Monday Mar 02, 2009

New DTrace Probes in PostgreSQL 8.4

DTrace probes were introduced in PostgreSQL starting in 8.2. Initially, only a handful were added, and they were mostly for developers. In 8.4, many more (46 to be exact) have been added, and they are targeted more toward database administrators as shown below.

query-parse-start(const char \*)
query-parse-done(const char \*)
query-rewrite-start(const char \*)
query-rewrite-done(const char \*)
query-plan-start()
query-plan-done()
query-execute-start()
query-execute-done()
query-start(const char \*)
query-done(const char \*)
statement-status(const char \*)
sort-start(int, bool, int, int, bool)
sort-done(unsigned long, long)
buffer-read-start(ForkNumber, BlockNumber, Oid, Oid, Oid, bool)
buffer-read-done(ForkNumber, BlockNumber, Oid, Oid, Oid, bool, bool)buffer-flush-start(Oid, Oid, Oid)
buffer-flush-done(Oid, Oid, Oid)
buffer-hit(bool)buffer-miss(bool)
buffer-checkpoint-start(int)
buffer-checkpoint-sync-start()
buffer-checkpoint-done()
buffer-sync-start(int, int)
buffer-sync-written(int)
buffer-sync-done(int, int, int)
buffer-write-dirty-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
buffer-write-dirty-done(ForkNumber, BlockNumber, Oid, Oid, Oid)
deadlock-found()
checkpoint-start(int)
checkpoint-done(int, int, int, int, int)
clog-checkpoint-start(bool)
clog-checkpoint-done(bool)
subtrans-checkpoint-start(bool)
subtrans-checkpoint-done(bool)
multixact-checkpoint-start(bool)
multixact-checkpoint-done(bool)
twophase-checkpoint-start()
twophase-checkpoint-done()
smgr-md-read-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
smgr-md-read-done(ForkNumber, BlockNumber, Oid, Oid, Oid, const char \*, int, int)
smgr-md-write-start(ForkNumber, BlockNumber, Oid, Oid, Oid)
smgr-md-write-done(ForkNumber, BlockNumber, Oid, Oid, Oid, const char \*, int, int)
xlog-insert(unsigned char, unsigned char)
xlog-switch()
wal-buffer-write-dirty-start()
wal-buffer-write-dirty-done()

Documentation should be available in 8.4 doc page soon, but if you don't want to wait, check out the doc patch I recently submitted. If you're using or plan to use the probes, I'd love to hear your feedback, both positive and constructive!

Special thanks to Theo Schlossnagle, Robert Treat, Zdenek Kotala, Alvaro Herrera and Simon Riggs for their contributions with the probes as well as reviewing them.

Sunday Nov 30, 2008

Time for a Home NAS!

Over the years I have accumulated quite a bit of digital content (music, pictures, videos, archived tax returns, etc) and have used external drives, CDs/DVDs, and even USB sticks to backup the data. After a while, it has become increasing inconvenience to locate the content let alone share it with multiple computers, so I decided to shop for a home NAS. But which one? There are quite a few of them on the market with varying prices ($200 to $2000+) and features. I liked the Netgear's ReadyNAS, but it was quite pricy. With 2TB of storage, it was around $1,500 at the time. This costed more than a PC with the same amount of storage, so I thought to myself, "Why not use a PC with ZFS."

After some Googling, I found a number of people had already done exactly this, and Simon's blog was particularly helpful , especially for the system config that worked well for him. So, I decided to build a similar system. I ordered the components from Newegg.com, and the total price was less than $900 for a pretty powerful system (2.6 Ghz AMD Athlon, 2GB ECC RAM, 4x500GB WD drives). It took me a couple of days to put the system together, install OpenSolaris 2008.05, setup ZFS pools, and CIFS server/client, and voila, I have a system that can be used as a NAS as well as a general purpose server. Now, I have all my data in one place and be able to get to it from all the computers on the network, and be assured that if a drive fails, I won't lose data thanks to the RAID support in ZFS.

