Messages in this thread | | | Date | Mon, 24 Aug 2009 16:22:22 -0600 | From | Zan Lynx <> | Subject | Re: [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."
| |