs
to allocate one block to store that value. If a process reads that
file, it will get all zeros from the OS until it reaches the date
stored at my uid offset.
Applications can't tell whether a given set of zeros in a file are
actual zeros on disk, or if they're simply parts of the file that
haven't been written to, so when you tell rsync to create sparse
files, it will do its best to identify blocks that are all zeros and
simply not write to those on the destination. Thus, if you use
/dev/zero to create a file on the source and then rsync it with -S,
the destination file will use zero blocks of storage. Naturally, that
can only be true with files whose contents are null bytes, as you get
from /dev/zero.
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Thank you for your answer. I appreciated it.
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