Re: [PATCH RFC] nilfs2: continuous snapshotting file system

[konishi . ryusuke]

On Fri, 29 Aug 2008 10:40:06 +0200, Arnd Bergmann wrote:
> On Friday 29 August 2008, Ryusuke Konishi wrote:
> > >Do you do wear leveling or scrubbing?
> > 
> > NILFS does not support scrubbing. (as you guessed)
> > Under the current GC daemon, it writes logs sequentially and circularly
> > in the partition, and as you know, this leads to the wear levelling
> > except for superblock.
> 
> I don't see how that would cope with file systems that have a lot
> of static data. The classic problem of most cheap devices that implement
> wear leveling in hardware is that they never move data in an erase block
> that is used for read-only data. If 90% of the file system is read-only,
> your wear leveling will only work on 10% of the medium, wearing it down
> 10 times faster than it should.
>
> Can the GC daemon handle this case, e.g. by moving around aging read-only
> erase blocks?

Yeah, exactly.  Thank you for this comment.

To minimize aging of the device itself, the userland GC daemon would
need another cleaning policy.  So, in that sense, the answer of the
above question is NO.

Since the primary purpose of NILFS is providing continuous
snapshotting, the GC is not necessarily designed with such requirement
in mind.

Regards,
Ryusuke Konishi
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