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Re: Finding the Bottleneck



On Friday 08 June 2001 12:25, Jason Lim wrote:
> The network is connected via 100Mb to a switch, so server to server
> connections would be at that limit. Even 10Mb wouldn't be a problem as
> I don't think that much data would be crossing the cable.. would it?

10Mb shouldn't be a problem for DNS.  Of course there's the issue of what 
else is on the same cable.

There will of course be a few extra milli-seconds latency, but you are 
correct that it shouldn't make a difference.

> As for the "single machine" issue, that would depend. If you're talking
> about either getting a couple of SCSI disks, putting them on a hardware
> raid, or getting an additional small server just for the queue, then I
> think the cost would end up approximately the same. This client doesn't
> have the cash for a huge upgrade, but something moderate would be okay.

However getting an extra server will not make things faster, in fact it 
will probably make things slower (maybe a lot slower).  Faster hard 
drives is what you need!

> BTW, just to clarify for people who are not familar with qmail, qmail
> stores outgoing email in a special queue, not in Maildir. Only incoming
> mail is stored in Maildir. The Maildirs are actually stored on Disk 1
> (along with the operating system and everything else except the queue).
> I know Maildir can be put in a NFS disk... BUT i've never heard of
> anyone putting the mail queue on NFS, so I'm not sure if the file
> locking issues you mention would pertain to that as well.

For the queue, Qmail creates file names that match Inode numbers (NFS 
doesn't have Inodes).  Qmail also relies on certain link operations being 
atomic and reliable, while on NFS they aren't guaranteed to be atomic, 
and packet loss can cause big reliability problems.

Consider "ln file file2", when an NFS packet is sent to the server the 
server will create the link and return success, if the return packet is 
lost due to packet corruption then the client will re-send the request.  
The server will notice that file2 exists and return an error message.  
The result is that the operation succeeded but the client thinks it 
failed!

There are many other issues with NFS for this type of thing.  NFS is only 
good for data that has simple access patterns (read-only files and simple 
operations like mounting a home directory and editing a file with "vi"), 
and for applications which have carefully been written to work with NFS 
(Maildir programs).

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