I currently have a btrfs filesystem that I am unmounting and it has been has been "unmounting" for the last 20 minutes.
I'm pretty sure I know exactly what is going on and in my current situation it's not a huge issues, but it would be a problem if this was a production system and I was trying to do a maintenance. Here is how I got into this situation: I am migrating my data from one pair of disks (mirrored with btrfs) to another pair of disks. I rsync'd my data from the original btrfs file system to the other. When it completed, my new filesystem showed 165GB used. The original show 1.8TB used. I came to the conclusion that it must be the daily snapshots I have that were using the majority of the space and because I was going to destroy the filesystem, I decided, what the heck, let me destroy the snapshots and see what it looks like. To my surprise, removing all the snapshots resulted in the usage dropping from 1.8TB to 1.7TB. I re-ran my rsync, it complete without transferring any new data. I then did a du -s in the mountpoint for the original filesystem and is reported back 165GB which agrees with what rsync and df on the new filesystem reports. My first thought was that I must have some sort of bizarre corruption on the original filesystem. And then I went to unmount it and it still has not returned. What I now suspect is going on is that while deleting the snapshots was quick, that probably kicks of a background thread which actually does the heavy lifting. I noticed a btrfs-cleaner process that was in an io wait state, which I presumed was the process in question. However, now 40 minutes later, my unmount is still hung and the btrfs-cleaner process is sleeping, so perhaps I am wrong. At this point I am going to powercycle my system, but I figured I would check and see if anyone else knew for certain it this was the type of behavior one would expect to see when removing large snapshots and then immediately trying to unmount the filesystem. If so, it seems like this is something that would need to change before someone would want to seriously consider using btrfs w/ snapshots in a production environment. I know btrfs is not considered production ready yet (well, at least not by the developers, regardless of what Oracle and Suse say). At the same time, I've not been able to find any mention of similar problems, so I figured it was worth mentioning. -- Michael Johnson - MJ -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
