Windows 10: Large file copying speed. Why does it slow down?

  1.    28 Sep 2017 #1

    Large file copying speed. Why does it slow down?


    When I copy a file about 2TB in size from 1 disk to another the speed starts of fast say 185MBps and then drops to about 80MBps. This happens both on my desktop. On my laptop copying between two partitions slows down to a similar speed. Why does this happen?
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  2. Fafhrd's Avatar
    Posts : 1,928
    Windows 10 x86 14383 Insider Pro and Core 10240
       28 Sep 2017 #2

    There is a copy buffer - stuff gets read into memory from the source storage, then written to the target storage, which is checked against the buffer in memory for errors, then the next block of data is read, so at first about 2x the final rate is being transferred, but when the buffer is reading new stuff and checking the last block that has been transferred simultaneously, it has only ~half of the initial rate.

    The data copy rate indicator shows the amount of data stored and checked against time - at the end of the operation, it seems to do nothing for a while as data already written to the target is checked against a buffer before the process shuts down.

    The reading and writing from memory is the fast part of the operation, but the storage read and write operations are the slowest parts of the operation.
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  3. Posts : 133
    Windows XP, 7, 8.1, 10, Knoppix [Debian] linux
       28 Sep 2017 #3

    machare said: View Post
    When I copy a file about 2TB in size from 1 disk to another the speed starts of fast say 185MBps and then drops to about 80MBps. This happens both on my desktop. On my laptop copying between two partitions slows down to a similar speed. Why does this happen?
    Wild guess: the initial, faster speed, reflects system buffering of the write operation.
    Once the buffer fills up, the hardware transfer rate is reflected for the rest of the file.
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  4.    28 Sep 2017 #4

    Fafhrd said: View Post
    There is a copy buffer - stuff gets read into memory from the source storage, then written to the target storage, which is checked against the buffer in memory for errors, then the next block of data is read, so at first about 2x the final rate is being transferred, but when the buffer is reading new stuff and checking the last block that has been transferred simultaneously, it has only ~half of the initial rate.

    The data copy rate indicator shows the amount of data stored and checked against time - at the end of the operation, it seems to do nothing for a while as data already written to the target is checked against a buffer before the process shuts down.

    The reading and writing from memory is the fast part of the operation, but the storage read and write operations are the slowest parts of the operation.
    Yeah - it is copying to RAM buffer at that speed but can only write to disk at its maximum speed. Once buffer is full, it can only copy as fast as it can write i.e. buffer empties at rate of writing to drive.

    That initial burst is an illusion really.
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  5.    28 Sep 2017 #5

    Using robocopy built in to windows is the fastest method Anti virus can slow it to a crawl as it will scan on read and write
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  6.    28 Sep 2017 #6

    These links may help to understand what is happening :

    Enable or Disable Disk Write Caching in Windows 10 - Winaero

    Also for Windows 10:

    Write-Caching - Enable or Disable - Windows 7 Help Forums


    Flashorn.
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