因之前的事（見：新換的硬碟很慢）了解到我買的平價硬碟原來寫入效能差是因為SMR技術所致，這種硬碟也不適合用在NAS。但剛才發現家中主力NAS的主硬碟正正就是SMR，幸好這部NAS加入了SSD CACHE，我猜問題也不大吧，日後有錢有時間就換掉它。
說回Shingled magnetic recording / 疊瓦磁記錄 (SMR)技術吧：
Shingled magnetic recording (SMR) is a magnetic storage data recording technology used in hard disk drives (HDDs) to increase storage density and overall per-drive storage capacity....While SMR drives can use DRAM, flash memory, and even a portion of their own platter reserved for use with CMR instead of SMR, as a cache to improve writing performance, continuous writing of large amount of data is noticeably slower than with CMR drives.
Wiki - Shingled magnetic recording
SMR可以簡單理解為一種在硬碟上利用增加磁軌密度來增加容量的技術，但缺點就是寫入速度比傳統磁記錄(Conventional magnetic recording, CMR)方式慢，NAS廠商不建議使用SMR技術的磁碟用於NAS中。
伸延閱讀：
- CMR vs. SMR Hard Drives: What's the Difference? (howtogeek.com)
- PMR 和 SMR 硬碟分別是什麼？ - Synology 知識中心
- Will the WD SMR based hard drives affect the RAID rebuilding speed? | QNAP (IN)
可能大眾對硬碟廠商存疑，所以硬碟廠商也公佈了哪些硬碟用哪種磁記錄技術。目前我找到一些：