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How can i add virtual SSD to DSM for cacheing? (ESXi 6.7 + Gen8 Microserver)


Hope someone can help me how can i add virtual SSD for cache to DSM.. 


My setup is:


Gen8 Microserver 

ESXI 6.7

Jun 1.02b loader


16GB MicroSD (ESXI installation) 

250GB SSD (50 Virtual disk to Syno, another 100 to Windows Server) 

120GB SSD (host swap, i would create 2x20GB virtual SSD to this drive) 

2TB HDD (1.5TB virtual HDD to Synology, what i would cache with 1*20GB virtual SSD)

4TB HDD (3.5TB virtual HDD to Synology, what i would cache with the another 1x20GB virtual SSD) 


Syno VM config: loader, 50mb as virtual SCSI device, virtual disks as SATA drives. 


Loader working in barmemetal mode, i cant see the 50mb loader disk, SATA mapping is good, everything is fine, it's upgrading good, no errors so far. 


Just i can't add drives to the VM as SSD, the DSM recognize all drives as vmware virtual HDD. 

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Posted (edited)

As far as I know, virtual SSD can't be used as SSD cache.

In my experience, SSD cache can only be used with pass-through-ing host SATA I/F and SSDs.

If you tried with success, please post about that.



BTW, My recommended configuration for ESXi system is following.


In the following config, whole system is hosted with SSD cache & (almost) all drives are managed by DSM.

It performs well and notifies me on any disk troubles. I'm satisfied with this configuration. 

(I think) It's not so complex, but has enough performance & good flexibility. 


I hope this helps.



My recommended XPEnology based configuration of ESXi system:

  • boot ESXi from USB drive (as you do)
  • Add 1 disk for VMFS datastore (& use it's disk inteface directly for ESXi). (this datastore is just for booting "Host DSM" VM) (*1)
  • Make 1 XPEnology VM as "Host DSM" VM and pass through All disk interfaces (other than above one) to the VM.
    (This VM is used only for ESXi datastore & ESXi host.)
  • Add all other HDDs / SSDs to make XPEnology VM & setup SSD cache & format disk group with ext4 (for VM performance). 
  • Create share folder for ESXi datastore using nfs export (better for performance & good maintainability with SMB access.
    you can add SMB access for direct maintenance of the datastore from client PCs.)
  • Add that nfs exported datastore from ESXi
  • Add your own VMs on that datastore (*2)
  • Add Another XPEnology VM ("User DSM" VM) with thick provisioned vmdk, formatting with btrfs (for usual file sharing, etc.).
    Add users & apps only on "User DSM". (*3)


*1) If you don't use USB sharing for VM, you can use USB disk for this datastore, perhaps.

*2) I can also add Windows/MacOS Desktop VM with pass-through-ing GPU and USB. (Choose ESXi 6.0 for hosting mac. You can still use vCenter 6.5 or later.)

*3) The only I wish but I can't is H/W encoding with DSM6.1 + DS916 VM.  (Perhaps, you have to pass-through host's iGPU. I don't have such iGPU system.)


Edited by benok
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Thanks for the answer.

But I'm using only 2TB-4TB HDDs, and if i want to use cache with pass-through, i have to use one SSD/HDD.

As i experienced before, the DSM used only max 20GB cache for one HDD, so a 120GB SSD could cache 3-4 HDDs..

I don't have place in my server to use 1SSD/HDD for caching :/

Tapatalkkal küldve az én SM-N950F eszközömről

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I don't know such limitation exists. Is there before 5.2 ?

I used 240gb x2 SSD for read/write cache for 3TBx4 RAID5 for around 2 years ? since DSM5.2.

Last year I upgraded to another system with 1TB x2 cache and it also works fine.

I think you should check the recent document again. It almost only says about the memory requirements.



DSM caches frequently used random access area automatically within the cache size.

(I also recommend not to enable sequential cache, as document says. It just works fast only for a while.) 


Optimal cache size depends on your workload.

I'm using 10-15 VMs constantly, but my cache usage is just 45%.

(Before upgrade, cache usage was very high. I can't recall the real number.)

My SSD cache seems to be over spec for my current workload.


If we can use Virtual SSD for SSD cache, we could share SSD for cache and datastore,

but I think it's not so good both for complexity & performance loss.

I didn't have investigated,  but I guess virtual SSD don't have required command for SSD cache

or returns bad response for some command, and DSM refused VSSD.

You should log in to console via ssh  and check logs around enabling SSD cache.

We might get work with flag tweak around VSSD, if such flags exists...


I don't have good idea for small factor server like your HP Gen8.

I built my system with mid-tower PC case as I wanted to use it as both for workstation & server.

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