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DragonFly users List (threaded) for 2012-07
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Re: Unable to mount hammer file system Undo failed

From: Francis GUDIN <fgudin+dfly@xxxxxxxxxxxxx>
Date: Thu, 19 Jul 2012 16:24:42 +0200

Wojciech Puchar writes:

>>> you may postpone fsck when using softupdates. It is clearly stated in 
>>> softupdate documents you may find (McKusick was one of the authors).
>>> that's what i do.
>> Then, you suffer a performance hit when fsck'ing in bg.
> once again - read more carefully :)
> I am NOT talking about background fsck which is implemented in FreeBSD and 
> i turn this off.
> I am talking about just not doing fsck of every filesystem after crash. 
> And doing it within same day but when pause is not a problem.
> This is legitimate method with UFS+softupdates.

OK, understood now, i think: you agree with temporarily loosing a bit of 
unreclaimed free-space on disk until time permits cleaning things up 
properly, afaiu softupdates (+journalling ? not really clear).

>>> as someone proposed doing tests with writing random disk blocks, i would 
>>> rather write make a program that would flip random memory bit every few 
>>> minutes.
>> If you're assuming that even the computer itself ain't reliable, how the hell
> Assuming hardware never fails is certainly wrong

And there's no practical point assuming it *always* fails, is there ?

>> could any FS be trustworthy then ??? IMHO, that's nonsense.
> No it isn't. Sorry if i wasn't clear enough to explain it.

Well, if the thing that you try to defend against is plain hardware failure 
(memory bits flipping, CPU going mad, whatever), i just doubt that any kind 
of software layer could definitely solve it (checksums of checksums ofâ?¦ i/o 
buffers, to be safe ? seriously ? do you trust your DMA chip, also ?): here, 
one answer is ECC-RAM. Any practical FS will have to use RAM as a cache, no 
matter what. If your cache can't be trusted, you're screwed. That's it. You 
could build whatever clever mecanism into your on-disk layout to 
(only) improve robustness, but you will have to trust your executing 

Well, that's just my view of the matter. I'm a happy hammer user since 
years, and i felt you made up strong arguments against it without much 
experience with it.
Please try it, break it and if you can report anything that can help enhance 
it, be welcome to post a bug report.


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