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Re: Shutdown behavior and flash storage devices

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On 7/25/2021 23:27, Don Y wrote:
> On 7/25/2021 12:51 PM, Dimiter_Popoff wrote:
> ......
> 
>> Of course again
>> you will be prone to data loss on unexpected power loss but updating
>> the write cache will be done at (much) longer intervals and in
>> relatively brief bursts, thus the probability to be hit gets lower.
> 
> The usual trick is to just have a mirror of the FLASH contents
> and flush that to the media when you're pretty sure the ship
> is sinking.
> 
> But, if this is *large*, then you risk only partial writes before
> power drops to an unusable/unreliable level.

Well I think tens of gigabytes for small drives nowadays, what can you
do. Cacheing it all just is not feasible.
What DPS does is cache directories and CATs on a 4k piece basis
(regardless of the disk block size, 512 2048, 4k) and maintains a
bitmap for its dirty regions - so when it updates to the medium
it writes at least 4k and no more than twice 4095 excessive bytes
or something like that.

> 
>> The rest can only be got rid of by having large enough caps on
>> the power supply and an early warning that power has been lost
>> (usually at the input of the stepdowns and you are running on fumes.
> 
> I'm not sure that will work with consumer devices.  You have no way of
> knowing how long it will take to reliably flush ALL of the data
> out to the physical store.

Oh with consumer devices you have not built yourself it will probably
never work, of course. They are unlikely to give you an early warning
in the first place. If you want to be safer you just use good rotating
disks.... I think I read somewhere they do their soft shutdown using the
energy in the rotor (and do that since times immemorial), but I have
never verified this.

Dimiter

======================================================
Dimiter Popoff, TGI             http://www.tgi-sci.com
======================================================
http://www.flickr.com/photos/didi_tgi/

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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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