RE: async vs. sync

["Roger Heflin" <rheflin@atipa.com>]

This might be the issue, (and someone correct this if I am incorrect), 
I know I ran into it a couple of years ago, and it is not the easiest
to understand exactly what is actually going on.

There are 2 places where you can put sync and async, one is the exports
and one is on the mount command.  They are different.

You want sync on the exports, this will allow a client to survive without
data loss if the server reboots.   You want async on the client mount end
and
this will generally give you the speed.   With async on the client end
the client is keeping track of what is outstanding if the server crashes,
so you won't lose data.  With async on both ends the server tells one that
the data is safe when it is not, and if the server crashes the client things
that the data was safe when it really was not.  

If you put sync in both locations then your NFS disk is fully synced and 
the application won't even start another write until the last one is
confirmed
finished and on the actual disk.   With async on the client end the next
write 
will start before the client has received an ack from the server, and this
will
be reasonably fast.

So basically:

exports          mount
sync             sync      -> really safe and really slow
sync             async     -> Safe and fast
async            either    -> unsafe and fast.

Running async exports and async mount did not appear (under my testing) to
be faster under a sustained load than did sync exports and async mount.
When
the initial test was started async/async was faster but that quick changed
once
the buffer cache filled up.

                       Roger


> -----Original Message-----
> From: nfs-admin@lists.sourceforge.net 
> [mailto:nfs-admin@lists.sourceforge.net] On Behalf Of jehan.procaccia
> Sent: Monday, November 22, 2004 11:55 AM
> To: Olaf Kirch
> Cc: nfs@lists.sourceforge.net; trond.myklebust@fys.uio.no; 
> mci-unix@int-evry.fr
> Subject: Re: [NFS] async vs. sync
> 
> Olaf Kirch wrote:
> 
> >On Tue, Nov 16, 2004 at 10:48:12AM -0800, Lever, Charles wrote:
> >  
> >
> >>i'm just looking for clarification so i can provide a good 
> explanation 
> >>in the Linux NFS FAQ about the evils of using "async."  i'll cruise 
> >>through the server code.
> >>    
> >>
> >
> >Just about the only reason for async I can think of is if 
> you have an 
> >incoming data stream you need to write at a constant rate 
> (think of a 
> >diskless set top box writing an mpeg2 stream)
> >
> >Olaf
> >  
> >
> OK, but using sync at my site is really really slow ... 
> compare to async ! here's a detailed (options printed) 
> demonstration for an untar operation that takes 13 minutes in 
> async mode and only 14 secondes in sync mode !!
> 
> 1) Export in sync mode
> NFS server  (RedHat ES3 kernel 2.4.21-4.ELsmp) options for 
> that export:
> $ cat /proc/fs/nfs/exports | grep arvouin
> /p2v5f1 arvouin.int-evry.fr(rw,no_root_squash,sync,wdelay,acl) #
> 157.159.21.55
> $ cat /var/lib/nfs/xtab  | grep arvouin
> /p2v5f1
> arvouin.int-evry.fr(rw,sync,wdelay,hide,nocrossmnt,secure,no_r
oot_squash,no_all_squash,subtree_check,secure_locks,acl,mapping=identity,ano
nuid=-2,anongid=-2)
> 
> Client running Fedora Core 2, kernel 2.6.8-1.521 
> [root@arvouin /mnt/cobra3/mci/test/Test-sync] $cat /proc/mounts
> cobra3:/p2v5f1 /mnt/cobra3 nfs
> rw,v3,rsize=8192,wsize=8192,hard,tcp,lock,addr=cobra3 0 0 
> $time tar xvfz /usr/src/redhat/SOURCES/httpd-2.0.51.tar.gz
> real    13m3.686s
> user    0m1.055s
> sys     0m4.354s
> 
> 2) Export in async mode:
> Same NFS server, options for that export:
> $ cat /proc/fs/nfs/exports | grep arvouin
> /p2v5f1 arvouin.int-evry.fr(rw,no_root_squash,async,wdelay,acl) #
> 157.159.21.55
> $ cat /var/lib/nfs/xtab  | grep arvouin
> /p2v5f2
> arvouin.int-evry.fr(rw,async,wdelay,hide,nocrossmnt,secure,no_
root_squash,no_all_squash,subtree_check,secure_locks,acl,mapping=identity,an
onuid=-2,anongid=-2)
> 
> Same client running Fedora Core 2, kernel 2.6.8-1.521
> cobra3:/p2v5f1 /mnt/cobra3 nfs
> rw,v3,rsize=8192,wsize=8192,hard,tcp,lock,addr=cobra3 0 0 
> [root@arvouin /mnt/cobra3/mci/test/Test-sync] $time tar xvfz 
> /usr/src/redhat/SOURCES/httpd-2.0.51.tar.gz
> real    0m14.802s
> user    0m0.867s
> sys     0m2.886s
> 
> My users won't accept the sync performances ! . I have no 
> choice, but is running in async mode is really evil as you 
> mentioned it ? is there a way to have better performances in 
> sync in my case ?  As anyone had the same gap in performance 
> as me ( here 55 times longer in sync mode !)  ?
> 
> Thanks.
> 
> 
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