Racker Hacker

On a Plesk server, the maximum size for an individual e-mail sent through qmail is unlimited. You can limit this size by adding a number to the /var/qmail/control/databytes file.

If you wanted to limit this to something like 10MB, you can just run the following command:

echo “10485760″ > /var/qmail/control/databytes

This will limit the size of messages (including attachments) to 10MB as a maximum.

It’s tough to find examples of dumps that can’t be properly reimported on other servers. However, if you have a 64-bit server, and you make a MySQL dump file from it, you may see this issue when importing the dump on a 32-bit MySQL server:

ERROR 1118 (42000) at line 1686: Row size too large. The maximum row size for the used table type, not counting BLOBs, is 65535. You have to change some columns to TEXT or BLOBs

You really don’t have any options in this situation. You’ll need to adjust your table on the 64-bit server for good and then make a new dump file, or you will just have to live with the fact that it can’t be imported into a 32-bit instance of MySQL.

I really dislike qmail. But, since I use Plesk, I’m stuck with it. However, I found a way to improve it’s awful mail queue performance by putting the mail queue onto a ramdisk. This is actually pretty darned easy to do.

First, toss a line like this into your /etc/fstab:

none    /mailqueue      tmpfs   defaults,size=100m,nr_inodes=999k,mode=775      0       0

This will make a 100MB ramdisk on /mailqueue. Now, just symlink /var/qmail/mqueue to /mailqueue and move your e-mail over:

# mount /mailqueue
# chmod 750 /mailqueue
# chown qmailq:qmail /mailqueue
# mv /var/qmail/mqueue /var/qmail/mqueue-old
# ln -s /mailqueue /var/qmail/mqueue
# rsync -av /var/qmail/mqueue-old /mailqueue

This has significantly cut the iowait on my server during heavy e-mail periods. In addition, tools like qmHandle now fly through my mail queue and give me reports very quickly.

One of my biggest Plesk gripes is dealing with the Plesk Professional Website Editor. One of the most common occurrences with PPWSE is that it hangs when you attempt to log into the server. Normally, this happens when a server is behind a firewall, and it is using private IP’s.

Plesk will actually query the DNS for the domain (rather than simply connecting to the localhost), try to reach the public IP, and the traffic will be blocked by the firewall. This creates a login session that appears to hang, and then it shows “FTP: not connected” in the interface.

The fix is actually quite easy:

1) Be sure that the ftp.domain.com CNAME/A record exists
2) Add a line to /etc/hosts that forces ftp.domain.com to resolve to the proper private IP address.

The third item should be to stop using PPWSE, but that’s the hardest one to work out. I’d recommend using something like TextMate on a Mac along with Transmit, but you can get some good results out of Dreamweaver as well. Whatever you do, don’t use Contribute.

Just in case some of you out there enjoy nomenclature and theory behind Linux filesystems, here’s some things to keep in mind. The modification time (mtime) of a file describes when the actual data blocks that hold the file changed. The changed time (ctime) of a file describes when the metadata was last changed.

Also, metadata is stored within a different location than the data blocks. The metadata fits in the inode while the file’s data goes within data blocks. The inode information contains the owner, owner’s group, time related data (atime, ctime, mtime), and the mode (permissions).

The name of the file itself is actually stored within the file that makes up the directory. And, the directory is simply a file that masquerades as a directory once the filesystem is mounted and read.

A question I’m asked daily is “How can I find out what is generating iowait on my server?” Sure, you can dig through pages of lsof output, restart services, or run strace, but it can be a frustrating process. I saw a process on this blog post, and I changed the regexes to fit Red Hat and CentOS systems a bit better:

# /etc/init.d/syslog stop
# echo 1 > /proc/sys/vm/block_dump
# dmesg | egrep "READ|WRITE|dirtied" | egrep -o '([a-zA-Z]*)’ | sort | uniq -c | sort -rn | head
1526 mysqld
819 httpd
429 kjournald
35 qmail
27 in
7 imapd
6 irqbalance
5 pop
4 pdflush
3 spamc

In my specific situation, it looks like MySQL is the biggest abuser of my disk, followed by Apache and the filesystem journaling. As expected, qmail is a large contender, too.

Don’t forget to set things back to their normal state when you’re done!

# echo 0 > /proc/sys/vm/block_dump
# /etc/init.d/syslog start

I saw a ticket the other day where a customer received this error from Horde when trying to expand items on the left pane of the interface:

Fatal error: Cannot use string offset as an array in /www/horde/lib/Horde/Block/Layout/Manager.php on line 389

It turns out that Plesk 8.1.1 bundles Horde 3.1.3 which has an occasional bug within the interface. Upgrading to Plesk 8.2.0 corrects the issue as Horde 3.1.4 is installed with the upgrade.

See Horde’s bug page for more information.

I’ve been busy with work and I haven’t had time to add some posts. Rest assured, I am still here and working on new content for you.