Cache Drives

[SLNetworks]

Why do my cache drives show up as one? I have two 250GB cache drives, but cache is only recognized as 250GB instead of 500GB.

[Hoopster]

46 minutes ago, SLNetworks said:

Why do my cache drives show up as one? I have two 250GB cache drives, but cache is only recognized as 250GB instead of 500GB.

Did you just install these cache drives?  How did you configure them?  The default (if you changed nothing) is to format a cache pool with more than one drive as a BTRFS RAID1 pool.  RAID1 is a redundant mirror configuration in which case two 250GB cache drives gives you 250GB of usable space.

 

There are other options if you want all the space available without redundancy.

 

Did you just do an upgrade and now they show up differently than before.  There is not a lot to go on so far.

 

Post your diagnostics (Tools --> Diagnostics) for more detailed assistance from one of the resident experts.

[SLNetworks]

I looked around to see if there was a pool setting, I didn't see anything.. but maybe I was looking in the wrong spot. I put them in when I built the server, changed nothing, so it's safe to say they're in Raid1.

 

I have mover set, but it seems to help very little. Trans... seems to use it as a spot to save to, but I can't verify that. My only hint was that the notifications popping up were saying I'm outta disk space, but I have 17TB left.

[trurl]

9 minutes ago, SLNetworks said:

I have mover set, but it seems to help very little. Trans... seems to use it as a spot to save to, but I can't verify that. My only hint was that the notifications popping up were saying I'm outta disk space, but I have 17TB left.

 

Sounds like you don't really know how cache is supposed to work.

 

16 minutes ago, Hoopster said:

Post your diagnostics (Tools --> Diagnostics) for more detailed assistance from one of the resident experts.

 

[SLNetworks]

It's not like I wanted my downloads to be defaulted to my cache.

 

I'm sure that the cache should be used to used to access commonly accessed files. But downloads though? Really? *facepalm* That kinda data should be going straight to mechanical drives.. unless your server is completely populated with SSDs.

 

So, rather than request diagnostic data, tell me how I can split the drives so they aren't in Raid1 along with telling me how I can change my downloads to be saved straight to mechanical drives.

 

Thanks.

[Hoopster]

27 minutes ago, SLNetworks said:

I'm sure that the cache should be used to used to access commonly accessed files

This is not what the cache drives in unRAID do.  There is no concept in unRAID of caching frequently accessed files

 

The cache drive/pool has the following functions:

1.  Cache initial writes the mechanical array drives so data can be written to the array faster, by bypassing parity.  The Mover then runs at a low-usage time (default is 2am) and moves the data to the mechanical drives and writes to parity which is a much slower write process.
2.  Provide a faster access location for appdata (docker applications) where frequently accessed application-specific data such as databases, config files, codecs, etc. can be located so that accessing these files does not cause mechanical array disks to be spun up.  The only thing that will spin up the disks is accessing the data (photos, movies, documents, etc.) that these applications manage.
3.  A potential fast access location for VM virtual disks and OSes although some prefer to have those on an unassigned device rather than on the cache drive.
4.  The location for cache-only shares which is the closest idea conceptually to your understanding of cache.  Some unRAID users choose to create special cache-only shares for subsets of data that they want to live on a cache SSD for fast access and never cause a mechanical disk to spin up.  For example, a subset of new or frequently watched movies with the larger collection living on the mechanical disk array.

Whether or not a share uses the cache disk/pool and how is determined by share settings which are:

Use cache - Yes

Use cache - No

Use cache - Prefer

Use cache - Only

 

Set each share to use cache in the way you wish.

 

If you want a share, such as downloads for example, to write directly to the array and never use the cache drive, the setting on that share should be Use cache - No.

 

Start with this forum post and read it and the post that follow it from JorgeB to get a better understanding about how to break the current cache pool and create it differently.

 

Edited March 4, 2021 by Hoopster

[SLNetworks]

Thanks for that information.

