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List Archive: gentoo-embedded
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To: gentoo-embedded@g.o
From: Ed W <lists@...>
Subject: Re: crash proofing a system from power cutoffs
Date: Mon, 23 Jan 2012 18:39:36 +0000
> I thought of creating 5 200mb paratitions of raid1. So 5 will be 
> mirrored.
> What do you think of this solution?

I don't think it will help much... Sorry.  Partial writes are partial writes

The main way you can kill flash drives is to power off while they are 
doing an internal write.  On many devices if you do this at the right 
moment then the flash drive completely disappears and is completely gone 
(not recoverable as far as I know, never can be used or data accessed 
ever again...)

I believe some very new drives has small capacitors to finalise a write 
if the power goes off suddenly, however, I think on everything else your 
flash is likely toast... The issue is corruption of the internal flash 
drive filesystem, NOT the filesystem that you create on top of that (I 
have one dead flash drive and I think it may well have been killed by 
this? Allegedly the latest SLC flash drives from PC Engines are somewhat 
resistant to this problem...)

With regards to corrupting the user filesystem due to sudden power loss, 
ie as per normal spinning drives, then yes, any journalling filesystem 
seems like a good idea.  Ext4 is chosen by google (albeit without a 
journal...).  XFS is an odd ball choice and may have massive write 
amplification problems that I don't know about, but what is nice is that 
it seems resiliant to corruption on powerloss and avoids a fsck step.  
This is only "better" in the sense that it doesn't require user input 
and for embedded, this may keep the device going longer between visits 
from support... However, bottom line is that if the power goes off 
during a write then at some point you loose data...


Good luck

Ed W


References:
crash proofing a system from power cutoffs
-- Kfir Lavi
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Updated Jun 25, 2012

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