Failover using wackamole?

Failover using wackamole?

[Vsevolod (Simon) Ilyushchenko]

Hi,

I had this neat idea that NFS failover of read-only filesystems can be 
achieved using wackamole (floating IP adresses providing very simple 
HA). This did not work with NFS 3 because the filehandles are different 
on the primary and the secondary servers, so I was getting a "stale NFS 
filehandle" error after the failover.

I tried this using NFS 4 nfs-utils-1.0.6-52 from FC3 updates. However, 
it still failed with the following error:
	ls: reading directory a: Input/output error
and this in the error log on the client:
	kernel: RPC: garbage, exit EIO

Is this a bug or a feature?

Thanks,
Simon
-- 

Simon (Vsevolod ILyushchenko)   simonf@cshl.edu
				http://www.simonf.com

Terrorism is a tactic and so to declare war on terrorism
is equivalent to Roosevelt's declaring war on blitzkrieg.

Zbigniew Brzezinski, U.S. national security advisor, 1977-81


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Re: Failover using wackamole?

[J. Bruce Fields]

On Mon, Feb 28, 2005 at 11:18:24AM -0500, Vsevolod (Simon) Ilyushchenko wrote:
> I had this neat idea that NFS failover of read-only filesystems can be 
> achieved using wackamole (floating IP adresses providing very simple 
> HA). This did not work with NFS 3 because the filehandles are different 
> on the primary and the secondary servers, so I was getting a "stale NFS 
> filehandle" error after the failover.
> 
> I tried this using NFS 4 nfs-utils-1.0.6-52 from FC3 updates. However, 
> it still failed with the following error:
> 	ls: reading directory a: Input/output error
> and this in the error log on the client:
> 	kernel: RPC: garbage, exit EIO
> 
> Is this a bug or a feature?

Hm, there may be a bug there.  But in any case v4 isn't going to help
you here.  It's constructing filehandles in exactly the same was as v3.

--b.


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Re: Failover using wackamole?

[Vsevolod (Simon) Ilyushchenko]

>>I tried this using NFS 4 nfs-utils-1.0.6-52 from FC3 updates. However, 
>>it still failed with the following error:
>>	ls: reading directory a: Input/output error
>>and this in the error log on the client:
>>	kernel: RPC: garbage, exit EIO
>>
>>Is this a bug or a feature?
> 
> 
> Hm, there may be a bug there.  But in any case v4 isn't going to help
> you here.  It's constructing filehandles in exactly the same was as v3.

Oh. :( Bummer. Thanks, though.

Is there then an official NFS 4 way of doing a transparent failover with 
non-shared storage?

Simon
-- 

Simon (Vsevolod ILyushchenko)   simonf@cshl.edu
				http://www.simonf.com

Terrorism is a tactic and so to declare war on terrorism
is equivalent to Roosevelt's declaring war on blitzkrieg.

Zbigniew Brzezinski, U.S. national security advisor, 1977-81


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Re: Failover using wackamole?

[J. Bruce Fields]

On Mon, Feb 28, 2005 at 03:24:07PM -0500, Vsevolod (Simon) Ilyushchenko wrote:
> >>I tried this using NFS 4 nfs-utils-1.0.6-52 from FC3 updates. However, 
> >>it still failed with the following error:
> >>	ls: reading directory a: Input/output error
> >>and this in the error log on the client:
> >>	kernel: RPC: garbage, exit EIO
> >>
> >>Is this a bug or a feature?
> >
> >
> >Hm, there may be a bug there.  But in any case v4 isn't going to help
> >you here.  It's constructing filehandles in exactly the same was as v3.
> 
> Oh. :( Bummer. Thanks, though.
> 
> Is there then an official NFS 4 way of doing a transparent failover with 
> non-shared storage?

NFSv4 does allow you to designate filehandles as "volatile on
migration", in which case the client can attempt to look up the
filehandles after migration.  This would only work if you actually told
the client about the migration (probably using NFSERR_MOVED and
FS_LOCATIONS, both added in v4).  If the failover was really
"transparent" to the client, then the client wouldn't be able to tell
there was a migration in the first place, so this wouldn't help.  Also,
the Linux client implementation of migration isn't finished yet anyway.

Also of course there are inherent problems with looking up the
filehandles again after migration--if the file that a filehandle refers
to has moved since you originally looked up the file, then looking it up
again under the path you originally got it from isn't going to help.

--b.


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