Hard Drives

From Leo's Notes
Last edited on 15 June 2020, at 00:47.

SMR[edit | edit source]

Shingled Magnetic Recording (SMR) drives writes tracks that partially overlap to increase the number of tracks per platter. The designed usage is for write once, read many (WORM) data since the cost of re-writing data is significant. Similar to a SSD, rewriting data also requires rewriting a larger superset of the data. In the case of SMR drives, tracks adjacent to the data being rewritten as well as adjacent tracks to those tracks need to be rewritten. As such, a rewrite operation will require the disk to re-read all affected tracks before the entire affected section is rewritten resulting in high latency and poor performance.

Drive-managed SMR (DM-SMR) drives mask the poor write performance by buffering writes to a non-SMR portion of the disk and then rewriting to shingled area of the disk when idle. Under heavy writes, this buffer may fill up and results in very time consuming write operations. The sudden decrease in write performance and substantial increase in disk latency especially when resilvering can cause certain RAID arrays to think the disk is faulty or failing and removes it from the array.

Although read performance is typically similar to that of Conventional Magnetic Recording (CMR) drives, it isn't always the case especially when write operations are in progress.

More recently, Western Digital and Seagate have admitted to using SMR on disks that were not advertised as SMR. Western Digital Reds which are promoted for NAS applications are also included and have resulted in many unhappy users. See also:

Some claim that WD SMR drives can be detected if a normal hard drive supports the TRIM function. See: https://github.com/openzfs/zfs/issues/10182

Manufacturers[edit | edit source]

See: https://en.wikipedia.org/wiki/List_of_defunct_hard_disk_manufacturers

The major players now are: Seagate, Western Digital, and Toshiba.