 

That leads me to this - why are cache drives pooled instead of being used individually? I mean, I at least expected that more cache space would be better than less.

[Hoopster]

1 minute ago, SLNetworks said:

 

That leads me to this - why are cache drives pooled instead of being used individually? I mean, I at least expected that more cache space would be better than less

You have the option of either configuration.

 

Lets say you have 4 SSDs you want to use for one or more of the above-mentioned uses.  You can configure them in the following ways in unRAID 6.9.0 (6.8.3 is more limited):

• Four separate, individual cache disks. You can assign different cache disks to different shares.  A "pool" can be just one disk.
• A single cache pool containing all 4 SSDs.  This pool can be configured as a RAID1 mirror with half the storage used for caching used and half as a redundant backup or a pool that has the capacity of all four SSDs combined with no redundancy (RAID0)
• A hybrid combination of the above with, for example, two individual single disk "pools" and one pool with 2 SSDs in one of the supported RAID configurations.

Given your situation, you might also find this thread to be of interest.

[trurl]

 

In 6.9 you can have multiple pools and each can have 1 or more drives.

 

If you want yours separate then create a separate pool.

 

Post Diagnostics for more detailed advice. 

[SLNetworks]

32 minutes ago, Hoopster said:

You have the option of either configuration.

 

Lets say you have 4 SSDs you want to use for one or more of the above-mentioned uses.  You can configure them in the following ways in unRAID 6.9.0 (6.8.3 is more limited):

• Four separate, individual cache disks. You can assign different cache disks to different shares.  A "pool" can be just one disk.
• A single cache pool containing all 4 SSDs.  This pool can be configured as a RAID1 mirror with half the storage used for caching used and half as a redundant backup or a pool that has the capacity of all four SSDs combined with no redundancy (RAID0)
• A hybrid combination of the above with, for example, two individual single disk "pools" and one pool with 2 SSDs in one of the supported RAID configurations.

Given your situation, you might also find this thread to be of interest.

 

btrfs balance start -dconvert=raid0 -mconvert=raid1 /mnt/cache
ERROR: cannot access '/mnt/cache': No such file or directory

 

For an older version of Unraid?

Edited March 4, 2021 by SLNetworks

[mgutt]

1 hour ago, SLNetworks said:

That leads me to this - why are cache drives pooled instead of being used individually?

Because files are on the cache OR on the array. If you use only one SSD and it fails, all these files will be lost.

[trurl]

8 hours ago, SLNetworks said:

For an older version of Unraid?

The usual method for separating these in versions before 6.9 was to set one up as an Unassigned Device. Unassigned Devices can't participate in user shares, but that isn't really a problem if all you want to do is have fast storage for dockers and VMs.

[RoboCanvas]

When does it make sense to have more then one pool of cache drives? I have 2 pools now (one for Vms and one for everything else). The first pool is 2x nvme (1tb each) and the second is 4x SSD (500gb each).

 

Would it make more sense to have 1 large pool and let unRaid figure it all out?

[trurl]

2 pools, one for caching user share writes, one for dockers and VMs and anything else you want to permanently keep on fast storage. That is exactly how I do it.

[babz]

Hi Sorry, for reviving an old thread. I have an issue with my Unraid cache pool.

 

I have 3 1TB drives running in BTRFS Raid 1 configuration. I was expecting the total stoarage to be 1.5TB. This seems to be correctly displayed in the Size column, however once adding up taken and free space, I only have 1TB of space. Am I doing something wrong?

 

 

Using the BTRFS pool calculator which I have found on one of the other threads, the suggestion is that I should have a total drive of 1.5TB. Am I doing something wrong here?

 

 

Any suggestions?

[ChatNoir]

9 hours ago, babz said:

Am I doing something wrong here?

No, BTRFS RAID is reporting free space wrong on pools with an odd number of devices. Not much you can do about it.