ReadyNAS 214 BTRFS corruption - appears to be out of memory related

Sorry for not replying quickly, I had a very long day at work so did not have chance to yesterday.

Replying to each question:

Was the volume still syncing?

No, that finished within a couple of days of me doing the factory reset:

03 Feb 2023 05:00:32 Volume: Volume data is resynced.

Also, did you have multiple threads running for the file restore?

Not sure what you mean, I copied each share back one-by-one from USB drives using a single rsync or cp command (over rsync or cifs respectively). This out of memory happened the same day but many hours (>10) after the restores had been finished - nothing was actively using the NAS at the time, as far as I know - this time there's nothing in the UI log (see below) so I have not yet worked out exactly what time it happened. As with last time, I only noticed when I tried to use it and found it was unresponsive (last time, I discovered it had failed when I found it was read-only).

This time I didn't get an email either, it just seems to have locked up with the message on the LCD. The first time (on 31st January) I had an email that said "The volume data encountered an error and was made read-only. It is recommended to backup your data.".

How large are the drives?

10TB

just searching for what did tip the memory usage over the top so you and others can avoid it

I've not had chance to examine the logs this time - will be doing that very shortly. Fortunately this time BTRFS seems to have survived, probably because nothing was accessing the NAS at the time so there were no writes "in flight" when it ran out of memory.

I am also wondering if the drives are SMR or CMR (since SMR can have very low write speeds, which might result in excessive memory buffering).

Errr... I had to google that one - they are Seagate IronWolf NAS drives, 7200RPM (all the same model but I sourced them from different retailers in the hope that reduce the risk of them being from the same batch) - according to scan's website they're CMR.

Drive health might also factor in.  Did you test the drives?

Yes, I did that the first time and again - health checks are reporting the drives are all fine. All are reporting zero ATA errors, which was the indication of a faulty disk last time I had a drive fault. Two of the drives have single-digit numbers of reallocated sectors but they have been stable like that for a long time and counts have not increased while I've been having these issues - my understanding is modern drives will reallocate sectors automatically and unless the numbers start getting large or start growing in a shortish time that is probably nothing to worry about?