First of all, thank you for sharing your test data.
Regarding your concerns, here is a detailed explanation and suggestion:
1. Does the cache only dump files when the SSD is full?
No, it doesn't wait until it's full.
In Read-Write Mode, when you write data to the NAS, the system acknowledges the write as "complete" the moment it hits the high-speed SSD (which is why your write speeds are so fast).
However, the system does not wait for the SSD to fill up to start moving data to the HDDs. Instead, it constantly and quietly trickles data from the SSD to the HDDs in the background when the system is relatively idle.
2. Why does the system "freak out" upon reboot then?
If it syncs continuously, why are there so many unsynced files after a reboot? There are two main reasons:
- Accumulated Cache: If you did heavy write operations shortly before the reboot, the background flusher might not have had enough "idle time" to finish syncing everything.
- Boot-up Flush & Integrity Check: When TOS boots up, its storage daemon prioritizes data integrity. To prevent any potential file system corruption, the system aggressively spins up the HDDs to flush all remaining "Dirty Blocks" from the SSD journal down to the platters as quickly as possible. This is why you see frantic 100% disk activity right after booting up.
Note: If your goal is for data to be immediately and simultaneously written to both the SSD and HDD, you should switch to Balanced Mode (which acts closer to Write-Through). However, please note that this will lower your sequential write speeds down to the HDD's native limit.
3. Would a smaller cache drive limit the busy time?
Technically yes, but it’s a double-edged sword.
A smaller SSD (e.g., 128GB or 256GB) has a lower maximum capacity, meaning the absolute volume of 95% dirty blocks is much smaller than that of a 1TB or 2TB drive. Therefore, the worst-case flushing time after a reboot would indeed be shorter.
However, we do not recommend downgrading for this reason. A smaller cache drive means a much lower cache hit rate, and it will trigger the system's active recycling mechanism much faster during daily use, which can hurt overall performance.
Recommendation:
Your 512GB SSD taking ~45 minutes to completely sync after a boot is entirely normal behavior for a Write-Back cache ensuring your data is safely committed to the hard drives.
If the noise and disk activity after a reboot bother you, the best approaches are:
- Minimize unnecessary reboots.
- Switch to Balanced Mode if you prefer immediate syncing and an instantaneous quiet boot, at the expense of peak write performance.