On Mar 9, 2011, at 1:52 PM, objectwerks inc wrote:
> > On Mar 9, 2011, at 1:46 PM, Chris Murphy wrote: > >> >> >> On Mar 9, 2011, at 1:35 PM, objectwerks inc wrote: >> >>>>> The integrity of the data with files after any issues such as these may >>>>> be suspect, especially if the fileystem was on a RAID5 which is very well >>>>> known for silent data corruption. This is why RAID5 should not be used. >>>> >>>> OK that's possibly a whole separate thread for qualifying such a >>>> statement. Apple has a supported product that uses RAID 5. I have clients >>>> with them and they've lost drives, and no data, and no data corruption. >>>> And an even larger sample size exists if filesystems other than jhfs+ are >>>> considered. RAID 5/6 are common with ext3, ext4, XFS and other file >>>> systems without anyone suggesting RAID 5 in particular is known for itself >>>> increasing the incidence of silent data corruption. >>>> >>> >>> >>> There is a reason why it is called silent data corruption. They may not >>> know they have it. It happens all the time and with HW raid 5 you may not >>> even know it for a long time. >>> >>> This is the whole reason why ZFS was made. >> >> ZFS was not made for combating this claim of RAID 5 specific silent data >> corruption, but rather silent data corruption in general. > > > http://blogs.sun.com/bonwick/entry/raid_z OK so are you going to take the definition of "silent data corruption" to mean any sort of error where the user isn't explicitly notified? What if they aren't even passively notified? I have had two completely fakaked Time Machine backups that continued to backup, with no complaints, and no complaints on startup or when mounting the disk. Upon restore, errors. Could not complete the restore. Disk Utility? With one of the drives, there were file system errors they were fixed, the restore still would not restore. The other drive, completely clean with Disk Utility, would not restore. I would not consider these silent data corruption events, despite not being notified of a problem in advance, if they resulted from file system or data corruption due to a kernel panic or power failure. That's the premise of the article and I don't agree with it. In any event, that one is expected to take precautions with RAID 5/6 or even RAID-Z with respect to power management does not mean the incidence of silent data corruption is higher. It simply means *IF* it happens to a RAID 5 array the problems can rapidly become magnified requiring significant contingencies. I might still be able to suck off a bunch of data from a non-array (or RAID 1 disk) despite file system corruption, or the corruption of even 10% of the disk - I will have *better* recovery from RAID 5 if that same event happens to 1 disk, but much worse recovery if that problem is propagated through the entire array. I don't think this is a secret. Again, RAID 5 is not a new thing. But I totally disagree with the wording used that implies there is a high incidence of silent data corruption inherent to a RAID 5 system. Usually those systems have better drives, better interfaces, better cables, and more resilient OS, with UPS systems. The net of that is silent data corruption would be less likely by far than a non-arrayed setup of the same capacity. Let me use perhaps an imperfect analogy. Cars and airplanes. Cars = non-array and airplanes = RAID 5. The likelihood you're going to have an accident with a car is astronomically higher than an airplane. But if you have an accident in an airplane the incidence of death is higher. Death is contingent on the other happening first, which is very unlikely with airplane travel. Chris Murphy_______________________________________________ MacOSX-admin mailing list [email protected] http://www.omnigroup.com/mailman/listinfo/macosx-admin
