Use a Chef recipe to modify a single line in a config file - mongodb

Question

I'm trying to automate disabling the Transparent Huge Pages (THP) Settings for MongoDB using a Chef Recipe.
The THP setting is explained here: MongoDocs THP Settings
I'm trying to follow the first option "In Boot-Time Configuration (Preferred)" by editing the grub configuration file at "/etc/grub.conf"
All I need to do is append "transparent_hugepage=never" to the end of the existing line that starts with "kernel "
I know I can replace a line with Chef::Util::FileEdit, using something like this:
ruby_block "replace_line" do
block do
file = Chef::Util::FileEdit.new("/etc/grub.conf")
file.search_file_replace_line("/kernel/", "kernel <kernel path> <kernel options> transparent_hugepage=never")
file.write_file
end
end
but I need to keep the existing kernel path and kernel options.
I've tried playing around with Chef::Util::Editor, but haven't been successful initializing the constructor. Chef::Util::FileEdit is initialized with a file path (per above), but the ruby docs say that Chef::Util::Editor is initialized with "lines". I've tried
lines = Chef::Util::Editor.new(<lines>)
where <lines> = file path, = Chef::Util::FileEdit.new(), and = 'test string', but nothing seems to work.
Does anyone have any experience with the Chef::Util::Editor? Or a better solution?
Thanks

Answer 1

I never figured out how to modify a single line in a config file using Chef, but here's the recipe I ended up using to disable THP settings for MongoDB.
Recipe: Install MongoDB
# Install MongoDB on Amazon Linux
# http://docs.mongodb.org/manual/tutorial/install-mongodb-on-amazon/
# 1: configure the package management system (yum)
# 2: install mongodb
# 3: configure mongodb settings
# 3.A: give mongod permission to files
# data & log directories (everything in /srv/mongodb)
# http://stackoverflow.com/questions/7948789/mongodb-mongod-complains-that-there-is-no-data-db-folder
execute "mongod_permission" do
command "sudo chown -R mongod:mongod /srv/mongodb"
#command "sudo chown mongod:mongod /var/run/mongodb/mongod.pid"
#command "sudo chown -R $USER /srv/mongodb"
end
# 3.B: edit Transparent Huge Pages (THP) Settings
# get rid of mongod startup warning
# http://docs.mongodb.org/manual/reference/transparent-huge-pages/#transparent-huge-pages-thp-settings
# 3.B.1: disable
execute "disable_thp_khugepaged_defrag" do
command "echo 0 | sudo tee /sys/kernel/mm/transparent_hugepage/khugepaged/defrag" # different b/c file doesn't have options list
end
execute "disable_thp_hugepage_defrag" do
command "echo 'never > /sys/kernel/mm/transparent_hugepage/defrag' | sudo tee --append /sys/kernel/mm/transparent_hugepage/defrag"
end
execute "disable_thp_hugepage_enables" do
command "echo 'never > /sys/kernel/mm/transparent_hugepage/enabled' | sudo tee --append /sys/kernel/mm/transparent_hugepage/enabled"
end
# 3.B.2: verify disabled on reboot
template "/etc/rc.local" do
source "init-rc.local.erb"
owner 'root'
group 'root'
mode '0775'
end
# 4: use upstart & monit to keep mongod alive
Template: init-rc.local.erb
touch /var/lock/subsys/local
if test -f /sys/kernel/mm/transparent_hugepage/khugepaged/defrag; then
echo 0 > /sys/kernel/mm/transparent_hugepage/khugepaged/defrag
fi
if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
echo never > /sys/kernel/mm/transparent_hugepage/defrag
fi
if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
echo never > /sys/kernel/mm/transparent_hugepage/enabled
fi

Answer 2

The problem with your own solution is the template can be overwritten by another recipe with it's own rc.local template.
To change that, I add the lines to the existing rc.local
execute "disable_thp_hugepage_defrag" do
command "sudo sed -i -e '$i \\echo never > /sys/kernel/mm/transparent_hugepage/defrag\\n' /etc/rc.local"
not_if 'grep -c "transparent_hugepage/defrag" /etc/rc.local'
end
execute "disable_thp_hugepage_enables" do
command "sudo sed -i -e '$i \\echo never > /sys/kernel/mm/transparent_hugepage/enabled\\n' /etc/rc.local"
not_if 'grep -c "transparent_hugepage/enabled" /etc/rc.local'
end
The grep makes sure that the line is not already in it.
Maybe chef has something better to manage that?

Answer 3

We can efficietly replace contents of file by grouping the elements
e.g.
appending "transparent_hugepage=never" to the end of the existing line that starts with "kernel "
ruby_block "replace_line" do
block do
file = Chef::Util::FileEdit.new("/etc/grub.conf")
file.search_file_replace_line(/kernel.*/, '\0 tansparent_hugepage=never')
file.write_file
end
end
\0 adds whole mached string
note: ' '(single quote)

Answer 4

I disabled hugepages by replicating the following in chef (looks the same as above but with the addition of a not_if statement):
echo never | sudo tee /sys/kernel/mm/transparent_hugepage/defrag
E.G
execute "disable_hugepage_defrag" do
not_if "grep -F '[never]' /sys/kernel/mm/transparent_hugepage/defrag"
command "echo 'never' > /sys/kernel/mm/transparent_hugepage/defrag"
end
I have also had success inserting lines with file.insert_line_if_no_match the ruby line replace feature will probably work for you.
search_file_replace_line(regex, newline) ⇒ Object
ruby_block 'replace_line' do
block do
file = Chef::Util::FileEdit.new('/path/to/file')
file.search_file_replace_line('/Line to find/', 'Line to replace with')
file.write_file
end
end