Date: Thu, 10 Nov 2011 13:16:21 +0100 From: Ivan Voras <ivoras@freebsd.org> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: freebsd-fs@freebsd.org Subject: Re: [RFC] Should vfs.nfsrv.async be implemented for new NFS server? Message-ID: <CAF-QHFXhBuT36KZiVXPUhmF0QT4cYfaMaAo=oi7V6e9bMYVLVQ@mail.gmail.com> In-Reply-To: <364937844.1458624.1320854968216.JavaMail.root@erie.cs.uoguelph.ca> References: <j9e26l$ht8$1@dough.gmane.org> <364937844.1458624.1320854968216.JavaMail.root@erie.cs.uoguelph.ca>
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On 9 November 2011 17:09, Rick Macklem <rmacklem@uoguelph.ca> wrote: > Ivan Voras wrote: > [good stuff snipped for brevity] >> >> This seems too extreme... doesn't NFSv4 have its own fsync()-like RPC >> that does that manually? If it does, then I don't think there are any >> differences between doing a write() on a local file system with e.g. >> soft-updates enabled and doing a write on a NFS file system - in both >> cases, no data is even remotely guaranteed to survive a crash unless a >> fsync (or equivalent operation) was issued. >> > I think you have to be a bit careful when an assumption about loss of > data after a crash is applied to a file server. If a client crashes, > everyone pretty much assumes that recently written data might have been > lost (in a Unix-like world, at least) and the crash is visible to users. > > However, take the following scenario: > - User on desktop client edits a document for an hour, then saves it. > =C2=A0--> NFS server crashes/reboots just after the "save". > - User on desktop client goes off reading email for an hour, using a > =C2=A0mail system not affected by the NFS server crash/reboot. > - User goes back to document and finds an old damaged document file. > =C2=A0--> User is very upset... > > My point is that a user on a client may not even realize the server > crashed/rebooted. The above case can happen if async!=3D0, but not if > async=3D=3D0. Thank you for the excellent explanation! I see the problem but I think it is still well within the judgement call of the system administrator. It should probably not be the default (though I recall from some benchmarks way back that it might be the default on Linux) but it should definitely exist as an option. Thanks again!
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