Disclosure: I'm the one who put all this together for media after purchasing three reds and finding repeatable results
DM-SMR drives aren't just "a little" slower - and to top it off
DM-SMR REDs have a serious firmware bug which makes them throw hard IO errors at the host system
How much slower?
Well if you put one in an existing parity RAIDset (RAID5/6 or RAIDZ1/2/3) to replace a failed or aging drive, you can see 90-98% degradation in rebuild times
RAID rebuilds which used to take a day or less can easily take 8-19 DAYS
RAID scrub/verification passes can take 8-12 DAYS
In the case of WD REDs, they won't get that far - because during the raid rebuild process they will generate drive internal errors:
smartctl -x will show drive internal error logs like this:
" 10 -- 51 00 00 00 00 1f 3b c0 b0 40 00 Error: IDNF at LBA = 0x1f3bc0b0 = 524009648 "
Linux reports them like this (samples):
"blk_update_request: I/O error, dev sdd, sector 734925768 op 0x1
WRITE) flags 0x700 phys_seg 1 prio class 0
blk_update_request: critical target error, dev sdd, sector 734925408 op 0x1
WRITE) flags 0x700 phys_seg 1 prio class "
And the results above ARE ON AN IDLE ARRAY - the only thing going on is the rebuild or the scrub
It goes well beyond DM-SMR in NAS drives.
Here's why DM-SMR in "desktop drives" is a spectacularly bad idea:
If you know what you're doing and treat them "just right", then DM-SMR drives will have a performance hit of between 40-60% - which isn't minor.
The problem is this: Virtually every filesystem (except FAT) writes a metadata update back to the filesystem every time a file is read, updating the last read time of that file.
ie: Every file read, causes a write
THIS is the WORST POSSIBLE CASE for DM-SMR drives - and why operators MUST know what they have, as you need to be able to turn off "last accessed" times.
On linux, that's easy: "noatime" mount options - but doing this on the OS mounts and in areas where atime monitoring is critical WILL cause secondary issues
On Windows, it appears to be an "all or nothing" proposition across the entire system, which destroys the functionality of a lot of system automation utilty builtins.