Sometimes You Need to Compile It Yourself

I posted an earlier version of this on the Puppet mailing list recently; it seemed worth expanding here.

An Ideal World

In an ideal world, for each piece of Linux software I use, a very recent version would be “in the box” in the distribution package repositories, for every distro and distro release I use. The package versions would be updated promptly, and backported to the most recent N distro releases. This would be done in such a way as to avoid unexpected breakage, offering a combination of original (as of the distro release), bug-fix-only, and all-updates.

We don’t live in such a world.

The Real World

I’ve found that, with at least Debian, Ubuntu, Red Hat, and Mandrake, typically only the most popular packages are prone to prompt updates for new upstream versions. Backports to not-the-current-distro are even more rare, for various good reasons.

Therefore, when adopting something less widely used, especially if I need the same (current) package version for various distro versions, I’m resigned to having to either package it myself, or find someone “out there” offering updated packages.

Example #1: Puppet

As I write this, the current Puppet version is 0.24.8. It contains a lot of bug fixes and enhancements relative to even the earlier 0.24.x versions. It would probably be only a slight stretch to say that in the Puppet community,  versions before 0.24.x really aren’t recommended for current use at all. Yet the most recent versions offered in-the-box for the last few releases for Debian / Ubuntu are all old enough to be in that category.

Example #2: udpcast

Udpcast is an extremely useful tool, both for cloning systems en masse, and for totally unrelated uses like the one I describe here; yet the version in the very latest Ubuntu is from 2004, and the same is true for Debian.

Example #3: Zabbix

To get good results with Zabbix, it’s necessary to have approximately the same, approximately current versions, on all machines. The versions in various current and past Ubuntu and Debian releases / backports are not even close.

But Please, Use Your Package System

Please, I ask you… and if you work for me on one of my projects, I require of you… do not take any of this to mean you should ever type “sudo make install”.  It’s a nightmare to untangle a system with a mix of packaged and ad hoc compiled code in /usr/bin and friends. Always install using the package system: for a one-off (one machine, ever) install, simply use checkinstall, it takes only an extra minute or two and make it trivial to back the install out later. For a set of production systems. dpkg and RPM are your friends. They won’t bite. Get to know them.

DRBD on Ubuntu 8.04

A while back I wrote about setting up DRBD on Ubuntu.

It is considerably simpler now: you don’t need to build the module anymore, a reasonably recent version is already in the box. You should still read the full directions at the DRBD web site, but to get it going, you only need to configure it and start it; don’t download anything, don’t compile any modules kernels, etc.

Rather, the module is already there; you could load it manually like this:

modprobe drbd

But you really want the utils package:

apt-get install drbd8-utils

… which sets things up the rest of the way. Simply proceed with the configuration file, /etc/drbd.conf. If you want a nice clean config file, remove all the existing resource sections in the drbd.conf installed by the Ub package, and put in something like so:

The hostnames here need to match the full, actual hostnames. I show my example disk configuration, you’ll need to do somethign locally appropriate, based on understanding the DRBD docs.

Also adjust the syncer section:

syncer {
rate 40M;  # the rate is in BYTES per second
}

If there was already a filesystem on the metadata partitions you’re trying to put under DRBD, you may need to clear it out:

dd if=/dev/zero of=/dev/sdb1 bs=1M count=1

Now you’re ready to fire it up; on both machines:

drbdadm create-md mwm
drbdadm attach mwm
drbdadm connect mwm

# see status:
cat /proc/drbd

You now are ready to make one primary; do this on only one machine of course:

drbdadm -- --overwrite-data-of-peer primary all

or

drbdadm -- --overwrite-data-of-peer primary kylesdrbd

In my case, I now see a resync starting in the /proc/drbd output.

You don’t need to wait for that to finish; go ahead and create a filesystem on /dev/drbd0.

It’s best to have a dedicated Gigabit-ethernet connection between the nodes; or for busy systems, a pair of bonded GigE. For testing though, running over your existing network is fine.

I found that an adjustment in the inittimeout setting helps to avoid long boot delays, if one of the two systems is down.

Of course I only covered DRBD here; typically in production you’d use it in conjuction with a failover/heartbeat mechanism, so that whatever resource you serve on the DRBD volume (database, NFS, etc.) cuts over to the other machine without intervention; there is plenty to read online about Linux high availability.

Move files into an existing directory structure, on Linux

I recently needed to move a large number of files (millions) in a deep directory structure, into another similar directory structure, “merging” the contents of each directory and creating any missing directories. This task is easily (though slowly) performed on Windows with Control-C Control-V in Explorer, but I could find no obvious way to do it on Linux.

