On 4/11/2012 9:23 PM, Emmanuel Noobadmin wrote:
On 4/12/12, Stan Hoeppner stan@hardwarefreak.com wrote:
On 4/11/2012 11:50 AM, Ed W wrote:
One of the snags of md RAID1 vs RAID6 is the lack of checksumming in the event of bad blocks. (I'm not sure what actually happens when md scrubbing finds a bad sector with raid1..?). For low performance requirements I have become paranoid and been using RAID6 vs RAID10, filesystems with sector checksums seem attractive...
Except we're using hardware RAID1 here and mdraid linear. Thus the controller takes care of sector integrity. RAID6 yields nothing over RAID10, except lower performance, and more usable space if more than 4 drives are used.
How would the control ensure sector integrity unless it is writing additional checksum information to disk? I thought only a few filesystems like ZFS does the sector checksum to detect if any data corruption occurred. I suppose the controller could throw an error if the two drives returned data that didn't agree with each other but it wouldn't know which is the accurate copy but that wouldn't protect the integrity of the data, at least not directly without additional human intervention I would think.
When a drive starts throwing uncorrectable read errors, the controller faults the drive and tells you to replace it. Good hardware RAID controllers are notorious for their penchant to kick drives that would continue to work just fine in mdraid or as a single drive for many more years. The mindset here is that anyone would rather spent $150-$2500 dollars on a replacement drive than take a chance with his/her valuable data.
Yes I typed $2500. EMC charges over $2000 for a single Seagate disk drive with an EMC label and serial# on it. The serial number is what prevents one from taking the same off the shelf Seagate drive at $300 and mounting it in a $250,000 EMC array chassis. The controller firmware reads the S/N from each connected drive and will not allow foreign drives to be used. HP, IBM, Oracle/Sun, etc do this as well. Which is why they make lots of profit, and is why I prefer open storage systems.
-- Stan