hi ya andy good summary ... thanx.. one more item :-) On Wed, 5 Jan 2005, Andy Smith wrote: .. > From what I can understand of the thread so far, Peter is saying the > following: > > RAID mirrors are susceptible to increasing undetectable > inconsistencies because, as we all know, filesystems sustain > corruption over time. > > On a filesystem that runs from one disk, corruption serious > enough to affect the stability of the file system will do so > and so will be detected. As more disks are added to the > mirror, the probability of that corruption never being seen > naturally goes up. > > Peter personally does not put the journal inside the mirror > because if he ever came to need to use the journal and found > that it was corrupted, it could risk his whole filesystem. > Peter prefers to put the journal on a separate device that > is not mirrored. > > I am not trying to put words into your mouth Peter, just trying to > summarise what your points are. If I haven't represented your views > correctly then by all means correct me but please try to do so > succinctly and informatively. > > Now, others are saying in response to this, things like: > > Spontaneous corruption is rare compared to outright or > catastrophic device failure, and although it is more > likely to go unnoticed with RAID mirrors, while it IS > unnoticed, this presumably correct data is also being rewritten > back to the filesystem. > > Mirrors help protect against the more common complete device > failure and so a journal should surely be on a mirror since > if you lose the journal then the machine needs to go down > anyway. It is unavailability of the server we're trying to > avoid; consistency of the data can be protected with regular > backups and possibly measured with other methods like > md5sum. some other issues ... how one can detect failures, errors would be completely up to the tools they use ... various tools does specific functions and cannot tell you anything about any other causes of the problems for swap ... i personally don't see any reason to mirror swap partitions ... - once the system dies, ( power off ), all temp data is useless unless one continues from a coredump ( from the same state as when it went down initially ) if a disk did fail, die, error, hiccup, then whatever cause the problem can also affect the data and the metadata and the parity and the "mirror" - which set of "bytes" on the disk "raid" trust to restore from is up to the code and its predefined set of assumptions of various failure modes - partially written data is very bad thing to have unless you know "exactly why and how if failed/eerrored, there is no sane way to bet the house on which data is more correct than the other - i'm excluding bad memory, bad cpu, bad power supply from the lsit of possible problems and yes, bad (generic) memory has corrupted my systems once in 10 yrs... have fun alvin - To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
