Best way to 'harden' embedded ext4 file server against unexpected loss of power?
- by Jeremy Friesner
Hi all,
First, a little background: my company makes an audio streaming device that is a headless, rack-mounted Linux box with a couple of SSDs attached. Each SSD is formatted with ext4. The users can connect to the system using Samba/CIFS to upload new audio files or access existing ones. There is also custom software for streaming out audio over the network.
This is all fine. The only problem is that the users are audio people, not computer people, and see the system as a 'black box', not as a computer. Which means that at the end of the day, they aren't going to ssh in to the box and enter "/sbin/shutdown -h"; they are just going to cut power to the rack and leave, and expect things to still work properly the next day.
Since ext4 has journalling, journal checksumming, etc, this mostly works. The only time it doesn't work is when someone uploads a new file via Samba and then cuts power to the system before the uploaded data has been fully flushed to the disk. In that case, they come in the next day and find that their new file has been truncated or is missing entirely, and are unhappy.
My question is, what is the best way to avoid this problem? Is there a way to get smbd to call "sync" at the end of every upload? (Performance on uploads isn't so important, since they only happen occasionally). Or is there a way to tell ext4 to automatically flush within a few seconds of any change to a file? (Again, performance can be sacrificed for safety here) Should I set a particular write-ordering mode, activate barriers, etc?