Monday Feb 04, 2008

Achieving High Availability with PostgreSQL using Solaris Cluster

Quick announcement: Sun has just released Solaris Express Developer Edition (SXDE) 1/08, a developer version that has all the latest and greatest Solaris features and tools. So what's new for PostgreSQL? The main updates/additions are 8.2.5, Perl driver, and pgAdmin III v1.6.3.

High availability (HA) is very important for most applications. PostgreSQL has an HA feature called warm standby or log shipping which allows one or more secondary servers to take over when the primary server fails. However, there are some limitations with PostgreSQL warm standby, two of which are:

  1. No mechanism in PostgreSQL to identify and perform automatic failover when the primary server fails
  2. No read-only support on the standby server

Fortunately, Solaris Cluster can be used to solve limitation #1. Solaris Cluster is a robust clustering solution that has been around for many years, and best of all it's open sourced and free.

Detlef Ulherr has recently implemented the Solaris Cluster agent to work with PostgreSQL warm standby. We discussed different possible use-case scenarios with PostgreSQL warm standby, and he came up with a design that I think will work well for non-shared storage clustering. Maybe not everything will be perfect initially, but as more people test out the agent, we'll know what can be improved. The agent is still in beta now and will be released soon, probably in a couple of months.

The cool thing with Solaris Cluster is that you can now setup a cluster on a single node using multiple Solaris Zones . This is extremely useful because it eliminates the need for multiple machines or complicated hardware setup if you just want to try it out or if you want a simple environment for doing development. Here's more details on the Solaris Cluster and Zones integration

Of course you wouldn't want to deploy your HA application on a single machine. In production environment, you should have at least a two nodes cluster. Please refer to the Solaris Cluster documentation for more info.

From my recent test, I setup the cluster on a single machine with two Solaris Zones. Here's how the automatic failover works in a nutshell:

  • Client connects to a logical host. The logical host is configured to have two resource groups, each containing a Zone.
  • The logical host initially points to the primary server (Zone 1) where PostgreSQL is running, and PostgreSQL is configured to ship WAL logs to the secondary server (Zone 2) where PostgreSQL is running in continuous recovery mode.
  • When primary server fails, the Solaris Cluster agent detects the failure and triggers the logical host to automatically switch to the IP address of the secondary server.
  • From the PostgreSQL client perspective, it's still using the same logical host, but the actual PostgreSQL server has moved to a different machine, all happens transparently.
  • If the client was connecting to the DB on the primary, the session would be disconnected momentarily and reconnected automatically to the DB on the secondary server, and the application would continue on its merry way.

The combination of PostgreSQL warm standby and Solaris Cluster can provide an enterprise class clustering solution for free (unless to need support services off course). So, please try it out and provide your feedback on what can be improved.

In my next blog, I will discuss how Solaris ZFS and Zones can be used in a clever way to overcome limitation #2. This idea has been used already, and some of you may have seen Theo's blog on this topic. I will provide a working sample code and step by step instructions for setting it up.

Tuesday Jun 12, 2007

PostgreSQL 8.2.4 in Solaris Expres Developer Edition

Today Sun announced the availability of Solaris Express Developer Edition 5/07. There are many new features in this release and among them is the inclusion of PostgreSQL 8.2.4 with SMF and DTrace integration.

Here's how you'd run Postgres 8.2:
    1) As root, su to postgres
    # su - postgres

    2) Create Postgres DB cluster
    $ /usr/postgres/8.2/bin/initdb -D /var/postgres/8.2/data

    3) As root, use the SMF's svadm command to start Postgres
    # /usr/sbin/svcadm enable postgresql:version_82

Note that Postgres 8.1 is also available. The binaries are located in /usr/bin and /usr/postgres/8.2/bin for 8.1 & 8.2 respectively. To use 8.2, make sure to add /usr/postgres/8.2/bin in the PATH. For more info see postgres_82 man page (e.g. run "man postgres_82" from the command prompt).

With 8.2.4 all the user-level DTrace probes are now enabled. To see the list of available probes, run "dtrace -l | grep postgres"

For more info on how to use the probes, refer to http://www.postgresql.org/docs/8.2/static/dynamic-trace.html

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