[OT] - Backing Up Windows XP to Linux

Brian Hawkins brianhks at activeclickweb.com
Thu Oct 5 15:57:47 MDT 2006


This works but you are missing the real benefit of rsync.  For example 
lets say you are synchronizing a very large log file.  The file has been 
appended to and needs to be synced.  If using an smb mount, rsync sees 
the file is bigger and newer so it can do one of two things: 1 copy the 
entire file. or 2 checksum the file and copy just the change.  In either 
case it means reading the entire file across the drive mount.

Now take the case of running rsync with a local and remote path.  Rsync 
uses ssh to start up an instance of rsync on the remote machine and the 
two instances communicate through the ssh tunnel.  The two instances 
then exchange file size, time stamps and checksum data. (this is very 
small compared to the actual file size).  Now they figure out that only 
the last few blocks of the log file need to be transfered.  This they do 
and the job is done. 

So in effect the same result is reached with the first as in the second 
approach except in the second approach only a fraction of the data is 
transfered across the network.  You can see this by adding the -v to the 
rsync command.  At the end it will tell you how much data has been 
transfered.  Try modifying small sections of a very large file and then 
rsync them and you will see for your self how well it works.

Brian

Bart Whiteley wrote:
> On 10/4/06, Michael L Torrie <torriem at chem.byu.edu> wrote:
>>
>> On Wed, 2006-10-04 at 15:12 -0400, brianhks at activeclickweb.com wrote:
>> > This is bad.  If you mount the drive you may as well use cp or copy or
>> > xcopy.  The real strength of rsync is its extremely low level of
>> > network traffic to sync the data.  To do this rsync does checksums on
>> > the data so if you run it over a mounted volume it will read all of
>> > the date across the wire.  Rsync runs best when there are two
>> > instances of rsync running, one on each machine to be synchronized.
>> > This way only checksums are passed over the wire before any data is
>> > copied.
>>
>> I've heard this before, but I don't think it's true, based on my
>> experience.  I can rsync between two disks physically in my system and
>> it doesn't read much data at all off the local disk except to copy new
>> files.  If what you're saying is true about how rsync works, then it
>> wouldn't work at all for local rsyncs (wouldn't save time).  I think
>> rsync only checksums if it has to.  Could someone who is an rsync expert
>> clarify this for me?
>
>
> Man page indicates that rsync uses file size and mod-time by default.
> Force checksums instead with -c.
>
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