Hello
I am not sure what is best in how to use the SSD on my NAS. I have a 4 HDD hard drives NAS also fitted with 2 SSDs. Should I use these as a cache or as a RAID drives? What is the associated risks? I heard if data is in the cache while SSD fails, data is lost. Is it true? Or is data just duplicated in SSD for quick access?
HDDs are 4x8Gb and SSD are 2x1Gb. I use my NAS for storing data, pictures, movies and some VM…
Thanks for advising 
What is the NAS model ? … Do not use cache on QTS OS. (broken)
What is the usage case ? … Cache only helps in limited situations. Simple file backups/reads do not benefit from cache.
I also doubt that you have 4x8GB HDDs (I guess you meant 4x8TB HDDs)
My advice is. setup the NAS with the SSD’s as system volume (QTS) or system pool (QuTS), so they will be the default storage space for apps and system files (the OS is on all drives). Also run your VM’s from a volume you place on these drives.
Setup the HDD’s as RAID5 and use those for bulk storage.
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Modele is TS464eU running QTS OS.
What do you mean by broken on QTS OS?
I use my NAS for storing data, pictures, movies and some VM…
You are right: it is 4x8To HDD and 2x0.5To SSD: my mistake!
Thanks for your pieces of advises: very helpfull
This what broken cache means
https://forum.qnap.com/viewtopic.php?t=124852
Basically destaging is not working for many years and differing implementations.
It could help if you had VM images on HDD, but just put the VM on SSD be done with it
how it has been a long time since this problem is happening! I will use the SSD only for VM and OS then. Thanks a lot.
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