Forum: Ruby FileUtils.mv behaving not as documented

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56c8db2891b2d2714ff22be005859285?d=identicon&s=25 Alex DeCaria (deke)
on 2009-02-23 02:07
The documentation for FileUtils.mv(src, dest) says that if src and dest
are on different disk partitions then it is supposed to act as a copy
instead.  But when I use it to copy a file from the "C:" drive to a
flash drive or mp3 player using another drive letter such as "E:" it
moves it and doesn't copy it as advertised.  Is this a bug, or is the
documentation incorrect?  I am using Ruby 186-26 on Windows.

-Alex
Ef3aa7f7e577ea8cd620462724ddf73b?d=identicon&s=25 Rob Biedenharn (Guest)
on 2009-02-23 03:29
(Received via mailing list)
On Feb 22, 2009, at 8:06 PM, Alex DeCaria wrote:

> The documentation for FileUtils.mv(src, dest) says that if src and
> dest
> are on different disk partitions then it is supposed to act as a copy
> instead.  But when I use it to copy a file from the "C:" drive to a
> flash drive or mp3 player using another drive letter such as "E:" it
> moves it and doesn't copy it as advertised.  Is this a bug, or is the
> documentation incorrect?  I am using Ruby 186-26 on Windows.
>
> -Alex


I think that the difference the documentation is attempting to point
out is that a mv on a single partition just changes names and pointers
in directory entries so it makes no difference the size of the file.
On different partitions, the size of the file will matter because the
effect is a copy followed by an unlink of the original file.

If you want to copy, call FileUtils.cp
If you want to move, continue to call FileUtils.mv

-Rob

Rob Biedenharn    http://agileconsultingllc.com
Rob@AgileConsultingLLC.com
56c8db2891b2d2714ff22be005859285?d=identicon&s=25 Alex DeCaria (deke)
on 2009-02-23 03:59
Rob Biedenharn wrote:
> On Feb 22, 2009, at 8:06 PM, Alex DeCaria wrote:
>
>> The documentation for FileUtils.mv(src, dest) says that if src and
>> dest
>> are on different disk partitions then it is supposed to act as a copy
>> instead.  But when I use it to copy a file from the "C:" drive to a
>> flash drive or mp3 player using another drive letter such as "E:" it
>> moves it and doesn't copy it as advertised.  Is this a bug, or is the
>> documentation incorrect?  I am using Ruby 186-26 on Windows.
>>
>> -Alex
>
>
> I think that the difference the documentation is attempting to point
> out is that a mv on a single partition just changes names and pointers
> in directory entries so it makes no difference the size of the file.
> On different partitions, the size of the file will matter because the
> effect is a copy followed by an unlink of the original file.
>
> If you want to copy, call FileUtils.cp
> If you want to move, continue to call FileUtils.mv
>
> -Rob
>
> Rob Biedenharn    http://agileconsultingllc.com
> Rob@AgileConsultingLLC.com

Thanks.  Can you explain another curious bit of behavior I've noticed?
I found that if I use the FileUtil.cp(src, dest) to copy a large file
from say the C: to the E: drive, it takes much longer than if I first
use FileUtil.cp(src, tmp) to copy the file to a temporary directory on
the C: drive and then use FileUtil.mv(tmp, dest) to move the file from
the temporary directory to the E: drive.  This has the same result as
just doing FileUtil.cp(src, dest), but takes much less time.  If I'm
doing this on a lot of files the time saving is significant, but I don't
understand why.

In other words, if I do

FileUtil.cp("C:/somefile", "C:/Temp/somefile")
FileUtil.mv("C:/Temp/somefile", "E:/")

it is faster than if I do

FileUtil.cp("C:/somefile", "E:/")

-Alex
Ef3aa7f7e577ea8cd620462724ddf73b?d=identicon&s=25 Rob Biedenharn (Guest)
on 2009-02-23 05:37
(Received via mailing list)
On Feb 22, 2009, at 9:59 PM, Alex DeCaria wrote:
>>> the
>>
> from say the C: to the E: drive, it takes much longer than if I first
> FileUtil.cp("C:/somefile", "C:/Temp/somefile")
> FileUtil.mv("C:/Temp/somefile", "E:/")
>
> it is faster than if I do
>
> FileUtil.cp("C:/somefile", "E:/")
>
> -Alex


I'm making an educated guess (but it is nonetheless speculation) that
the underlying operating-system-level code that implements mv is using
a better buffer or at least tighter code which the implementation of
cp uses some higher-level Ruby code (or a different/smaller buffer).
It's also possible that Windows does something quite different between
cp and mv.

If you're truly interested, dig into the code and find out the
differences yourself.  (I don't work on the Windows platform so I have
no direct access [or interest].)

-Rob

Rob Biedenharn    http://agileconsultingllc.com
Rob@AgileConsultingLLC.com
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