Message-ID: <40D28950.7030500@namesys.com>
Date: Thu, 17 Jun 2004 23:18:56 -0700
From: Hans Reiser <reiser@namesys.com>
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To: Ken Ryan <linuxryan@leesburg-geeks.org>
CC: Timothy Miller <miller@techsource.com>, linux-kernel@vger.kernel.org,
       pla@morecom.no
Subject: Re: mode data=journal in ext3. Is it safe to use?
References: <40D1B110.7020409@leesburg-geeks.org> <40D1C18B.1030907@techsource.com> <40D1D2F0.7080102@namesys.com> <40D1EDB7.3030401@leesburg-geeks.org>
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Ken Ryan wrote:

> Hans Reiser wrote:
>
>> Timothy Miller wrote:
>>
>>> Doesn't Reiser4 do wear-leveling for flash?
>>
>>
>>
>> No, we don't.  We do have wandering logs, so it would be feasible to 
>> code, but bitmap blocks and super blocks get written to the same 
>> locations repeatedly.
>>
>> Actually, most compact flash devices DO do wear leveling, from what I 
>> have heard.
>
>
>
> The ones I've seen, only sort of.  They'll allocate writes from 
> available erased pages to try to distribute their use, but if you
> have a disk that's, say, 70% read-only data and 30% read-write then 
> the wear-levelling will only happen on that
> 30% of the disk.  True wear levelling will actually scrub read-only or 
> rarely-written data, forcing it to get off its
> duff so the flash cells they're sitting on can get some exercise, and 
> give the more worn cells a rest (that scrub
> helps ECC fix soft errors from weak cells too).  True wear-levelling 
> is really hard, and obviously requires
> budgeting extra bandwidth and storage devices for safely shuffling 
> around data that the application has no
> intention of moving (picture losing power in the middle of a scrub).  
> It's not worth it for the consumer CF
> usage model of "take photos until the card is full, then copy them all 
> to the PC and wipe the card clean".
>
> [Yes, I tend to see this from the inside-out: I'm actually an 
> FPGA/ASIC weenie not a kernel hacker.  One of my current
> projects is part of a controller chip for a solid-state storage system 
> with ${bignum} NAND flash chips.  Alas, my specialty
> is video and graphics, so I'm still coming up the learning curve on 
> storage systems].
>
>               ken
>
>
>
>
>
Interesting.  Thanks for educating me. 

No existing general purpose filesystem that I know of will address your 
needs.  We could of course write one if someone paid for it....
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