We are migrating from our old mail server to a new one with a completely different internal structure and many new features. I am using imapsync to migrate the data over (slow, but it works well). Before we do the last sync, my boss asked me to figure out a way to run the servers side by side so we could have the early adopters try the new mail server. The kicker is that both servers are set up to serve the same domain and we need to be able to have the early adopters get to their mail from either server.

This solution works for us, but I am not sure if this is the best solution and would love to hear your ideas.

I was asked recently to find a low cost setup for a small software development company (about 30 people) for their infrastructure that had growth potential. I came up with the following:

- One server running Open-E DSS software SAN/NAS: This holds 12 1TB disks in a raid 10 mirror and shares out the disks via iscsi, smb, and nfs (I am not using the other protocols available such as AFP, FTP...). It is certified for VMware. They use it for home directories and as an iscsi SAN for ESXi.

- Two servers running ESXi software attached via iscsi to the SAN.

along with a UPS, rack, switch, and firewall.

ulimit can be used to limit memory utilization (among other things)

Here is an example of setting memory usage so low that /bin/ls (which is larger than /bin/cat) no longer works, but /bin/cat still works.

$ ls -lh /bin/ls /bin/cat
-rwxr-xr-x 1 root root 25K May 24 2008 /bin/cat
-rwxr-xr-x 1 root root 88K May 24 2008 /bin/ls
$ date > test.txt
$ ulimit -d 10000 -m 10000 -v 10000
$ /bin/ls date.txt
/bin/ls: error while loading shared libraries: libc.so.6: failed to map segment from shared object: Cannot allocate memory
$ /bin/cat date.txt
Thu Mar 26 11:51:16 PDT 2009

Here are my notes how I set up SVN over SSH to an SVN server that had been set up to run SVN as a single user, "svn".

So, we have multiple users on the remote side, all logging in as "svn" on the SVN server side.
Authentication is done via a dedicated key-pair, with special options to make SVN+SSH work.

See http://svn.collab.net/repos/svn/trunk/notes/ssh-tricks for more details.

These are just my notes, the above link is authoritative.


Procedure:

1. Generate a dedicated key pair

	[SVN Client]$ ssh-keygen -f ~/.ssh/svnssh
	Generating public/private rsa key pair.
	Created directory '/home/tsalolia/.ssh'.


  

It is trivial to set things up so you are able to select memtest86 as a boot option in GRUB:

Download the latest memtest source from http://www.memtest86.com.

Extract and follow the instructions in the README that comes with it to compile. Copy the resulting memtest.bin to /boot and edit /etc/grub.conf to have the following 3 lines:

title Memtest86
        root (hd0,0)
        kernel /boot/memtest.bin

Make sure you reboot to test.

In theory this should work with any x86 OS + bootloader. I am doing this with RHEL 4 + GRUB.

I've recently had a number of projects at work that want to mix multiple volume managers on a single server, specifically ZFS and VxVM for SAN volumes (actually, three including SVM for internal boot disk mirroring). The projects generally are for database servers, and want to use VxVM for database volumes because ZFS currently has some serious limitation on database size (limited number of devices recommended in a single zpool) and performance (single threaded checksumming, for one). However, at the same time, they want to have access to some of ZFS's features (in particular, the ability to oversubscribe filesystems, dynamic resize, snapshots and rollback) for some of the other filesystems.

Read an interesting discussion on the lkml list where Linus and friends talked about atime's performance impacts (http://kerneltrap.org/node/14148). Ingo phrased the problem best with:

' For every file that is read from the disk, lets do a ... write to
the disk! And, for every file that is already cached and which we
read from the cache ... do a write to the disk! '

Solutions are to mount your file systems with the noatime, nodiratime options. The only time this may cause a problem is if you have a local mail spool (the mailer will not know that new email arrived) or possibly with some backup software.