appdata / domains / isos and system shares

[JoshFink]

I must have just read 20 different threads and other than seeing that appdata should be located as Cache:Only I'm confused on the others. 

 

At first I had planned on using my 500gb SSD for a true cache drive but then I noticed that write speeds to the array are good enough for me so I'm planning on just using the cache drive for dockers and maybe VM's. 500GB is probably too big but I can tackle that later. 

 

Should domains, isos and system shares be Cache:Only? If yes or no then why? 

 

Since I haven't started using any containers or vm's on here yet I'd love to get this taken care of before i do anything that then requires rework. 

 

Thanks for the help and sorry for the most likely easy question. 

[Squid]

Domains:  standard storage place for vm vdisks   should be cache only or cache prefer

ISO  used for installation media for the VMs    no real reason why it needs to be stored on the cache at all.   Only used once during initial install of the VM.    Set to use cache no or yes

System  used as the default locations for the docker image and the libvirt image.   Only because of the docker image it should be use cache only or prefer

 

 

Edited July 23, 2017 by Squid

[JoshFink]

Awesome.. Thank you

[mgutt]

Additional question:

Should I backup domains and/or system to the array if cache only/prefer is set?

Edited April 29, 2020 by mgutt

[itimpi]

2 hours ago, mgutt said:

Additional question:

Should I backup domains and/or system to the array if cache only/prefer is set?

I would think this would de a good idea  as it protects you against cache failure

[frakman1]

On 4/29/2020 at 1:06 PM, itimpi said:

I would think this would de a good idea  as it protects you against cache failure

 

That's what I thought too. But it's interesting that UnRaid's own documentation recommends against this:

 

Quote

IMPORTANT: Do NOT store your active virtual machines on a share where the Use Cache setting is set to Yes. Doing so will cause your VMs to be moved to the array when the mover is invoked.

 

But that's exactly what I want. To store the VM on the array. I mean, why wouldn't I want that? Is there something bad about this that I am missing?

Edited January 26, 2021 by frakman1

[itimpi]

28 minutes ago, frakman1 said:

But that's exactly what I want. To store the VM on the array. I mean, why wouldn't I want that? Is there something bad about this that I am missing?

The reason you do not normally want a VM on the array is that it badly affects performance as every write has the performance penalty of having to do additional writes to update parity.   Unraid will not stop you putting the VM on the array if you are prepared to take the performance hit.   If you really want the VM there then enabling ‘Turbo Write’ mode will help a little, but at the expense of keeping your drives all spun up any time the VM is running.

[frakman1]

6 minutes ago, itimpi said:

it badly affects performance as every write has the performance penalty of having to do additional writes to update parity.  

I don't understand this. If it is cached, then the reads and writes go to the cache and not the array and therefore not to parity. The mover moves it to the array once a day. That doesn't sound like a performance hit.

[itimpi]

2 minutes ago, frakman1 said:

I don't understand this. If it is cached, then the reads and writes go to the cache and not the array and therefore not to parity. The mover moves it to the array once a day. That doesn't sound like a performance hit.

If  the vdisk is on the array it is NOT cached. The caching works at the complete file level, not at writes within a file.
 

[mgutt]

3 hours ago, frakman1 said:

If it is cached, then the reads and writes go to the cache and not the array and therefore not to parity

"Yes" means "Create on cache, move to Array (later)". That's why you should choose "Prefer" which means "Create on cache if possible, if not, move to cache (later)" or "Only" which means "Create on cache". As "Only" can fail if the Cache is full, I suggest "Prefer".