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SubjectRe: [patch] ext2/3: document conditions when reliable operation is possible
Ric Wheeler wrote:
> Pavel Machek wrote:
>> Degraded MD RAID5 does not work by design; whole stripe will be
>> damaged on powerfail or reset or kernel bug, and ext3 can not cope
>> with that kind of damage. [I don't see why statistics should be
>> neccessary for that; the same way we don't need statistics to see that
>> ext2 needs fsck after powerfail.]
>> Pavel
>>
> What you are describing is a double failure and RAID5 is not double
> failure tolerant regardless of the file system type....

Are you sure he isn't talking about how RAID must write all the data
chunks to make a complete stripe and if there is a power-loss, some of
the chunks may be written and some may not?

As I read Pavel's point he is saying that the incomplete write can be
detected by the incorrect parity chunk, but degraded RAID-5 has no
working parity chunk so the incomplete write would go undetected.

I know this is a RAID failure mode. However, I actually thought this was
a problem even for a intact RAID-5. AFAIK, RAID-5 does not generally
read the complete stripe and perform verification unless that is
requested, because doing so would hurt performance and lose the entire
point of the RAID-5 rotating parity blocks.

--
Zan Lynx
zlynx@acm.org

"Knowledge is Power. Power Corrupts. Study Hard. Be Evil."


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