linux-kernel - Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

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Date:	Fri, 26 Jun 2009 18:56:59 +0200
From:	Marco <marco.stornelli@...il.com>
To:	Jamie Lokier <jamie@...reable.org>
CC:	Linux Embedded <linux-embedded@...r.kernel.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Daniel Walker <dwalker@....ucsc.edu>
Subject: Re: [PATCH 00/14] Pramfs: Persistent and protected ram filesystem

Jamie Lokier wrote:
> Marco Stornelli wrote:
>> 2009/6/24 Jamie Lokier <jamie@...reable.org>:
>>> Marco wrote:
>>> If it loses power when doing atomic rename (to replace config files,
>>> for example), it's likely that the whole /pramfs/configs/ directory
>>> will be corrupt, because the rename is writing to the directory inode,
>>> so you lose access to all names in that directory?
>>>
>>> That sounds like it can't be used for persistent configuration data.
>> It's true from this point of view currently there is a lack for this
>> and it needs a bit of effort to resolve this problem.  >From this
>> point of view I'd like to point out that I know that there was some
>> aspects to study in a deeper way, so I'll need of more then one
>> review :) but since this fs has been abandoned since 2004 and it
>> hadn't ever reviewed, it was important to do a serious review with
>> the kernel community to understand all the problems.
> 
> That's reasonable.
> 
> What do you think of my suggestion to double-buffer writes using a
> single fixed position block, as explained elsewhere in this thread?
> 
> It should give the power fail safety with very little code.  I don't
> know how much it would slwo down writing.  That probably depends on
> whether it's the checksum which is slow (which only needs to be done
> once when double-buffering), or the writing.
> 
> -- Jamie
> 

Yeah it can be a choice. For this fs it's important to use "simple" but
useful mechanism. What do you exactly mean with "fixed position block"?
A fixed position in the fs? For example superblock+inodetable+in-use
bitmap+blocks+"double-buffering block"? Using a temp block of the same
size of blocks used, isn't it? I agree, but I think it needs more then
100 lines of code. Even with this simple schema it needs a mechanism
with a timeout to do the "commit" of the temp block, it needs a
mechanism to read the temp block instead of the "old" block or a
mechanism to write-back the temp block. So it can be implemented but it
needs a bit of effort. I think I'll implement it in the next release.

Marco
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