As anyone who has been running HAMMER1 or HAMMER2 has noticed, snapshots and copy on write and infinite history can eat a lot of disk space, even if the actual file volume isn’t changing much. There’s now an ‘emergency mode‘ for HAMMER2, where disk operations can happen even if there isn’t space for the normal history activity. It’s dangerous, in that the normal protections against data loss if power is cut go away, and snapshots created while in this mode will be mangled. So definitely don’t leave it on!
2 Replies to “HAMMER2 emergency space mode”
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Looking at the details, it isn’t snapshots created in the mode which is the issue (the mode preventing the creation of new data), but rather that old snapshots are likely to be corrupted. Basically, it seems to disable data growth and allow invasive surgery to free up space, copying of old snapshots to other storage beforehand being a good idea.
Just in case, as I don’t want to leave false information around: The more pessimistic interpretation was based on reading the source comments added, but I don’t know enough about HAMMER2 meta data to know whether it’s always going to differ for contents of older snapshots and current contents. If it does, what I posted is wrong, but if it doesn’t, it’s not. Wherever what’s attached to current files and directories is also part of a snapshot, emergency removal could corrupt the snapshot, according to the info in the commit.