I'm in the process of building and migrating my first Unraid install and so far everything is working great.  However, I did have a few noob questions on the use of the cache pool.  Currently I'm moving my HDD's over one at at time and I have a 2 SSD drives (one at 1TB, the other at 500GM, see second point below) that I set as a cache pool.  My question is mostly a clarification of the doc below, specifically the last bullet:

https://docs.unraid.net/unraid-os/manual/storage-management/#using-a-cache-drive

 

Quote

If there is not sufficient free space on the cache then writes will start by-passing the cache and revert to the speeds that would be obtained when not using the cache.

 

1) Does this mean that if the amount of data that is being written to a share (set to use Cache -> Array) exceeds the cache size it will automatically start writing directly to the array?  This makes sense and is what I'd like to confirm b/c I'm currently migrating en masse all of the data so I want to make sure that if I exceed the cache size I won't lose any data in the migration.  (even though I'm being careful to only transfer in smaller-chunks to try and stay below the cache size)

 

2) Re the 1 TB + 500GB pool, for some reason Unraid reports the total size as "750 GB" but the available space (i.e. used + remaining) at 500 GB, and based on my understanding 500 should be correct for a RAID1-like capacity.  Any thoughts why this is?

 

Thanks!

https://docs.unraid.net/unraid-os/manual/storage-management/#minimum-free-space-for-a-pool

also

https://docs.unraid.net/unraid-os/manual/shares/user-shares/#minimum-free-space

 

I usually recommend not caching the initial data load. If you run without parity until the initial load is complete it will be much faster.

 

 

Also

https://docs.unraid.net/unraid-os/manual/shares/user-shares/#primary-and-secondary-storage-unraid-612

1 hour ago, darwinsbeard said:

Does this mean that if the amount of data that is being written to a share (set to use Cache -> Array) exceeds the cache size it will automatically start writing directly to the array?  This makes sense and is what I'd like to confirm b/c I'm currently migrating en masse all of the data so I want to make sure that if I exceed the cache size I won't lose any data in the migration.  (even though I'm being careful to only transfer in smaller-chunks to try and stay below the cache size)

In effect yes.   It is a bit more subtle in that before even trying to put a file on the cache pool Unraid checks to see if the current free space on the pool is more than the Minimum Free Space value set for the pool (if not it bypasses the pool).   You should set the Minimum Free Space value for the pool to be larger than the biggest file you expect to transfer to avoid Unraid trying to use the pool and then finding the file will not fit.

Thanks both!  Very helpful links and responses.  Reading through those links and with respect to:

Quote

You should set the Minimum Free Space value for the pool to be larger than the biggest file you expect to transfer to avoid Unraid trying to use the pool and then finding the file will not fit.

 

What happens when a transfer 'fails' due to out of disk space (i.e. will not fit)?  For example, say I am moving (not copying) a 10 GB file from my Windows desktop to a cache-enabled share. The cache only has 9 GB free space, based on the above the transfer will fail due to out of disk, but:  Will the file disappear? or will it automatically just start writing to the array instead, or will it just fail but the move aborts and the file still resides on my desktop?  The latter 2 options being preferable obviously to the first which would be bad!

 

In a related scenario, if I'm moving 10 files, each 1 GB, would 9 of the files transfer and the last one is subject to the above? (I assume that's the case, each file is independently transferred)

10 minutes ago, darwinsbeard said:

instead, or will it just fail but the move aborts and the file still resides on my desktop?

This.  
 

Once Unraid selects a disk for a file it does not change its mind if it does not fit - it gives an out of space error.