Hi AfroDieter, thank you for your detailed posts and brining this to QNAP's attention. I also have noticed that QNAP is not flushing my cache acceleration properly. I have ticket open, and they've escalated the ticket on Thursday, and I'm waiting for HQ to connect to my QNAP remotely to try and help resolve the issue. However, I bought a new QNAP TVS-672XT to handle Plex and storage (I wanted a new one anyway) and I STILL can't get the cache acceleration to flush on it's own. On top of that, when I visit the cache acceleration page the hit rate is always super high. 100% for hours and hours, and there is NO activity going on the NAS. The first tech support person wanted me to to change cache acceleration to Random I/O with 32MB sectors, but that defeats the purpose of me being able to use this as a file sharing storage and copy large files at 1GB/sec with my 10Gbe connection. The speed dropped down to 200MB/sec.AfroDieter wrote: ↑Mon Sep 23, 2019 7:46 am OK guys, I'm sorry this post is going to be so long, but if you actually give a ** about getting this cache performance thing solved, you may want to plow through it anway.
TL;DR: I think I'm coming to an understanding of why the performance starts out great with the cache and then hits a brick wall after some time, and I think I may be on the right path for finding a solution that avoids this performance crash. Read on for the gory details.
@joshuacant, my personal belief right now is as follows, and I'm doing research and investigation (and learning) to confirm or deny it, so here goes:
I believe that the QNAP device is not properly flushing the cache of dirty blocks. Over time the number of dirty blocks grows and grows until the entire cache is full of dirty blocks, upon which all subsequent writes will force the dirty blocks to first be written out to disk, then the blocks are free to be overwritten with new data. This is probably what people are seeing when they say that upon filling the cache performance plummets.
I'm new to all of this, so I've been doing some intense Googling, reading, and poking around on my TS-963X.
...........
At this point, I don't know what to do. I've been living with the issue for about a year, and I have to manually login to the NAS and flush the cache acceleration weekly. It's really annoying, and I wish they would solve the issue.
If QNAP doesn't help, or they can't solve the problem, here is going to be my work around:
1. Disable cache acceleration
2. Create a new mount/drive with my 1TB Samsung 970 EVO
3. After I'm done downloading large files, copy them to the Samsung SSD for the day. (Add these folders to Plex.)
4. Every night I will have "SyncBackSE" move the files from the Samsung SSD to my regular QNAP storage.
5. Plex will see the file(s) moved from the SSD to the regular storage and update the file locations for my video files.