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- Feb 6, 2014
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Testing with Windows there was a huge difference between those two.
No surprise here. Windows might be bloated but even the worst of 10/11 would be able to fit the vast majority of itself into the cache area of an SSHD. The nature of single-user access and the persistence of the cache also means hit rates will be much higher there vs. a multi-user access scenario like TrueNAS.
Can TrueNAS handle those SSD Cache from those disk? How to enable it?
System has it's own UPS.
Much like DM-SMR the MLC cache on these drives is handled entirely by firmware voodoo and there's no way for ZFS or any other software solution (outside of something vendor-controlled and probably under NDA) to adjust its behaviours.
The read side will behave as an "L3ARC" victim cache, in that frequent reads from the same (unchanging) disk LBAs will end up getting served from the MLC cache, but if that data is ever updated, the copy-on-write nature of ZFS will mean the disk will consider it as "new data" because it'll be written to a new LBA, invalidating any kind of internal MFU/MRU listing the firmware has.
Random writes might be accelerated by the ability of the drive to potentially soak larger amounts of writes into that NAND buffer, and it could then spin the writes off to the disk later; but again, this is all supposition inside the black-box of a vendor-supplied solution.
TL;DR the caching is all done on-disk via firmware, we can't really see what it's doing, and it's likely not optimized for a multi-user copy-on-write filesystem.