A Different Spin On the max_allowed_packet Problem

Back in November, I filed MySQL bug 67448, talking about a different type of max_allowed_packet problem.

See, an application had put data into the database, but could not retrieve it without getting max_allowed_packet. With the help of some really smart community folks (named Jesper Hansen, Brandon Johnson and Shane Bester), we determined that MySQL actually has 2 different max_allowed_packet settings: client and server.

When you change the max_allowed_packet variable, you are changing the server variable if it is in [mysqld] and the client variable if it is in [client] or [mysql] or whatever client you have. As far as we can tell, there’s no way to actually view what the client variable is, as looking at both the session and global max_allowed_packet variable shows you the server variable.

If max_allowed_packet is not set by the client, it defaults to 16M. The proposed solution is to allow it to be increased for non-interactive clients, and the bug has been verified as a “feature request”, though it has not been implemented yet.

ulimits and upgrading from Oracle MySQL 5.0 to Percona patched MySQL 5.1

After upgrading to Percona’s patched MySQL 5.1*, end users were having connectivity problems, and reporting errors such as:

OperationalError: (2003, "Can't connect to MySQL server on 'db-amo-ro' (110)")

TimeoutError: Request timed out after 5.000000 seconds

OperationalError: (1135, "Can't create a new thread (errno 11); if you are not out of available memory, you can consult the manual for a possible OS-dependent bug")

We had these same problems a while back, before increasing ulimit settings in /etc/sysconfig/mysqld. Oracle’s MySQL startup script specifically sources this file:

[ -e /etc/sysconfig/$prog ] && . /etc/sysconfig/$prog

However, we saw these errors again when we upgraded to Percona’s MySQL 5.1. At first we thought that it was because Oracle’s startup script is /etc/init.d/mysqld and Percona’s is named /etc/init.d/mysql (so we would put ulimits in /etc/sysconfig/mysql). However, when we looked, we saw that Percona’s startup script does NOT source anything in /etc/sysconfig.

So then we put the following in /etc/security/limits.d/99-nproc-mysql.conf:
root soft nproc 32768
root hard nproc 65535

We restarted MySQL and all was good. Even though we are long past having this problem, I thought it was important enough to blog about.

* We finished upgrading all of our servers to MySQL 5.1 at the end of 2012. We ran into this interesting snag that I wanted to blog about, even though we’re in the middle of upgrading to MySQL 5.5 right now (and by the end of the year, we will upgrade to MySQL 5.6 – the performance schema stuff is definitely something we want to utilize).

Upgrading support.mozilla.org databases

A while ago (November 2012 to be exact), we upgraded the support.mozilla.org databases from Percona 5.1 to MariaDB 5.5 (the next step, happening soon, is upgrading them to Oracle’s MySQL 5.6). One of the engineers and I had a conversation where he mentioned that “one of our worst performing views on SUMO is doing waaaayyy better with the upgraded databases”, that it “seems more stable” and that “I stopped receiving ‘MySQL went away or disconnected emails’ which came in once in a while.”

It’s always nice to see upgrades actually making a difference. In our case we saw a lot less CPU wait, though that might also be partially due to tuning the memory settings on the machines and adding in another read slave to handle queries. As a result, network traffic throughput went from less than 1 Mb/sec to about 18 Mb/sec, because the machines were just handling more queries per second, period.

(I had this e-mail as a draft for a while and decided to clean it up and publish it now!)

MySQL User Group Video – Determinism and Databases

The May Boston MySQL User Group featured John Hugg of VoltDB talking about determinism and databases. I have uploaded the hour-long video to http://www.youtube.com/watch?v=mTDLyRauJtw. Seasoned MySQLers will nod their heads because the problems described are familiar, and those who are not exactly sure what “determinism and databases” means will learn a lot.


(As always, videos are free on YouTube with no login or attempt to solicit your e-mail address or any other information)

RFO: DNSSEC Resolution failures (mozilla.org) 20130515@1800 PDT [872818]

On May 15 at 1800 PDT Nagios  alerted the start of sporadic DNS resolution failures.  This post summarizes the events, the impact and specific steps Mozilla IT is taking to avoid future disruptions of this nature.

This post is intended to be technical in nature.  DNSSEC is fairly technical and DNSSEC failures tend to be similarly technical. As we’ve done before, we hope to share the failures we encounter in production so you don’t have to experience the same.


An SOA mismatch between SVN and the nameservers was caused by the DNSSEC signer refusing to sign with an expired ZSK. This was misdiagnosed as a KSK issue, leading to a DNS outage for DNSSEC-verifying resolvers.


In the afternoon of May 15, the nameservers refused to load SOA update 2013051500 for the mozilla.org DNSSEC-signed zone.

