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Date: Mon, 31 Jan 2022 21:36:15 +0000
From: Adam Dinwoodie <adam@dinwoodie.org>
To: cygwin@cygwin.com
Subject: Re: Renaming (with 'mv') very large files is SLOW
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On Mon, Jan 31, 2022 at 10:17:59AM -0500, cygwin@kosowsky.org wrote:
> Eliot Moss wrote at about 09:59:17 -0500 on Monday, January 31, 2022:
>  > On 1/31/2022 9:52 AM, cygwin@kosowsky.org wrote:
>  >  > I tried renaming some very large files (20-40 GB) using:
>  >  >     mv <oldname> <newname>
>  >  > without changing the directory of course.
>  >  >
>  >  > The process took about 10-20 minutes with Task Manager showing disk
>  >  > activity of 100+ MB/s.
>  >  >
>  >  > Is there something about such large 'renaming' that actually results
>  >  > in the file being really moved (aka copied) rather than just renamed?
>  > 
>  > The two places are probably on different volumes (loosely, different disks).
>  > That requires a physical move, even under Linux.
> 
> No my point is I am just *renaming*, not physically moving the file!!
> i.e., I am not changing the directory location of the file, let alone
> the volume/disk location.
> (I am well aware that 'mv' does a copy when changing volumes/disks).
> 
> I literally am typing something like:
>   mv foo bar
> 
> In Linux, that just edits the file system table & inode...
> 
> UPDATE...
> I just tried a second 'mv' and it was near instantaneous.
> (and similarly with subsequent renaming of the same file)
> So perhaps not a 'Cygwin' thing but something going on within Windows.
> 
> Could it be that the first 'mv' triggered an anti-virus read of the file since
> perhaps it detects it as a new/changed file?
> 
> But if so, would 'mv' (under Task Manager) be showing the 100+ MB/s disk activity?

That definitely seems plausible; there's a reason a significant number
of the applications that are known to interfere with Cygwin operation
(see [0]) are antivirus applications.  But what would trigger your
antivirus to want to scan a file, and how much work is required to do
that, is something you'll need to take up with your antivirus vendor,
I'm afraid.

[0]: https://cygwin.com/faq/faq.html#faq.using.bloda

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