Andrei Borzenkov posted on Tue, 03 Jul 2018 07:25:14 +0300 as excerpted: > 02.07.2018 21:35, Austin S. Hemmelgarn пишет: >> them (trimming blocks on BTRFS gets rid of old root trees, so it's a >> bit dangerous to do it while writes are happening). > > Could you please elaborate? Do you mean btrfs can trim data before new > writes are actually committed to disk?
No. But normally old roots aren't rewritten for some time simply due to odds (fuller filesystems will of course recycle them sooner), and the btrfs mount option usebackuproot (formerly recovery, until the norecovery mount option that parallels that of other filesystems was added and this option was renamed to avoid confusion) can be used to try an older root if the current root is too damaged to successfully mount. But other than simply by odds not using them again immediately, btrfs has no special protection for those old roots, and trim/discard will recover them to hardware-unused as it does any other unused space, tho whether it simply marks them for later processing or actually processes them immediately is up to the individual implementation -- some do it immediately, killing all chances at using the backup root because it's already zeroed out, some don't. In the context of the discard mount option, that can mean there's never any old roots available ever, as they've already been cleaned up by the hardware due to the discard option telling the hardware to do it. But even not using that mount option, and simply doing the trims periodically, as done weekly by for instance the systemd fstrim timer and service units, or done manually if you prefer, obviously potentially wipes the old roots at that point. If the system's effectively idle at the time, not much risk as the current commit is likely to represent a filesystem in full stasis, but if there's lots of writes going on at that moment *AND* the system happens to crash at just the wrong time, before additional commits have recreated at least a bit of root history, again, you'll potentially be left without any old roots for the usebackuproot mount option to try to fall back to, should it actually be necessary. -- Duncan - List replies preferred. No HTML msgs. "Every nonfree program has a lord, a master -- and if you use the program, he is your master." Richard Stallman -- 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