Investigation found that the DNSSEC signer was refusing to sign the zone, providing only the error “fatal: cannot find SOA RRSIGs“. In hindsight, this undocumented error indicates that the zone’s ZSK has expired.

Mozilla’s domain registrar publishes DS records for the mozilla.org KSK. When the expired key was found at 16:44, it was misunderstood to be a KSK, rather than a ZSK. A new KSK was generated and its DS record added to Mozilla’s domain registrar.

The new KSK did not resolve the signing errors. Mozilla’s domain registrar was found to rate-limit DS record changes, preventing the new KSK from being reverted. DNS lookups began showing invalid DS records from Mozilla’s domain registrar, but this was later found to be internal DNS only.

After examining the keys (both current and expired) more closely, the expired key was found to be a ZSK, rather than a KSK. Renewing the ZSK fixed the DNSSEC signer. The mozilla.org SOA 2013051500 was signed by both KSKs and the new ZSK, and then published.

Comcast users began reporting DNS resolution issues of mozilla.org, complicating access to various Mozilla properties. DNSSEC validation tools showed unexpected issues with the signed mozilla.org zone.

The DS records were confirmed to be correct externally, so the mozilla.org zone was re-signed without the old KSK, leaving only the new KSK and new ZSK. This resolved the validation issues for reasons unknown, and Comcast users reported DNS working correctly again.

Bugs have been filed to document the KSK/ZSK renewal process, to monitor the expiration times of those keys, and to monitor that the zones validate.

  • 872818: mozilla.org SOA mismatch, DNSSEC signer refusing to sign
  • 872831: alarm when DNSSEC signing keys are expiring soon
  • 872884: document ZSK and KSK renewal/rollover process
  • 872832: regenerate mozilla.org DNSSEC ZSK (resolved)
  • 872885: regenerate mozilla.org DNSSEC KSK (resolved)
  • 872927: monitoring: add full validation of DNSSEC zones
  • 15:32 – SOA mismatch detected between nameservers 2013051402 and svn 2013051500.
  • 16:03 – Found DNSSEC signer refusing to sign mozilla.org 2013051500
  • 16:44 – Found expired key preventing signing of mozilla.org
  • 16:52 – Added new KSK to Mozilla’s domain registrar alongside existing KSK to renew expired key
  • 17:06 – Found that expired key was ZSK, not KSK as previously thought.
  • 17:27 – Signed mozilla.org with both KSKs and new ZSK
  • 17:45 – Mozilla’s domain registrar publishing incorrect hash for new KSK (misleadingly, for internal lookups only)
  • 18:00 – Comcast users reporting sporadic DNS resolution failures
  • 18:20 – Validation issue found with signed zones
  • 18:25 – Signed mozilla.org with new KSK and new ZSK
  • 18:30 – Comcast users reporting DNS resolving successfully
  • 18:35 – Validation issue confirmed resolved
  • ZSK and KSK are “zone signing key” and “key signing key” for mozilla.org. DNSSEC permits multiple KSKs and autoselects the latest ZSK. We sign with a single KSK, outside of 17:30-18:25 above.
  • There is no filesystem difference between ZSKs and KSKs. The distinction is the word “zone” or “key” in the comment in the first line of the keyfile.

60 Seconds with Mozilla IT – April 29

From the April 29 weekly Project Meeting:

  • hg push server had an issue last week, failover to a redundant push node happened with little impact

    Details: the way hg is setup there is a single server for changes over ssh (pushes) and a whole pool of servers for consumption through http. This single server has a passive replica that we’ve never had to rely on before last week. Failover to the backup node worked fine as did failing back to the primary node when it was fixed.

  • Mozilla is now contributing to pool.ntp.org

    We make use of GPS based time servers in our primary data centers to ensure that our infrastructure is as close to being in sync as possible. These servers are now contributing back to the greater Internet community

Women in Science and Engineering (WISE) Computing Skills Boot Camp

Software Carpentry is running a 2-day software skills boot camp in Boston, June 24-25th 2013, for women in science, engineering, medicine, and related research
areas. Registration is $20.

Boot camps alternate short tutorials with hands-on practical exercises. You are taught tools and concepts you can use immediately to increase your productivity and improve confidence in your results. Topics covered include the Unix shell, version control, basic Python programming, testing, and debugging — the core skills needed to write, test and manage research software.

This boot camp is open to women at all stages of their research careers, from graduate students, post-docs, and faculty to staff scientists at hospitals and in the public, private, and non-profit sectors.

Registration is $20; to sign up, or find out more, please visit the announcement at http://software-carpentry.org/blog/2013/04/announcing-wise-bootcamp.html. If you have questions, there is an e-mail link on the announcement page.

