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Re: Shutdown behavior and flash storage devices

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On 7/25/2021 5:49 AM, Dave Nadler wrote:
> On 7/24/2021 9:10 PM, Don Y wrote:
>>> So, how do you avoid this kind of corruption in your designs?
>>
>> Two (transient) copies of each "chunk" of data.  Update the second
>> copy (on or before impending power fail).  When complete, toggle
>> a pointer to reference it as the most recent (assuming you don't
>> have metadata that automatically provides that sort of information).
>>
>> The "first" copy can now be removed and its space reallocated to
>> some other "second copy"
>>
>> Then, move on to the next "chunk".  Design your algorithms so that
>> you can tolerate some chunks being "stale" (not updated in time)
>> wrt the others.
> 
> Don, I don't think that works. The entire contents of the volume can
> be corrupted if power is removed during wear-leveling operations that are 
> invisible to the host OS. And I guess you are assuming no file system is in use 
> (because file system internals would still get corrupted)?

[I don't use filesystems; do you use a filesystem to manage your *RAM*?  :> ]

I use "raw" devices and have the FTL (and wear-leveling) in my firmware.
So, I know what the hardware is being asked to do when I talk to it;
there's no "black magic" sitting in the middle that I can't quantify/qualify.

If you're going to use a "smart" device, then you have to come up with a
strategy that lets you recover from arbitrary levels of corruption
(because you don't know what that device is going to be doing when the
fit hits the shan).

I suspect you will see your losses limited to a single block (or so) as
it would require more resources to transfer a block's contents into
RAM while another block is nominated to receive them.

There might be different strategies employed by different device vendors.
E.g., one may defer the program/erase of that block until a later time;
another may opt to do it "now".

I don't see how you can expose enough of the internals of a "smart" device
in a manufacturer/model-independent manner.  I.e., even storing a hash with
each "file", there's no guarantee as to WHERE that will reside on the device.
And, when it might "move" (be exposed to loss).

[You also have to be prepared for "good data" being lost in such a reshuffling;
in my case, I can ensure "what I have, I keep (but may lose something NEW)"]

If your needs are for things like configuration parameters (presumably few),
can you consider an EEPROM?  Use FLASH for bigger things -- that tend to
change less frequently (and in more user-visible ways)?

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Shutdown behavior and flash storage devices Dave Nadler <drn@nadler.com> - 2021-07-24 11:19 -0400
  Re: Shutdown behavior and flash storage devices Theo <theom+news@chiark.greenend.org.uk> - 2021-07-24 16:37 +0100
  Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-24 18:10 -0700
    Re: Shutdown behavior and flash storage devices Dimiter_Popoff <dp@tgi-sci.com> - 2021-07-25 14:18 +0300
      Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-25 09:20 -0700
    Re: Shutdown behavior and flash storage devices Dave Nadler <drn@nadler.com> - 2021-07-25 08:49 -0400
      Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-25 09:30 -0700
      Re: Shutdown behavior and flash storage devices Dimiter_Popoff <dp@tgi-sci.com> - 2021-07-25 22:51 +0300
        Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-25 13:27 -0700
          Re: Shutdown behavior and flash storage devices Dimiter_Popoff <dp@tgi-sci.com> - 2021-07-26 00:00 +0300
            Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-25 17:35 -0700
  Re: Shutdown behavior and flash storage devices kegs@provalid.com (Kent Dickey) - 2021-07-25 13:53 -0500
    Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-25 13:20 -0700
  Re: Shutdown behavior and flash storage devices Jaded Hobo <badboy@heaven.org> - 2021-07-27 19:44 +0200
    Re: Shutdown behavior and flash storage devices Don Y <blockedofcourse@foo.invalid> - 2021-07-27 11:27 -0700

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