Can someone say whether unRAID is allowing drives to cache writes, or does it write through the cache?  I know from syslogs that it appears to query the drives' cache abilities, but what decisions -- if any -- does it make after that w/r/t using drive caches?

I thought that is something set up with drive defaults or a forced change with hdparm.

I know there is a switch to enable/disable this. It used to be marked as dangerous.

Not sure if it is write through/write back.

 

root@Atlas ~ #hdparm -W /dev/sdd

 

/dev/sdd:

write-caching =  1 (on)

The drive setup all seems to occur during the kernel setup of them, well before unRAID wakes up and inventories the drives.  The write caching is reported during the kernel setup, and appears to me to be 99% 'enabled', with only a rare case of 'disabled', probably the drive's decision (something faulty detected, or too old for caching, or previously configured that way).  I have never seen any evidence that unRAID uses, configures, or even acknowledges the drive caching.