Centos on Evan Hoffman

The not-so-secret secret to Postgres performance

Mon, 07 Oct 2013 10:19:05 +0000

I manage a bunch of Postgres DBs and one of the things I almost always forget to do when setting up a new one is set the readahead up from the default of 256. I created this script and run it out of /etc/rc.local and sometimes cron it too. The 3 commands at the top are only really relevant on systems with “huge” memory - probably over 64 GB. We ran into some memory problems with CentOS 6 on a box with 128 GB ram which we ended up working around by reinstalling CentOS 5, but the /sys/kernel/mm/redhat_transparent_hugepage/ options below should fix them in 6.x (though we haven’t actually tried it on that DB, we haven’t seen any problems in other large DBs).

Problems with PostgreSQL 9.0 on CentOS 6.3 with a 2.6.32-series kernel

Tue, 25 Sep 2012 13:20:36 +0000

Note: this is cross-posted from the Yodle Tech Blog. We recently migrated our Postgres database to a significantly more powerful machine. While our existing server was doing okay load-wise, we were approaching 90% full on its Postgres volume. We’d also recently acquired an awesome new PureStorage SSD-based SAN and were excited to move our database onto it. For comparison, here are the the specs for the two systems:


	Old server	New server
Nickname	DB1	DB2
CPU	2x 6-core Xeon X5690 CPUs at 3.47GHz	2x 8-core E5-2690 CPUs at 2.90 GHz
Memory	96 GB 1333 MHz	128 GB 1600 MHz
Storage	- Dell PERC H800 SAS HBA - Dell MD1220 drive enclosure - 24x 15k RPM SAS drives configured in RAID 10	- QLogic QLE2562 dual-port 8 Gbit FC HBA - PureStorage SSD-based SAN
OS	CentOS 5.6 x86_64	CentOS 6.3 x86_64
Kernel	2.6.18-238.9.1.el5	2.6.32-279.2.1.el6.x86_64

As you can probably guess, moving from spinning disk onto SSD was the centerpiece of the upgrade. We’d tested the PureStorage system pretty extensively and used it in some secondary databases and seen some amazing improvements. We were excited to see how it would handle our production workload.

Rescan SATA bus (aka hot-adding a SATA disk on a Linux guest in VMware without rebooting)

Mon, 24 Oct 2011 15:20:51 +0000

Linux supports hot-adding disks but whenever I add a new vdisk in VMware the new disk doesn’t show up unless I reboot, which defeats the purpose of hot-add. This command forces a rescan of the bus:

echo "- - -" > /sys/class/scsi_host/host0/scan

dmesg shows the new disk has been found:

Vendor: VMware Model: Virtual disk Rev: 1.0 
 Type: Direct-Access ANSI SCSI revision: 02
 target0:0:2: Beginning Domain Validation
 target0:0:2: Domain Validation skipping write tests
 target0:0:2: Ending Domain Validation
 target0:0:2: FAST-40 WIDE SCSI 80.0 MB/s ST (25 ns, offset 127)
SCSI device sdd: 1048576000 512-byte hdwr sectors (536871 MB)
sdd: Write Protect is off
sdd: Mode Sense: 03 00 00 00
sdd: cache data unavailable
sdd: assuming drive cache: write through
SCSI device sdd: 1048576000 512-byte hdwr sectors (536871 MB)
sdd: Write Protect is off
sdd: Mode Sense: 03 00 00 00
sdd: cache data unavailable
sdd: assuming drive cache: write through
 sdd: unknown partition table
sd 0:0:2:0: Attached scsi disk sdd
sd 0:0:2:0: Attached scsi generic sg3 type 0

Now, why there’s no “rescan_sata” command is something I can’t fathom, but that’s Linux for you.

64-bit Firefox 6 on CentOS 6 x86_64 (or Fedora) with Flash 11

Wed, 31 Aug 2011 16:15:14 +0000

So I’ve gone back to Linux from Mac, due to the SSD issues I had with my Macbook Pro basically making VMware unusable. A Win7 VM would grind the guest and host to a halt on the 7200 RPM SATA OEM drive, and the SSD wouldn’t work, so I put the SSD back in my HP and installed CentOS 6 x86_64. Not really ideal, but at least it works.

CentOS 6 ships with Firefox 3.6.9, which is really old by now. Fine for a server, but I wanted FF6. I grabbed the Firefox 6.0.1 bz2 from VoxCast, unzipped it and copied it to /usr/lib64/firefox-6. Then I ran yum remove firefox to remove 3.6.9 and avoid any issues. I tried the instructions from my older post on this subject but for whatever reason it didn’t work - I’m guessing because I’m not using the distro’s Firefox RPM.

MongoDB logrotate script

Thu, 21 Jul 2011 10:10:26 +0000

Daily mongodb log rotation via logrotate.

Setting up InspIRCd as an internal IRC server for companywide chat

Thu, 28 Oct 2010 17:16:12 +0000

I got this idea (from this blog post) that it would be cool and great if we had an IRC server to facilitate communication within our team. I started checking out some different IRC servers to see which ones supported the main features I wanted (LDAP auth & SSL). I started out with ratbox but I didn’t really like it. I found InspIRCd and after some config tweaking it’s working well. Here’s the info from modules.conf for authentication against Active Directory (so people login with their AD usernames):

HOWTO: Install Flash plugin on Firefox on Fedora / CentOS / RedHat Linux

Wed, 09 Jun 2010 01:36:54 +0000

Updated 8/31/2011 See my newer post on this subject.

I know I’ve been through this before because I’ve setup Fedora before and it’s working. But I just installed FC12 on my desktop at home (since my Seagate drive bricked itself a few days ago and I had to reinstall anyway) and while I finally got the Adobe Flash plugin working, it didn’t have any audio, which made YouTube kind of suck. Anyway, this of course was a solved problem, I just forgot where the fix was located… but duh, it’s on the Fedora website. A simple

I put Fedora Core 11 on my work laptop

Wed, 21 Oct 2009 00:25:19 +0000

My work laptop, which I got around the end of 2006, was starting to run like crap. Pretty sure it started around the time I put antivirus on it, which reinforces my theory that antivirus is a virus itself. Anyway, being bored with Windows XP and having already played with Windows 7 at home, I decided to install Linux. The last time I tried Linux desktop was around RedHat 7.3, which was a long time ago… pre-Fedora even. I’m pretty happy with CentOS on the server side and was set to try CentOS 5.3 on my laptop when someone suggested I go with FC due to more bleeding-edge driver support. Even though the hardware is pretty old at this point I figured it was worth a shot.