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Thread: rsync problem

  1. #1
    Join Date
    Jan 2009

    rsync problem

    hi there

    I have recently used rsync to backup my data through NTFS file system and I used rsync as my backup software.

    The procedure used by me is as below;

    rsync -arclpog --delete --ignore-errors /source_path /NFS_remote_path

    The first time I tried to backup all data which took me about 5 hours.
    In real sense it should be taking less time because rsync just check the time stamp for update but now also I still get every time 5 hours to backup.
    Am I using wrong option with this result?

    Please provide some help regarding this

  2. #2
    Join Date
    Jan 2008

    Re: rsync problem

    Try this one :

    It seems that rsync deletes before the sync by default. You may want to try --delete-after rather than --delete.

  3. #3
    Join Date
    May 2008

    Re: rsync problem

    Read 'man rsync' about the implications of using checksumming (-c). Run your current rsync run with -v added to see where the delay is. My guess is the checksum generation on both ends.

  4. #4
    Join Date
    Apr 2008

    Re: rsync problem

    The rsync utility uses an algorithm (invented by the Australian computer programmer Andrew Tridgell) for efficiently transmitting a structure (such as a file) across a communications link when the receiving computer already has a different version of the same structure.

    The recipient splits its copy of the file into fixed-size non-overlapping chunks, of size S, and computes two checksums for each chunk: the MD4 hash, and a weaker 'rolling checksum'. It sends these checksums to the sender. Version 30 of the protocol now uses MD5 hashes rather than MD4.

    The sender computes the rolling checksum for every chunk of size S in its own version of the file, even overlapping chunks. This can be calculated efficiently because of a special property of the rolling checksum: if the rolling checksum of bytes n through n + S − 1 is R, the rolling checksum of bytes n + 1 through n + S can be computed from R, byte n, and byte n + S without having to examine the intervening bytes. Thus, if one had already calculated the rolling checksum of bytes 1–25, one could calculate the rolling checksum of bytes 2–26 solely from the previous checksum, and from bytes 1 and 26.

    The rolling checksum used in rsync is based on Mark Adler's adler-32 checksum, which is used in zlib, and which itself is based on Fletcher's checksum.

    The sender then compares its rolling checksums with the set sent by the recipient to determine if any matches exist. If they do, it verifies the match by computing the MD4 checksum for the matching block and by comparing it with the MD4 checksum sent by the recipient.

    The sender then sends the recipient those parts of its file that did not match any of the recipient's blocks, along with assembly instructions on how to merge these blocks into the recipient's version. In practice, this creates a file identical to the sender's copy. However, it is in principle possible that the recipient's copy differs at this point from the sender's: this can happen when the two files have different chunks that nonetheless possess the same MD4 hash and rolling checksum; the chances for this to happen are in practice extremely remote.

    If the sender's and recipient's versions of the file have many sections in common, the utility needs to transfer relatively little data to synchronize the files.

    While the rsync algorithm forms the heart of the rsync application that essentially optimizes transfers between two computers over TCP/IP, the rsync application supports other key features that aid significantly in data transfers or backup. They include compression and decompression of data block by block using zlib at sending and receiving ends, respectively, and support for protocols such as ssh that enables encrypted transmission of compressed and efficient differential data using rsync algorithm. Instead of ssh, stunnel can also be used to create an encrypted tunnel to secure the data transmitted.

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