Re: soft update vs journaling?

[John Richard Moser]

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Jan Engelhardt wrote:
>>Unfortunately, journaling uses a chunk of space.  Imagine a journal on a
>>USB flash stick of 128M; a typical ReiserFS journal is 32 megabytes!
>>Sure it could be done in 8 or 4 or so; or (in one of my file system
>>designs) a static 16KiB block could reference dynamicly allocated
>>journal space, allowing the system to sacrifice performance and shrink
>>the journal when more space is needed.  Either way, slow media like
>>floppies will suffer, HARD; and flash devices will see a lot of
>>write/erase all over the journal area, causing wear on that spot.
> 
> 
>  - Smallest reiserfs3 journal size is 513 blocks - some 2 megabytes,
>    which would be ok with me for a 128meg drive.
>    Most of the time you need vfat anyway for your flashstick to make
>    useful use of it on Windows.
> 
>  - reiser4's journal is even smaller than reiser3's with a new fresh
>    filesystem - same goes for jfs and xfs (below 1 megabyte IIRC)
> 

Nice, but does not solve. . .

>  - I would not use a journalling filesystem at all on media that degrades
>    faster as harddisks (flash drives, CD-RWs/DVD-RWs/RAMs).
>    There are specially-crafted filesystems for that, mostly jffs and udf.
> 

Yes.  They'll degrade very, very fast.  This is where Soft Update would
have an advantage.  Another issue here is we can't just slap a journal
onto vfat, for all those flash devices that we want to share with Windows.

>  - You really need a hell of a power fluctuation to get a disk crippled.
>    Just powering off (and potentially on after a few milliseconds) did
>    (in my cases) just stop a disk write whereever it happened to be,
>    and that seemed easily correctable.

Yeah, I never said you could cripple a disk with power problems.  You
COULD destroy a NAND in a flash device by nuking the thing with
10000000000000 writes to the same area.

> 
> 
> Jan Engelhardt

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