Re: ext4 features (checksums)

[Bill Davidsen]

Avi Kivity wrote:
> Neil Brown wrote:
> > 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.
>
> A company called Isilon does this on a cluster.  They claim (IIRC) a one 
> hour rebuild time for a failure.  AFAIK they rebuild into cluster free 
> space, so they are not bound by the spare's bandwidth; they can utilize 
> all cluster resources for a rebuild.
>
> (You don't need spare disks, just spare free space; so you don't have 
> idle disk heads)
Readers of the RAID list will recognize this description, it matches my 
comments on RAID5E (distributed hot spare) very well. And I suppose 
there could be RAID6E as well, although I haven't really thought about it.

--
Bill Davidsen <davidsen@xxxxxxx>
  Obscure bug of 2004: BASH BUFFER OVERFLOW - if bash is being run by a
normal user and is setuid root, with the "vi" line edit mode selected,
and the character set is "big5," an off-by-one errors occurs during
wildcard (glob) expansion.

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