On 26/11/2023 11:55, Hannes Reinecke wrote:
On 11/26/23 11:37, Andy Smith wrote:
Hello,
On Sun, Nov 26, 2023 at 11:18:39AM +0100, Gandalf Corvotempesta wrote:
i'm doing some maintenance replacing some not-yet-failed HDD with WD
RED SSD.
I've read the warning on the homepage
https://raid.wiki.kernel.org/index.php/Linux_Raid
Are SSD drives still subject to SMR ? I've thought that it was related
only to magnetic drives not on SSD.
SMR is not applicable to flash-based storage. I expect the warning
on the wiki was written at a time when the only drives bearing the
WD Red brand were HDDs, not any SSDs.
Yes. The WD Red line was always advertised as "optimised for raid"
(which was debatable, but okay), then they switched it to SMR. So the
notice was saying "keep away !!!". I think several new SMR Reds failed
badly when people tried to rebuild their array.
Correct. Typically SATA SSDs do not expose SMR capabilities; it's all
hidden by the FTL there.
If you look at what SMR is, it's only relevant to spinning rust. It
relies on the fact that a read head can be much smaller than a write
head, so provided you shingle your writes (hence the name), you can
over-write half the previous track (so saving space) without rendering
the data unreadable.
The problem is you have to stream your writes, you can't go back, which
is why these drives - especially the early ones - had a habit of
stalling for minutes on end, as they shuffled stuff around.
And the warning really is outdated. What matters is that the HDD needs
to support TLER (ie scterc capabilities); the brand or series really is
immaterial. There are plenty of non-SMR HDDs which do not support it.
Sadly it's not well advertised, so you really have to test
(or invest in the HDD range which is geared up for this kind of usage.)
I'm not sure how true that is now, I've not been keeping up. But about
that time (2020), most desktop drives - the sort that don't have TLER -
were transitioning to SMR. Hence the warning following immediately -
don't use desktop drives! I suspect non-SMR is now a premium feature
which will usually come with TLER as a matter of course.
As I say, I'm not up-to-date any more. If anybody can give me some
facts, I'll happily update the site.
Cheers,
Wol
