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List:       unison-users
Subject:    Re: [unison-users] Windows System vs. Filesystem
From:       "Adrian Klaver adrian.klaver () aklaver ! com [unison-users]" <unison-users-noreply () y
Date:       2014-10-07 17:46:27
Message-ID: 543426F3.7030803 () aklaver ! com
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On 10/07/2014 07:46 AM, Brian services@increa.com [unison-users] wrote:
> Adrian,
> 
> Oops.  Typo.  What you said is what I meant to say.  Below I typed "fastcheck \
> -false" when I should have typed "fastcheck -true" 
> I usually run with fastcheck = auto, and so Unison will full scan "when running on \
> a Windows system".  My observation was that "on a Windows system" is different than \
> "when synchronizing a Windows filesystem mount".  The manual is written from a \
> mindset of synching over a network and this doesn't exactly map to the situation of \
> synchronizing a local USB drive, where "Windows system" <> "Windows filesystem \
> mount".  Unison also seems to slow down when synchronizing a Windows filesystem \
> mount when running on a NON-windows system. 
> It seems the manual should state that full scan is done whenever Unison is run on a \
> Windows system OR is synchronizing a Windows filesystem mount.

What the manual says is:

"If your replicas are large and at least one of them is on a Windows 
system.."

It might be semantics, but I would consider FAT32 a Windows system. In 
the end what Unison is dealing with is the filesystem and that is what 
counts. You can see this noted here:

Changes since 2.32:

"Fastcheck is now the default under Windows. People mostly use NTFS 
nowadays and the Unicode API provides an equivalent to inode numbers for 
this filesystem."

USB drives are not speedy and one of the uses of fastcheck=true is to 
deal with that.

> 
> ~ Brian
> 
> On Oct 7, 2014, at 7:18 AM, Adrian Klaver wrote:
> 
> > > I use the fastcheck option usually in -auto mode, but on the Windows mount it \
> > > gets really slow if Unison decides to scan every byte of every file for many \
> > > GB, so sometimes I put fastcheck -false and only periodically put it back to \
> > > -auto (following the procedure the manual recommends).  However, here again is \
> > > another manual ambiguity I am struggling with.  When the manual says "Windows \
> > > system", I think it really means "any system with a Windows FAT32 mount", which \
> > > really are different ideas that the manual blurs together.  I say this because \
> > > for me Unison kicks into the "scan every byte" mode when working with a FAT32 \
> > > mount on a non-Windows system, which shouldn't happen if the Unison manual is \
> > > literally true (windows system, not windows mount).
> > 
> > I think you have this backwards. fastcheck=no/false will cause all file content \
> > to be scanned. If you want Unison to scan the Windows file system quickly you \
> > want fastcheck=true, which is what the manual suggests: 
> > http://www.cis.upenn.edu/~bcpierce/unison/download/releases/stable/unison-manual.html#fastcheck
> >  
> > "If your replicas are large and at least one of them is on a Windows system, you \
> > may find that Unison's default method for detecting changes (which involves \
> > scanning the full contents of every file on every sync—the only completely safe \
> > way to do it under Windows) is too slow. Unison provides a preference fastcheck \
> > that, when set to true, causes it to use file creation times as 'pseudo inode \
> > numbers' when scanning replicas for updates, instead of reading the full contents \
> > of every file." 
> > Unison is seeing the FAT32 mount as a Windows file system and is doing the right \
> > thing. fastcheck when set to default does a slow file content check on Windows \
> > systems and a fast check on Unix-like systems.
> 
> 
> 


-- 
Adrian Klaver
adrian.klaver@aklaver.com


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