Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: Zan Lynx
Date: Mon Aug 24 2009 - 18:22:53 EST

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.]
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

"Knowledge is Power. Power Corrupts. Study Hard. Be Evil."
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at