Re: ext4 features (checksums)

[Bill Davidsen <davidsen@tmr.com>]

Avi Kivity wrote:
> Neil Brown wrote:
>>
>> On Tuesday July 4, avi@argo.co.il 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@tmr.com>
   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.