linux-kernel - Re: [PATCH] scsi: sd: stop SSD (non-rotational) disks before reboot

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Message-ID: <20200705221929.GD8285@khazad-dum.debian.net>
Date:   Sun, 5 Jul 2020 19:19:29 -0300
From:   Henrique de Moraes Holschuh <hmh@....eng.br>
To:     Ming Lei <tom.leiming@...il.com>
Cc:     Damien Le Moal <Damien.LeMoal@....com>,
        Simon Arlott <simon@...iron.net>,
        "James E.J. Bottomley" <jejb@...ux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Jonathan Corbet <corbet@....net>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>
Subject: Re: [PATCH] scsi: sd: stop SSD (non-rotational) disks before reboot

On Tue, 30 Jun 2020, Ming Lei wrote:
> On Wed, Jun 24, 2020 at 5:01 AM Henrique de Moraes Holschuh
> <hmh@....eng.br> wrote:
> > Cache flushes do not matter that much when SSDs and sudden power cuts
> > are involved.  Power cuts at the wrong time harm the FLASH itself, it is
> > not about still-in-flight data.
> >
> > Keep in mind that SSDs do a _lot_ of background writing, and power cuts
> 
> What is the __lot__ of SSD's BG writing? GC?

GC, and scrubbing.

> > during a FLASH write or erase can cause from weakened cells, to much
> > larger damage.  It is possible to harden the chip or the design against
> > this, but it is *expensive*.  And even if warded off by hardening and no
> > FLASH damage happens, an erase/program cycle must be done on the whole
> > erase block to clean up the incomplete program cycle.
> 
> It should have been SSD's(including FW) responsibility to avoid data loss when
> the SSD is doing its own BG writing, because power cut can happen any time
> from SSD's viewpoint.

Oh, I fully agree.  And yet, we had devices from several large vendors
complaining about unclean shutdowns.  So, "it should have been", as
usual, amounts to very little in the end.

> > When you do not follow these rules, well, excellent datacenter-class
> > SSDs have super-capacitor power banks that actually work.  Most SSDs do
> > not, although they hopefully came a long way and hopefully modern SSDs
> > are not as easily to brick as they were reported to be three or four
> > years ago.
> 
> I remember that DC SSDs often don't support BG GC.

And have proper supercap local power banks, etc.  I'd say they're not
really relevant to this thread.

-- 
  Henrique Holschuh