Date: Sun, 6 Nov 2011 11:01:55 -0500 (EST) From: Rick Macklem <rmacklem@uoguelph.ca> To: Xin LI <delphij@gmail.com> Cc: freebsd-fs@freebsd.org, Josh Paetzel <jpaetzel@freebsd.org>, zkirsch@freebsd.org Subject: Re: [RFC] Should vfs.nfsrv.async be implemented for new NFS server? Message-ID: <858445532.1240736.1320595315792.JavaMail.root@erie.cs.uoguelph.ca> In-Reply-To: <CAGMYy3tT3vmqGq6ju7GVpA1WKZ=VdFSbvZH1FAezaxtNUMDnvQ@mail.gmail.com>
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Xin Li wrote: > Hi, Rick, > > On Sat, Nov 5, 2011 at 6:18 PM, Rick Macklem <rmacklem@uoguelph.ca> > wrote: > > Hi, > > > > Josh Paetzel pointed out that vfs.nfsrv.async doesn't exist > > for the new NFS server. > > > > I don't think I had spotted this before, but when I looked I > > saw that, when vfs.nfsrv.async is set non-zero in the old server, > > it returns FILESYNC (which means the write has been committed to > > non-volatile storage) even when it hasn't actually done that. > > > > This can improve performance, but has some negative implications: > > - If the server crashes before the write is committed to > > non-volatile storage, the file modification will be lost. > > (When a server replies UNSTABLE to a write, the client holds > > onto the data in its cache and does the write again if the > > server crashes/reboots before the client does a Commit RPC > > for the file. However, a reply of FILESYNC tells the client > > it can forget about the write, because it is done.) > > - Because of the above, replying FILESYNC when the data is not > > yet committed to non-volatile (also referred to as stable) > > storage, this is a violation of RFC1813. > > > > I wouldn't want this to be the default, but am willing to > > patch head based on the "backwards compatibility" argument. > > My concern with these types of patches is that some people > > will enable them without realizing the risk of data loss > > that they introduce. > > > > So, how do others feel with respect to whether or not this > > patch should be committed to head? > > I think the default of old NFS server was async=0? In general I'd > prefer seeing this as an option but disabled by default, so > administrators can override the option. Having async=1 by default > doesn't seem to be a good idea in my opinion. > Yes, I agree. If I commit it, it will definitely not be enabled by default. The question I was asking was "Should it be committed at all?". > Another thought is the async flag should be a per-mountpoint flag > rather than a global flag, but that might be over-complicating things > so just my $0.02. > Well, since it doesn't affect server file system behaviour and only the reply the NFS server sends to the client, I don't see it as a per-mountpoint flag on server file systems, but I'm really looking for what others think? > Cheers, > -- > Xin LI <delphij@delphij.net> https://www.delphij.net/ > FreeBSD - The Power to Serve! Live free or die
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