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Len,
It's been a while since we've spoke. Good to
hear from you again & see that you're still on the Imail listserv
here. See my response below to your comments, in red:
----- Original Message -----
Sent: Thursday, June 26, 2003 10:18
AM
Subject: Re: [IMail Forum] Imail High
Availability Setup
>I'm working on a project with a goal of making Imail a high
availability
>service. I'm aware that Imail does come with support for
"peering"
>(setting up multiple Imail servers to listen to each others
messages & use
>store & forward when a Imail server goes down),
but I'd like to take it a
>few steps further.
Imail peering is
not HA.
No, it's certainly not, but
it's currently Ipswitch's "best attempt" at HA. But, it's a long shot at
best.
>I'm strongly considering setting up the
following:
>
>1) Two Imail 8.0 servers will be setup, but
completely unaware of each
>other (in other words, Imail's peering
feature will not be used).
>
>2) Two load balancing devices, such
as Cisco's LocalDirector will be
>placed in front of the two Imail
servers.
>
>3) Behind the two Imail servers, an iSCSI disk array
will be setup. Both
>Imail servers will have direct access to a common
storage pool
what's going to manage file collisions?
Good question. That's the
kind of input I was looking for, as I knew there was something that I was
overlooking.
>on the iSCSI disk array and will essentially be able to see the
same set
>of files (i.e. shared storage or similar to mapping several
computers to a
>single drive on a server).
>
>4)
Appropriate DNS records will be setup so that both the MX records point
>to the load balancers.
you have more that one load
balancer?
Yes. I have two Cisco
LocalDirectors. I currently use them for HA web site
serving.
>But, here's the question: I have little doubt
that both servers can
>effectively load balance the SMTP
traffic.
The load balancer balances the SMTP traffic. Imail servers
don't load
balance. Each Imail server has to be powerful enough to handle
the entire
traffic.
Yes, you're correct. I didn't
phrase my original statement well. To be more specific, the customers
would point their email clients outgoing mail server directly to the load
balancers, via DNS, not directly at the Imail servers. Each Imail server
will be a pretty beefy machine, which will be able to take over in the event
that one machine goes down for whatever reason. In reality, I've yet to
see a dedicated Imail box go down completely. Typically, it tends to
only be one or two Imail related services. So, in all likelihood, I
don't anticipate one machine needing to completely take over for the other,
but on the otherhand if I don't plan for that possibility, HA goes out the
window.
> However, when it comes to the Pop3 or Imap4 service, Imail
tends to put
> a lock on a user's mailbox file while a message is being
delivered.
IMail uses OS-level file locking for all mailbox access,
read or
write. This is why you will see Imail logging failures in
mailbox access
when SMTP attempts to write to the mailbox while the mbx
file is locked for
POP/IMAP/webmail read, or vice versa.
From what I've observed before with
Imail's locking mechanism (via the OS), is that it tends to create a LCK file,
or something similar to that, for exclusive write access. My theory is
based on the idea that in the event that Imail server 1 creates a lock file on
a mailbox, and Imail server 2 tries to write to that mailbox that it'll see
the lock file & simply delay attempting to the file. Or would
Imail server 2 completely fail & never write the new message to the
file. That's the question. I just don't have enough info available
about how the Windows OS handles locking to be certain.
> What happens when one of the Imail servers has locked a
user mailbox for
> writing & the other server tries to
simultaneously write a message to the
> same mailbox?
This is
called a file collision.
You're going to have two parallel OS's
accessing the same files at the
physical file level. What is going
to manage collisions? Imail can't, Win
OS of the Imail server can't,
so what will?
If you want file locking by the shared storage, you'd
have to have Imail
access the files through an MS SMB "file server" that
provides file
locking, not a "physical disk server".
What about having both Imail
servers access a common storage pool via a NAS (network attached storage)
appliance? For example, some of the NAS devices I've learned about from
Dell have a Windows OS on board that supposedly controls file locking.
Using gigabit Ethernet between the NAS & the Imail servers, I think that
would potentially offer an adequate level of speed & prevent file
collisions.
>I'm taking an educated guess that the second
Imail server will see a lock
Your education is not complete. :)) How
can one OS see the file open status
by the other OS?
>
& will just hold the message until the lock is removed by the other
> Imail server, then try to deliver it's message.
IMail/OS 1 has
no idea what Imail/OS 2 is doing, unless they access the
shared disk via
some file-smart manager.
> But, I'm just not certain. I
spoke with Ipswitch tech support earlier
> today & learned that
they "don't officially support this type of setup,"
And still you
persist? :))
Of course! Back when
Imail 6 was the current release, I wasn't satisfied with the lack of
control of the appearance in the web email templates, so I reverse engineered
Ipswitch's web email tags & figured out the actual tags so they could be
broken out for customization. For example, the following KB article was
copy & pasted from one of my earlier messages about that
topic:
Then Ipswitch ended my fun in the next release
& finally broke out the tags for customization. :)
> however the tech admitted to me that she has had
customers call in & say
> that they've been able to successfully
set up this type of configuration.
The only way that shared external
file storage can work for two Imail
servers is that there is some
mechanism to manage file access
collisions. Either use a MS
Networking file server (this is horribly
slower than local disk access, so
it doesn't scale well but maybe your
scale will fit into your hardware
budget), or a "disk server" that has
file-collision management.
In
order for load balancing to work, you have to have mirroring of the
Imail
config on both machines. How do you plan to do that?
There are
ways to do it. Check the list archives.
I'm thinking that a Windows powered
NAS from Dell, using gigabit ethernet connectivity might be the answer
for file collision management. As for the configuration mirroring, I'm
planning on writing some code to use the command line utilities & Imail
APIs to keep both Imail configurations in sync with each other.
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