lionceau Posted December 23, 2016 Share Posted December 23, 2016 I'm no expert on the matter but surely it can't be a good idea to use SMR drives as parity? They have two levels of cache (128MB DRAM + 20GB on-disk) managed by a firmware that's optimized for archival (few, large sequential writes). The screenshots suggest Parity workload is the exact opposite. Any time a data disk is written to' date=' the parity disk needs to be updated as well. Each write to a parity-protected unRAID data disk results in 4 disk operations: a read and write for parity, and a read and write for data. The platter of each disk has to make a full revolution after reading to position the disk head back over the sector being written.[/quote'] That seems like a huge headache waiting to happen once the SMR drives have been populated with data that needs to be overwritten while the cache is full. My ST8000AS0002 dropped to <10 MB/s sequential write speed in those cases. Quote Link to comment
Karyudo Posted December 24, 2016 Author Share Posted December 24, 2016 Which screenshots? Right now, my parity is being rebuilt on SMR drives at 88 MB/s. Not the speediest, but not the "less than 10 MB/s" you were getting. Whether I wait 24 hours or just 12 hours (say) for the parity to be written doesn't really affect me. Day-to-day, I've never noticed an issue with write speeds to SMR-based parity. Downloading (even at 150 Mbps) and unpacking seems to happen much more slowly than moving, so I guess I never fill up the cache on the drives? I figure if I start to notice abysmal speeds "on the regular," I'll consider spending some more money and getting some non-SMR 8TB parity drives instead. But SMR drives are so much cheaper that I think I'll be willing to tolerate quite a lot of speed drop before that happens. The aversion to SMR drives in all the forums I read to me feels a little, "sure it works in practice, but can you make it work in theory?" I can't disagree that on paper, using SMR drives for parity looks sub-optimal. But in practice, I've never noticed or been bothered by it. Quote Link to comment
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