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mattie112

Moving from Synology, what cache disk to use?

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Allright, first my current situation (synology on custom hardware) 

2x 240GB SSD (OS / VM)

3x 3TB HDD (data)

2x 120GB SSD (read/write cache for data volume above)

1x 4TB (offsite backup for a friend)

1x 1TB (temp stuff / spare )

 

I am looking into moving to Unraid as I see the benifits of adding more storage in an easy way (I am reaching capacity). I understand the whole process of having the biggest disk as parity and I see the unassigned drive plugin that I could use for my friend's disk (that won't need parity).

 

But I'm a bit in doubt of the cache disk. I currently have the disks for my VM's and the cache for the data volume separated but if my understanding is correct in Unraid you can only have a single (raid1) cache volume. I want my VM's to have the best performance (AND be protected against failure) so they must go on the cache volume. Let's say that I have 200GB of VM's. The will leave me with only 40GB as write-cache. But I also have 2x 120GB SSD left-over.

 

So what can I do?

- 240GB cache that leaves me with little write-cache space

- 120GB cache and 240GB 'unassigned' but that leaves me with no drive failure protection for my VM's

- 120GB cache and adding the 240GB SSD's to the array and force the VM disks on that SSD

 

The last option seems the most economical but while the read speeds would be good I can only assume the write speeds would still be bad?

 

Is there any other way to configure this? Or should I just accept it and go for 2x 500GB SSD's that will have plenty of VM storage and write-cache storage. Thanks in advance for any information. Really starting to like Unraid!

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2x 240GB SSD (OS / VM)

3x 3TB HDD (data)

2x 120GB SSD (read/write cache for data volume above)

1x 4TB (offsite backup for a friend)

1x 1TB (temp stuff / spare )

 

You have to have a 3tb as the Parity drive at a minimum... and if you have the 4tb as an unassigned "spare/unprotected drive for a friend" I would do the following

 

3tb Parity

Data Drives

2x 240 VM drives

2x 3tb 

1tb 

 

Cache

2x 120 SSD

 

the VMs would read data from the fast SSD in protected data store... and any changes to those files would be written to the Parity... IMO that would be your best configuration... 

 

There is no way around it... you have to use the 3tb as parity due to design... the parity has to be = to or more then the rest of the raid...

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Yes the parity bit I understand. Currently the 3x 3TB are in use with a RAID 5 configuration so there I am also losing 1 disk. It is just that I am doubting a bit on the cache. Is having SSD's in the array for VM's a good way or will they still suffer from slow writing speed. Of course I can just test it but perhaps someone already have some experience with this?

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Posted (edited)

The cache pool by default uses btrfs RAID 1, which means you can mix different sized SSDs. You just lose 1/2 of your usable capacity. So you can put your 2x 240GB and 2x 120GB SSDs into the same cache pool, and you will have 360 GB of usable cache pool space.

 

Use this site to calculate usable capacity:

https://carfax.org.uk/btrfs-usage/

 

 

Edited by Phoenix Down

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the reason I put your VM SSDs into the data drive pool is that you said you wanted them protected... but if there is something im not understanding... so be it...

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16 hours ago, Phoenix Down said:

The cache pool by default uses btrfs RAID 1, which means you can mix different sized SSDs. You just lose 1/2 of your usable capacity. So you can put your 2x 240GB and 2x 120GB SSDs into the same cache pool, and you will have 360 GB of usable cache pool space.

 

Use this site to calculate usable capacity:

https://carfax.org.uk/btrfs-usage/

 

 

Ah that is great, I thought only 2 disks in a single pool could be added. I don't mind "wasting" a bit of extra space because for now I don't need to purchase larger disks. (But when I upgrade those I will go for larger of cource)

12 hours ago, mathomas3 said:

the reason I put your VM SSDs into the data drive pool is that you said you wanted them protected... but if there is something im not understanding... so be it...

Yes correct, but the cache pool with 2 disks (and so it seems also 4 disks) uses (a) RAID1 (solution) so that also has protection.

12 hours ago, trurl said:

Not correct.

 

See here for the different btrfs raid configurations available:

https://forums.unraid.net/topic/46802-faq-for-unraid-v6/?do=findComment&comment=480421

 

Also, latest beta allows multiple cache pools.

 

Thank you for this information! I will definitely try it out!

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