There is quite a bit of discussion about this on the web, including:

  • Suggestions to do with with tar; this is a poor idea because it copies all the file data, taking an enormously long time.
  • Suggestions to do it with “mv -r”… but as far as I can tell, mv does not have a -r option.

After a little thought I came up with the script below. I’d love to have a Linux/Bash guru out there point out how awful it and and send me something better!

A critical feature for me is that it does not overwrite files; if a source file name/path overlaps a destinate file, the source file is left alone, untouched. This can be changed easily to overwrite instead: remove the [[-f]] test.

Large, economical RAID: 10 1 TB drives

I recently needed a file server with ample capacity, on which to store data backups. The typical data access pattern for backups is that data is written once and read rarely, so bulk backup storage has much lower performance requirements than, for example, disks used for database files.

I need to store a great number of files, and I had an old server to recycle, so I ended up with:

  • 4U ASUS case with room for many internal drives
  • Qty1, leftover 320GB drive (to boot from)
  • Qty 10, 1 TB drives for data storage: WD Caviar Green
  • An extra SATA controller (plus the 8 SATA ports on the motherboard)
  • Ubuntu Linux 8.04.1 LTS
  • Software RAID6

The separate boot drive is for simplicity; it contains a trivial, vanilla Ubuntu install; if availability mattered more I could replace it with a RAID1 pair, or flash storage – even a cheap USB “key drive” would be sufficient, if I went to the trouble of setting up /var and /tmp to not write to it (thus avoid premature wearout).

The terabyte drives have one large RAID container partition each (quick work with sfdisk). The 10 of them in a RAID6 yield 8 drives worth of capacity. Adjusting also for the difference between marketing TB/GB and the real thing, plus a bit of filesystem overhead, I ended up with 7.3 TB of available storage. Here it is, with some data already loaded:

I went with software RAID for simplicity, low cost, and easy management:

I chose RAID6 over RAID5 because:

  • This array is so large, and a rebuild takes so long, that the risk of a second drive failing before a first failure is replaced and rebuilt seems high.
  • 8 drives of capacity for 10 drives is a decent value. 5/10 (with RAID10) is not.

It turns out that certain default settings in Linux software RAID can yield instability under RAM pressure in very large arrays; after some online research I made the adjustments below and it appears solid. The sync_speed_max setting throttles back the RAID rebuild, helpful because I was able to start populating the storage during the very long rebuild process.

7.3T is far beyond the size limit for ext2/3, so I went with XFS. XFS appears to deal well with the large size without any particular tuning, but increasing the read-ahead helps with my particular access pattern (mostly sequential), also in rc.local:

I was happy to find that XFS has a good “defrag” capability; simply install the xfsdump toolset (apt-get install xfsdump) then schedule xfs_fsr to run daily in cron.

Power consumption seems reasonable at 3.0 amps under load.

Marvell: Not Marvellous

In this machine I happen to have an Intel D975XBX2 motherboard (in retrospect an awful choice, but already installed) which includes a Marvell 88SE61xx SATA controller. This controller does not get along well with Linux. Again with some online research, the fix is just a few commands:

This works. But if I had it to do over, I’d rip out and throw away that motherboard, and replace it with any of the 99% of other motherboards that work well with Linux out of the box, or disable the Marvell controller and add another extra SATA controller on a card.

Is this as big as it gets?

Not by a long shot; this is a secondary, backup storage machine, worth a blog post because of the technical details. It has barely over $1000 worth of hard drives, and a total cost of under $3000 (even less for me, since I reused some old hardware). You can readily order off-the-shelf machines with much more storage (12, 16, 24, even 48 drives). The pricing per-byte is appealing up to the 16 or 24-drive level, then escalates to the stratosphere.

Analyzing PostgreSQL logs with pgFouine (on Ubuntu 8.04)

pgFouine is a slick, useful, and free tool for analyzing PostgreSQL query workloads. It works without any impact on the running PostgreSQL: it analyzes the PG log output. The caveat is that it needs PG configured to write the right kind of log output.

Sadly, as of version 8.3 PG has a wrinkle in how it writes its logs: multi-line queries can get jumbled together in the stderr-based log, resulting in erroneous output from pgFouine. Hopefully a future PG will be able to write its logs without this issue, but in the meantime, the answer it to use syslog logging instead of native PG logging. This isn’t a bad idea anyway, since syslogd and friends are well proven.

On our project where this need arose, we use the Ubuntu Linux distribution, currently version 8.04. Ubuntu’s PG package sets up native stderr logging; here are the steps needed to change that to syslog logging instead. These steps are about the same for other distributions (or for manual compiles), but with different paths.

The setting shown here for log_min_duration_statement will log all queries that take more than 4 seconds to complete. Depending on your server, workload, and type of workload (OLTP vs. OLAP), this might be too high or too low.

