On Mar 9, 2011, at 2:14 PM, Chris Murphy wrote: > > 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. >
A power failure or sudden crash is not the only place corruption can enter. But on the Time Machine side, if you want a more reliable time machine backup solution, build or buy a box running Nexenta that uses ZFS and then create AFS services on it that you can target your Time Machine to. I am in the process of implementing this in my office. My hosting business actually also uses Nexenta and has two HW RAID Cards (Areca with battery back NVRAM). One HW Raid is a Raid 6 and one a Raid 5 using bigger base disks. Then I use these two volumes in a ZFS mirror, along with a mirrored SSD ZIL. The server sits in a UPS and generator backed data center as well. Provides good performance and I should be able to tolerate a lot of disks going out at once. _______________________________________________ MacOSX-admin mailing list [email protected] http://www.omnigroup.com/mailman/listinfo/macosx-admin
