Re: [gentoo-user] [OT] SMR drives (WAS: cryptsetup close and device in use when it is not)

[Rich Freeman]

On Fri, Jul 30, 2021 at 1:14 AM William Kenworthy <billk@×××××××××.au> wrote:
>
> 2. btrfs scrub (a couple of days)
>

Was this a read-only scrub, or did this involve repair (such as after
losing a disk/etc)?

My understanding of SMR is that it is supposed to perform identically
to CMR for reads.  If you've just recently done a bunch of writes I
could see there being some slowdown due to garbage collection (the
drive has a CMR cache which gets written out to the SMR regions), but
other than that I'd think that reads would perform normally.

Now, writes are a whole different matter and SMR is going to perform
terribly unless it is a host-managed drive (which the consumer drives
aren't), and the filesystem is SMR-aware.  I'm not aware of anything
FOSS but in theory a log-based filesystem should do just fine on
host-managed SMR, or at least as well as it would do on CMR (log-based
filesystems tend to get fragmented, which is a problem on non-SSDs
unless the application isn't prone to fragmentation in the first
place, such as for logs).

Honestly I feel like the whole SMR thing is a missed opportunity,
mainly because manufacturers decided to use it as a way to save a few
bucks instead of as a new technology that can be embraced as long as
you understand its benefits and limitations.  One thing I don't get is
why it is showing up on all sorts of smaller drives.  I'd think the
main application would be for large drives - maybe a drive that is
14TB as CMR could have been formatted as 20TB as SMR for the same
price, and somebody could make that trade-off if it was worth it for
the application.  Using it on smaller drives where are more likely to
be general-purpose is just going to cause issues for consumers who
have no idea what they're getting into, particularly since the changes
were sneaked into the product line.  Somebody really needs to lose
their job over this...

-- 
Rich