A question about large file copies and the cache drive.

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So I'm about to start copying a fairly large amount of data (around 2TB) to my unRAID server, and it has a 128GB SSD for a cache drive.

My question is: Should I leave the cache drive enabled? Once its filled the cache drive, will unRAID automatically start writing directly to the array, or will it error with a 'disk full' message or something? Or am I better off disabling the cache and writing directly to the array, albeit at a much slower rate?

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Personally, I would turn off the cache until you are finished with this transfer.  (I think it handles it gracefully but why take a chance when you already know that the cache is not going to gain you much in the total process.)   However, I would go to     Settings   >>>   Disk Settings     and change the "Tunable (md_write_method):" method to 'reconstruct write'.  While this will spinup all of the disks in the array for the duration of the copy, it will about double the speed of the array write process.  That increase will make the process much more bearable.  

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I concur with everything Frank1940 said, but I am going to say a little more about your questions and about how unRAID works anyway. See especially what I have to say below about Minimum Free since that may also come into play with a large transfer such as you are doing.


1 hour ago, Frank1940 said:

I think it handles it gracefully


Whether or not it handles it gracefully depends on several settings.


Each user share has its own settings, some of which control whether and how that user share will use cache. Here is a good explanation of the Use cache settings:




As you can see at that post, cache-yes user shares do overflow to the array.


But it is also important to have Minimum Free set for each user share. If a disk has less than Minimum Free when unRAID is beginning to write to a user share, it will choose a different disk. Mimimum Free should be set larger than the largest file you expect to write to the user share or it could begin a write to a disk that it cannot finish because it runs out of space and you will get an error.


Cache also has a Minimum Free setting in Global Share Settings which comes into play when cache needs to overflow.


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