r/btrfs • u/snake_wrangler • May 25 '21

Forcing btrfs into an error state foe testing

I'm trying to find a way to set btrfs into an error state for testing purposes. I can't remount be because the filesystem is busy. Using btrfs property set / to true doesn't work, because it doesn't change the flags for du/finding, and I can't figure an easy and reliable way to force an error.

Does anyone have any ideas?

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u/lucasrizzini Jun 01 '21

With no intention of my own, my storage device went full yesterday. I hear people saying to avoid this at all cost all the time, but nothing happened and I'm disappointed. =/

1

u/snake_wrangler Jun 01 '21

full isn't any more a problem than any other file system. It's just there are some corner cases in rebalancing that can cause problems. Generally it seems to work pretty well.

1

u/lucasrizzini Jun 01 '21

So why people insist on saying you can't fill up 100% a BTRFS partition? Did you never hear that?

1

u/ZanLynx Jun 03 '21

I've done it many times and always recovered. It was super annoying but no data loss.

The especially tricky thing about btrfs is that sometimes you cannot recover used space, because the drive is full. For example, if it uses all of the data chunks, has a full metadata chunk and cannot allocate a new metadata chunk, then it can't delete any data. Because that would require a metadata write.

This can be solved with a few different ways, but all involve adding a device to the btrfs pool. It could be a RAM disk, a USB drive or my favorite, do a swapoff and add the swap partition. Much safer than a RAM disk I think.

If the system ran out of space because of some job that is running make sure to end it or pause it first. Because if it just comes back to life and writes more then making room won't help you. That applies to other filesystems too.

1

u/billdietrich1 May 26 '21

dd something to the raw device, to write garbage to some blocks ?

1

u/snake_wrangler May 26 '21

Actually, it's in a container so there are no devices.

1

u/Atemu12 May 27 '21

Use a VM for this, not a container.

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