Yorick
Wizard
- Joined
- Nov 4, 2018
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Hard drives that write data in overlapping, "shingled" tracks, have greater areal density than ones that do not. For cost and capacity reasons, manufacturers are increasingly moving to SMR, Shingled Magnetic Recording. SMR is a form of PMR (Perpendicular Magnetic Recording). The tracks are perpendicular, they are also shingled - layered - on top of each other. This table will use CMR (Conventional Magnetic Recording) to mean "PMR without the use of shingling".
SMR allows vendors to offer higher capacity without the need to fundamentally change the underlying recording technology.
New technology such as HAMR (Heat Assisted Magnetic Recording) can be used with or without shingling. The first drives are expected in 2020, in either flavor.
SMR is well suited for high-capacity, low-cost use where writes are few and reads are many.
SMR has worse sustained write performance than CMR, which can cause severe issues during resilver or other write-intensive operations, up to and including failure of that resilver. It is often desirable to choose a CMR drive instead. This thread attempts to pull together known SMR drives, and the sources for that information.
There are three types of SMR:
- Drive Managed, DM-SMR, which is opaque to the OS. This means ZFS cannot "target" writes, and is the worst type for ZFS use. As a rule of thumb, avoid DM-SMR drives, unless you have a specific use case where the increased resilver time (a week or longer) is acceptable, and you know the drive will function for ZFS during resilver. See (h)
- Host Aware, HA-SMR, which is designed to give ZFS insight into the SMR process. Note that ZFS code to use HA-SMR does not appear to exist. Without that code, a HA-SMR drive behaves like a DM-SMR drive where ZFS is concerned.
- Host Managed, HM-SMR, which is not backwards compatible and requires ZFS to manage the SMR process.
I am assuming ZFS does not currently handle HA-ZFS or HM-ZFS drives, as this would require Block Pointer Rewrite. See page 24 of (d) as well as (i) and (j).
The list of SMR drives known to the community is in the "Overview" tab. This is also where the referenced sources are.
SMR allows vendors to offer higher capacity without the need to fundamentally change the underlying recording technology.
New technology such as HAMR (Heat Assisted Magnetic Recording) can be used with or without shingling. The first drives are expected in 2020, in either flavor.
SMR is well suited for high-capacity, low-cost use where writes are few and reads are many.
SMR has worse sustained write performance than CMR, which can cause severe issues during resilver or other write-intensive operations, up to and including failure of that resilver. It is often desirable to choose a CMR drive instead. This thread attempts to pull together known SMR drives, and the sources for that information.
There are three types of SMR:
- Drive Managed, DM-SMR, which is opaque to the OS. This means ZFS cannot "target" writes, and is the worst type for ZFS use. As a rule of thumb, avoid DM-SMR drives, unless you have a specific use case where the increased resilver time (a week or longer) is acceptable, and you know the drive will function for ZFS during resilver. See (h)
- Host Aware, HA-SMR, which is designed to give ZFS insight into the SMR process. Note that ZFS code to use HA-SMR does not appear to exist. Without that code, a HA-SMR drive behaves like a DM-SMR drive where ZFS is concerned.
- Host Managed, HM-SMR, which is not backwards compatible and requires ZFS to manage the SMR process.
I am assuming ZFS does not currently handle HA-ZFS or HM-ZFS drives, as this would require Block Pointer Rewrite. See page 24 of (d) as well as (i) and (j).
The list of SMR drives known to the community is in the "Overview" tab. This is also where the referenced sources are.
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