For those curious, they are using sqlite, not MySQL or PostgreSQL, and I will be helping out with the SQL parts. There are about 2 months left but the boot camp is about 2/3 full right now, so I wanted to make sure this opportunity was spread to as many people as possible so they do not hear about it too late.

Percona Live Has No Code of Conduct

I am not at Percona Live this week because I opted to stay home after a crazy year of travel (41 talks in 11 different countries on 3 continents in the past year). However, I realized today that Percona Live has no Code of Conduct.

I will not be attending any Percona Live events until there is an acceptable Code of Conduct. MySQL is the world’s most popular open source database; the community deserves a Code of Conduct.

ETA: I have contacted Kortney, the conference organizer for Percona Live, and asked for a Code of Conduct to be put in place ASAP.

ETA: If you want to know why this is an issue, see http://adainitiative.org/what-we-do/conference-policies/

ETA: This is my personal statement, and not a statement of what any of my Mozilla colleagues may feel. Other colleagues, including employees under me, may choose to attend or even present at any events they wish. I personally do not feel comfortable at a conference with no Code of Conduct, this is not a reflection on the technical merits of any conference.

BBLISA Lightning Talks

At this month’s Back Bay LISA, Matt Simmons (aka Standalone Sysadmin) set up Lightning Talks. There were 9 presentations, and the videos are up! The playlist is on YouTube but here is a list of all the videos, with the descriptions taken from Matt’s blog post:

Back Bay LISA Lightning Talks
April 2013

  • Mentoring by Matt Finnigan
    Matt Finnigan gave a talk discussing the LOPSA Mentorship program. If you aren’t familiar, the mentorship program is a free service offered by LOPSA, where any admin who needs help, either with a project or just general career guidance, can sign up to be connected to someone with experience in their target area. You need to be a LOPSA member in order to be a mentor, but being a protege is open to anyone, regardless of LOPSA membership.

  • Cooking by Adam Moskowitz
    Adam Moskowitz gave a talk discussing cooking for system administrators. He appealed to our sense of making things as well as our need of healthy food and good value. Adam encouraged us to try cooking, and although most people thought it was expensive to property outfit a kitchen, he reminded us that it was actually a fraction of the price of our new laptops, and the kitchen gear would last a lot longer.

  • Amazon SMS by KM Peterson

    This talk is a result of KM Peterson’s search for a provider-agnostic method to send SMS messages that didn’t break the bank or involve maintaining an array of modems. He ended up setting up a script to talk to Amazon’s SMS service, and provided us example code in his slides.

  • SmartOS by Nahum Shalman
    Nahum Shalman gave a really nice introduction to SmartOS, a derivative of OpenSolaris which is maintained by Joyent. Interestingly, the Linux-native KVM was ported to the SmartOS kernel, allowing creative and secure uses of jails and virtual sandboxes, all taking advantage of native ZFS, dtrace, and all kinds of delicious Solaris-y goodness.

  • MySQL and Puppet by Sheeri Cabral
    Sheeri Cabral came from Mozilla to talk with us about how they’re deploying MySQL using Puppet. Her slides had example code, and she walked us through the abstracted object and up to the deployment on the actual nodes.

  • Secrets by KM Peterson
    KM Peterson’s”second talk was on Shamir’s Secret Sharing Scheme, aka ‘SSSS’. The idea behind this crypto tech is that you have a secret which you want to ensure can only be recovered by the collaboration of a minimum number of involved people – say three of your team of five. You encrypt the plaintext and generate as many keys as people you have, and tell the app how many should be required to release the information. To pull the data out, you provide any of the generated keys, as long as the number of different keys meets the minimum determined when the data was encrypted.

  • Stick Destroyer by John Jarvis
    John Jarvis talked to us about a creative use for his Raspberry Pi – he securely erases flash media using Stick Destroyer. He rigged up a light so that you have a nice visual indicator of when the stick is being erased, and when it’s done.

  • Sensu by Pat Cable
    Pat Cable showed up to talk about Sensu, a ruby-based monitoring solution that uses AMQP queues to distribute tasks around a monitoring infrastructure that can scale out horizontally to monitor extremely large numbers of machines. It’s definitely a “next gen” monitoring solution that you should be aware of.

  • Sysadmins and Doctors by Matt Simmons
    I got up in front of everyone and talked briefly about something that I’ve noticed – mainly about how I see our profession splintering, but that the splintered elements (such as network and storage administrators) aren’t actually specialties of “system administrators”, it’s much more like the specialized administrators are specialist doctors, and system administrators are like general practitioners. The idea is still half baked, but that’s the fun of a lightning talk, right? I didn’t offer any answers, but I asked a lot of questions.