How long does it take data to be written to the hard drives in a Read/Write SSD Cache


I have a 920+ and have a R/W Cache with NVME SSDs.
When I write a file and it ends up in the cache how long will it take for that file to be written to the main hard disk volume?

I couldn’t think of any information regarding that on Synology’s pages.

It would be interesting to know how much of a buffer the SSDs are when writing lots of small files to the NAS.


It’ll be transferred to the RAID as soon as the file copy to the SSD Cache is complete.

One thing to understand here is that it’s not the entire file thats being written to the NVMe SSD when you are using a Read / Write cache, but only parts of it that are showing up as random writes.

If you were to write a sequential file to the NAS, without anything else going on, none of the data would end up being written to the write cache.

But that does not mean that file that has just been written to the HDD volume is safe, if the write cache crashes. The write cache will likely have important pool metadata written to it, that has not yet been written to the HDD volume. If this metadata is lost then the pool will go into a read only mode (at best) or completely crash at worse!