Edit your postgresql.conf file:

With PostgreSQL 8.2, set redirect_stderr instead of logging_collector:

Next, setup where syslog will store the data, and add “local0,none” to the ;-separated list of what goes in to var/log/message.  On my system it ended up looking like this, but of course it may vary depending on what else you’ve set up in syslog:

Restart syslogd to make the change take effect:

Then restart PG so it starts logging there:

Note that we are putting these new logs in the existing /var/log/postgresql directory which the Ubuntu PG package creates; if you install PG manually, create such a directory yourself, or set up syslog to write to the pg_log directory. The existing logs there will remain, holding only the messages from PG startup and shutdown (via /etc/init.d/postgresql). I find this unhelpful but harmless.

Log Rotation

By putting the files in this preexisting location, we take advantage of the log rotation already set up in /etc/logrotate.d/postgresql-common. On a busy server, you may want to adjust the rotation setting therein from weekly to daily, or add a line with “size 1000k” or so. Take a look at “man logrotate” to learn about many useful options, such as the ability to have these logs emailed to your DBA as they rotate.

pgFouine

Finally, you are ready to analyze logs. If you plan to analyze them on the same machine where you run your database (probably not a great idea), proceed (on Ubuntu) to get the PHP command line executable:

Then download the pgFouine tarball, quietly curse the lack of an Ubuntu package, put it in your $PATH, and run it. Don’t be alarmed by its .php file extension; PHP is a usable (though not particularly charming) language for writing command line tools, as well as dynamic web pages.

View the HTML file in your web browser, and dig in to your worse queries. Good luck.

Multicast your DB backups with UDPCast

At work we have a set of database machines set up so that one is the primary machine, making backups once per day, and several other machines restore this backup once per day, for development, ad hoc reporting, and other secondary purposes. We started out with an obvious approach:

  • back up on server1, to a file on server1
  • SCP or rsync the file from server1 to server2
  • restore the DB on server2

… but over time as the data has grown the inefficiency of such an approach become equally obvious: the backup data goes back and forth across the network and to/from disk repeatedly. These steps only count the backup data, not the live storage in the DBMS:

  1. On to the disk on server1  (putting additional load on the primary DB machine)
  2. Off the disk on server1  (putting additional load on the primary DB machine)
  3. On to the disk on server2
  4. Off the disk on server2

This is also wasteful from a failure-recovery point of view, since the place we are least likely to need the backup is on the machine whose failure would lead us to need a backup.

Pipe it over the network instead

The project at hand uses PostgreSQL on Linux, so I’ll show example PG commands here. The principles apply equally well to other DBs and platforms of course, though some DBMSs or platforms might not offer backup and restore commands that stream data. (I’m looking at you, MS SQL Server!)

What we need is a pipe that goes over the network.  One way to get such a pipe is with ssh (or rsh), something like so, run from server1:

This variation will simultaneously store the backup in a file on server1:

This variation (or something close, I last run this several days ago) will store the backup in a file on server2 instead:

To reduce the CPU load from this, adjust SSH to use less CPU-intensive encryption, or avoid that entirely with rsh (but only if you have a trusted / local network).

Multicast / Broadcast it over the network instead

The above commands are good for point-to-point streaming backup / restore, but the scenario I have in mind has one primary machine and several (3, 4, or more) secondary machines. One answer is to run the above process repeatly, once for each secondary machine, but that sends the whole backup over the network N times. Inefficiency! (==Blashphemy?)

To avoid that, simply use UDPCast. It’s a trivial install on Debian / Ubuntu:

(Be warned though: there is at least one annoying bug in the old (2004) UDPCast offered off-the-shelf in Debian / Ubuntu as of 2008. You might need to the latest UDPCast source from its web site above.)

Run this on the server1:

Run this on the server2 .. serverN:

With this approach, the backup data will be multicast (or broadcast, if multicast does not work and if all the machines are on the same segment), only traversing the network once no matter how many receiving machines are set up. udp-receiver has a –pipe option, but I found that I occasionally get corruption with huge (50GB+) transfer, when using –file or –pipe. So I recommend this, to save a copy on the receiving end:

Or perhaps you want to just receive and store the backup on a file server, with this:

To make all this happen automatically, you’ll set the sender to start at the same time as the receivers in “cron” on the relevant machines. Use NTP to keep their clocks in sync, and adjust the udp-sender and udp-receiver options as needed to get the whole process to start smoothly in spite of minor timing variations (–min-wait t, –max-wait t).

As with the previous suggestion for rsh, the data will travel unencrypted over your network, so do this only if you trust your network (such as a LAN segment between your database servers).

Multicast / broadcast is very useful technology, and with UDPcast it is quite easy to use. UDPcast also implements a checksum/retransmit mechanism, it is not a “bare”, loss-prone UDP transmission.