Linux – btrfs on SD card, what does “Btrfs detected SSD devices, enabling SSD mode” mean exactly

From wiki:

Extent based file storage
2^64 byte == 16 EiB maximum file size
Space-efficient packing of small files
Space-efficient indexed directories
Dynamic inode allocation
Writable snapshots, read-only snapshots
Subvolumes (separate internal filesystem roots)
Checksums on data and metadata
Compression (gzip and LZO)
Integrated multiple device support 
    RAID-0, RAID-1 and RAID-10 implementations 
Efficient incremental backup
Background scrub process for finding and fixing errors on files with redundant copies
Online filesystem defragmentation 

Explanation for desktop users:

• Space-efficient packing of small files: Important for desktops with tens of thousands of files (maildirs, repos with code, etc).
• Dynamic inode allocation: Avoid the limits of Ext2/3/4 in numbers of inodes. Btrfs inode limits is in a whole different league (whereas ext4's inodes are allocated at filesystem creation time and cannot be resized after creation, typically at 1-2 million, with a hard limit of 4 billion, btrfs's inodes are dynamically allocated as needed, and the hard limit is 2^64, around 18.4 quintillion, which is around 4.6 billion times the hard limit of ext4).
• Read-only snapshots: fast backups.
• Checksums on data and metadata: essential for data integrity. Ext4 only has metadata integrity.
• Compression: LZO compression is very fast.
• Background scrub process to find and to fix errors on files with redundant copies: data integrity.
• Online filesystem defragmentation: autodefrag in 3.0 will defrag some types of files like databases (e.g. firefox profiles or akonadi storage).

I recommend you the kernel 3.0. Also btrfs is a good FS for SSD.

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.