QNAP NAS Community Forum

bokr71 wrote: ↑Sat May 18, 2019 11:02 am I am not saying this is how it works. I am saying this is what the message from QNAP means. I also have seen it take a decent amount of time to turn caching off, which it says is due to the flushing of the write cache. But I think there is more than that going on...

Yes... something is going on. I'd just like a clear/good answer... which I haven't been able to get yet.

QNAP is using the flashcache linux kernel module from Facebook.

Dirty blocks (blocks that are in cache only) are synchronized to disk after 15 minutes of inactivity (no read/write of the block for 15 mins) or when 60% of the cache is used by dirty blocks.
dev.flashcache.CG0.fallow_delay = 900
dev.flashcache.CG0.dirty_thresh_pct = 60

Block are not synchronized at reboot/shutdown:
dev.flashcache.CG0.shutdown_fast_remove = 1

You can get info about the cache with:
more /proc/flashcache/CG0/flashcache_stats
dmsetup status CG0ssddev

To sync dirty blocks manually, set dev.flashcache.CG0.do_sync to 1:
sysctl -w dev.flashcache.CG0.do_sync=1

To enable sync at shutdown:
sysctl -w dev.flashcache.CG0.shutdown_fast_remove=0

(CG0 is the cache device name)

Did anyone ever get to a real answered on the question "when is SSD caching written to RAM and then to disk"? As CommonSenseAI mentioned, cache flushing takes ffoorreevveerr - literally on the 30th minute right now flushing my 1TB NVMe SSD cache...