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RE: [Linux-cluster] High availability mail system



-----Original Message-----
From: linux-cluster-bounces redhat com [mailto:linux-cluster-bounces redhat com] On Behalf Of Rainer Duffner
Sent: den 23 september 2005 12:58
To: linux clustering
Subject: Re: [Linux-cluster] High availability mail system

>Robert Olsson wrote:

>>Ok, how about performance when using NFS? I´m thinking about the overhead when accesing NFS filesystems. 
>>


>The trouble is that GFS also has an overhead - especially for Qmail.
>In fact, we have what I would call a "long-time evaluation" of qmail + GFS running.
>While some features (no SPOF) are nice, others (way too many concurrent directory-accesses to gain any performance gain in comparison to NFS) are not nice at all.
>I'm really not a GFS-expert at all, but the way I see it (and was told) is that everytime a directory-access occurs, GFS >must synchronize this to the other cluster-members.
>Now, when Qmail delivers a mail, it already takes great care not to produce conflicts on (NFS-)shared filesystems, by copying the message first to "tmp", then to "new", with timestamp as part of filename etc.
>For every file created on the shared directory, though, GFS creates locks and lockfiles - which easily doubles or triples >the load on the SAN. Even though, no two files of the same name are ever created by different hosts.
>In addition, GFS doesn't seem to have "directory hashing" like FreeBSD UFS and others have, as a result access to large directories with many files is slow. Running find(1) on the mail-store can bring the cluster to halt, so does du(1).

>Due to the fact that we also will have to move from cdb-backend to mysql, there will be a SPOF anyway and we will be actively evaluating going to NFS (more or less back).
>But only on a "sane" NFS-platform, most likely Solaris, or FreeBSD - don't waste your time with Linux-NFS...


>>Do you know about any mailsystem that distribute mail over NFS and do you have any links to performance data?
>>  
>>

>The shared-storage mail-systems only scale to a certain point. The numbers are in the 300k-500k ballpark. After that, you have to go distributed.
>If you have millions of users, go qmail-ldap.

How do you mean with "go distributed"?

>These things are very difficult to benchmark, every site has an individual use-profile..




cheers,
Rainer

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