Fresh Elastic Block Storage volumes have first-write overhead
At my employer I architect Big Data hybrid cloud platforms for global audience that have to be FAST. In our cluster provisioning I find we frequently overlook doing an initial write across our volumes to reduce write time during production compute workloads (called pre-warming the EBS volumes). Per Amazon (http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-prewarm.html) failure to pre-warm EBS volumes incurs a 5-50% loss in effective IOPS. Worst case that means you could DOUBLE the IO portion of your HDFS writes until each sector has been touched by the kernel. Amazon asserts that this performance loss, amortized over the life of a disk, is inconsequential to most applications. For one of our current clusters we have a portion with 8-1TB drives in each of 10 compute nodes as a baseline. Our estimated pre-warm time is 30 hours on each mount point so if done sequentially that’s 2,400 hours to touch each drive block.
What does this imply? Without pre-warming we would have added as much as 2,400 additional hours of write latency during initial HDFS writes and that latency could appear in many different places in the stack (HDFS direct writes, Hive postgresql/mysql metadata writes and management, log writes, etc.)
Steps to optimize your EBS writes
Read the AWS document above carefully as it will ERASE EVERYTHING ON THE DISK if you use the first method in their article. The steps below will execute this safely on disks with existing content.
To pre-warm the drives on your cluster:
- stop the cluster services
- ssh into each server
- execute lsblk and note the mount points (they likely start from /dev/xvdf and go down from there increasing the letter at the end, such as /dev/xvdg, /dev/xvdh, etc.)
- unmount each one at a time with sudo umount /ONEMOUNTPOINT
- Continue until all mount points are unmounted, meaning there’s nothing shown after the ‘disk’ column as below:
- CAUTION: DO NOT DO THE FOLLOWING ON A MOUNTED DISK AND MAKE SURE YOU USE THE SAME MOUNT FOR BOTH if AND of
- execute the following, changing the if= and of= to the same mount pointsudo dd if=/YOURMOUNTPOINT of=/YOURSAMEMOUNTPOINT conv=notrunc bs=1MExample: sudo dd if=/dev/xvdf of=/dev/xvdf conv=notrunc bs=1M
- Wait. It’ll be a few minutes for a 32GB drive as shown in the Amazon write-up above or 1 day+ for a 1TB drive.
- After ALL the processes on the server complete, reboot the server
If you’d like to check the process or if your ssh session has expired and you want to ensure you’re still warming execute ps aux|grep YOURMOUNTPOINT , example: ps aux |grep /dev/xvdf
A far better approach, of course, would be to automate this as part of your cluster deployment process using Chef or equivalent infrastructure automation tool.