Tag Archives: BSD

Verify backup integrity with rsync, sed, cat, and tee

Recently it became apparent that a large data transfer I did might have had some errors. I wanted to find an easy way to compare the source and destination to make sure that they were identical. My solution: rsync, sed, cat and tee

I have used rsync quite a bit but did not know about the –checksum flag until recently. When you run rsync with –checksum, it takes much longer, but it effectively does something similar to what a ZFS scrub does – it runs a checksum of every source file and compares it with the checksum of each destination file.  If there is a mismatch, rsync will overwrite the destination file with the source file to correct it.

In my situation I performed a large data migration from my old mdadam-based RAID array to my brand new ZFS array. During the transfer the disks were acting very strange, and at one point one of the disks even popped out of the array. The culprit turned out to be a faulty SATA controller. I bought a cheap 4 port SATA controller from Amazon for my new ZFS array. Do not do this! Spring the cash out for a better controller. The cheap ones, this one at least, only caused headache. I removed it and used the on-board SATA ports on my motherboard and the issues went  away.

All of those shennanigans made me wonder if there was corrupt data on my new ZFS array. A ZFS scrub repaired 15.5G of data! While I’m sure that fixed a lot of the issues, I realized there probably was still some corruption. This is how I verified it

rsync -Pahn --checksum /path/to/source /path/to/destination | tee migration.txt

-P shows progress, -a means archive, -h is for human readable measurements, and -n means dry run (don’t actually copy anything)

Tee is a cool utility that allows you to redirect output of a command both to a file and to standard output. This is useful if you want to see the verification take place in real time but also want to analyze it later.

After the comparison (which took a while!) I wanted to see the discrepancies. the -P flag lists each directory rsync checks as well as which files it detected. You can use sed in conjunction with cat to weed out the unwanted lines (directory listings) so that only the files with discrepancies are left.

 cat pictures.txt | sed '/\/$/d' | tee pictures-truncated.txt

The sed regex simply looks for any line ending in a / (directory listing) and removes that line. What is left is the files in question. You can combine the entire thing into one line like so

rsync -Pahn --checksum /path/to/source /path/to/destination | sed '/\/$/d' | tee migration.txt

In my case I wanted to compare discrepencies with rsync and make decisions on if I wanted to actually fix the issues. If you are 100% sure the source is OK to remove the destination completely, you can simply run

rsync -Pah --checksum --delete /path/to/source /path/to/destination

Find out free disk space from the command line

The du command can be used in any Unix shell to determine how much space a folder is taking. It’s a quick way to determine which directory is using the most space.

If you run du on the root drive “/” then you will get an idea of how much space the drive is using. One unfortunate side effect of that command is if you have any mounted drives or other filesystems, it will search the disk usage of those folders as well. Aside from taking a long time that method provides data we don’t necessarily want.

Fortunately, there are a few switches you can use to fix this problem. The -x switch tells du to ignore other filesystems. Perfect.

There are a few other switches that prove useful. Below is a list of my favorites:

  • -x Ignore other filesystems
  • -h Use human readable numbers (kilobytes, megabytes, and gigabytes) instead of raw bytes
  • -s Provide a summary of the folder instead of listing the size of each file inside the folder
  • * (not a switch, but rather an argument to put after your directory.) This summarizes subfolders in that directory, as opposed to simply returning the size of that directory.

For troubleshooting low disk space errors, the following command will give you a good place to start:

du -hsx /*

You can then dig further by altering the above command to reflect a directory instead of /, or simply do * if you want a breakdown of the directory you are currently in.

Check hard drives for bad sectors in Linux/BSD

It turns out that when hard drives fail, they don’t all fail completely. In fact, most fail silently, getting worse and worse as time moves on, causing bitrot and other issues.

I had a suspicion that one of my drives was failing so I thought I would test it. The tool for the job: badblocks.

badblocks writes data to the drive and then reads it back to ensure it gets the expected result. I have learned a lot about hard drive failure lately and now subscribe to running badblocks on every new hard drive I receive to ensure it is a good drive. The command I use is:

badblocks -wsv <device>

This is a destructive write test – it will wipe the disk. You can also run a non-destructive test, but for new disks you can go ahead and wipe them. I also use badblocks to ensure old disks can still be trusted with data. It’s great for “burn in” testing to ensure a drive won’t fail.