How to view the btrfs checksum of a file

cat file.gz file.gz.sig > transfer_me.signed

Then, on the remote host, you split out the appended checksum using

tail -c length_of_the_checksum_info transfer_me.signed > file.gz.sig
head -c -length_of_the_checksum_info transfer_me.signed > file.gz

Note the minus in the second command. The length depends on the type of checksum you use. You can have just

md5sum file.gz | cut -d ' ' -f 1 > file.gz.sig

Notice also, that in this way you can also unpack the transfer_me.signed file straight away without splitting, because tar will ignore the trailing "garbage".

A similar solution, with additional encryption or scrambling mechanisms, is used for signing updates for many mobile devices - for example the Amazon Kindles. But in their case, archive file identification is undesired, so they put all the fingerprinting information at the beginning of an update file.

Btrfs uses crc32c checksums to check the integrity of blocks. If the checksum doesn't match the block when it's read then an alternative block is read. This is assuming there is an alternative (RAID1). If that block also fails or if there is no alternative an EIO (error input/output) is returned.

I do not know of any way to automatically detecting errors, but all errors are logged to syslog. Try dmesg | grep btrfs. You should be looking for something like this:

[ 2993.114213] btrfs: sda2 checksum verify failed on 272228352 wanted 1A0FCFD3 found 119281BE level 0
[ 2993.114527] btrfs: sda2 checksum verify failed on 272228352 wanted 1A0FCFD3 found 119281BE level 0
[ 2993.114795] btrfs: sda2 checksum verify failed on 272228352 wanted 1A0FCFD3 found 119281BE level 0
[ 2993.115097] btrfs: sda2 checksum verify failed on 272228352 wanted 1A0FCFD3 found 119281BE level 0

You could probably make a script or a that looks through the logs and notifies you of errors at regular intervals. Or you could filter these log entries and trigger an rsyslog action.