Re: ext4 features (checksums)

[Neil Brown]

On Tuesday July 4, avi@xxxxxxxxxx wrote:
> Neil Brown wrote:
> >
> > To my mind, the only thing you should put between the filesystem and
> > the raw devices is RAID (real-raid - not raid0 or linear).
> >
> I believe that implementing RAID in the filesystem has many benefits too:
>  - multiple RAID levels: store metadata in triple-mirror RAID 1, random 
> write intensive data in RAID 1, bulk data in RAID 5/6
>  - improved write throughput - since stripes can be variable size, any 
> large enough write fills a whole stripe

Maybe....

Now imagine what would be required to rebuild a whole drive onto a
spare after a drive failure.

I'm sure it is possible, and I believe ZFS does something like that.
I find it hard to imagine getting reasonable speed if there is much
complexity.  And the longer it takes, the longer your data is exposed
to multiple-failures.

There may well be room there to come up with a really clever idea that
makes it both flexible and fast....

Note that 'resync' wouldn't be a problem.  Having the filesystem know
about the raid means that resync (after unclean shutdown) can be quite
trivial (I believe there is a paper related to this at OLS this year).

NeilBrown
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