Re: ext4 features (checksums)

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Avi Kivity wrote:
Neil Brown wrote:

On Tuesday July 4, [email protected] 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 <[email protected]>
  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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