Large file copying speed. Why does it slow down?


  1. Posts : 125
    Windows 8
       #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 : 1,981
    Windows 10 x86 14383 Insider Pro and Core 10240
       #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 : 191
    Windows XP, 10; Knoppix [Debian] linux
       #3

    machare said:
    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. cereberus's Avatar
    Posts : 12,013
    Windows10
       #4

    Fafhrd said:
    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. Samuria's Avatar
    Posts : 6,045
    windows 10
       #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. Flashorn's Avatar
    Posts : 81
    Windows 7 Ultimate
       #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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  7. Posts : 7
    Windows 10 Pro 64-bit (1909)
       #7

    Samuria said:
    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
    I'm surprised you recommend RoboCopy. I've found RichCopy much better, and Microsoft themselves made it available as an improvement over RoboCopy.
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  8. Berton's Avatar
    Posts : 10,525
    Win10 Pro Versions 2004 and 2009/20H2, Win10 Pro IP_Dev, Win10 Home 1909
       #8

    thoatswold said:
    I'm surprised you recommend RoboCopy. I've found RichCopy much better, and Microsoft themselves made it available as an improvement over RoboCopy.
    Things did change after that last post 3 years ago. One thing about computing and it's need for precision, for better or worse, is the constant change and users having to deal with it.

    Just looked for RichCopy, has been discontinued, latest download available from MajorGeeks is

    Microsoft RichCopy 4.0.211
    Author: Microsoft Corp.
    Date: 10/14/2009 11:40 AM
    Size: 5.79 MB
    License: Freeware
    Requires: Win 10 / 8 / 7 / Vista / XP
    Last edited by Berton; 11 Sep 2020 at 10:43.
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  9. Posts : 7
    Windows 10 Pro 64-bit (1909)
       #9

    Berton said:
    Just looked for RichCopy, has been discontinued, latest download available from MajorGeeks is
    Microsoft RichCopy 4.0.211
    Author: Microsoft Corp.
    Date: 10/14/2009 11:40 AM
    Size: 5.79 MB
    License: Freeware
    Requires: Win 10 / 8 / 7 / Vista / XP
    Yes, RichCopy was discontinued by Microsoft, I don't know why, but the 2009 version is, as you say, available elsewhere and still works perfectly well on the latest edition of Windows 10. I've been using it this week to copy large numbers of files from one disk to another, and with the right settings it's really fast and reliable.
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