On Apr 13, 2010, at 11:57 AM, nate wrote:
John R Pierce wrote:
well, IF your controller totally screams and can rebuild the drives at wire speeds with full overlap, you'll be reading 7 * 2TB of data at around 100MB/sec average and writing the XOR of that to the 8th drive in a 8 spindle raid5 (14tb total). just reading one drive at wirespeed is 2000,000MB / 100MB == 20,000 seconds, or about 5.5 hours, so thats about the shortest it possibly could be done.
More likely your looking at 24+ hours, because really no disk system is going to read your SATA disks at 100MB/second. If your really lucky perhaps you can get 10MB/second.
With the fastest RAID controllers in the industry my own storage array(which does heavy amounts of random I/O) averages about 2.2MB/second for a SATA disk, with peaks at around 4MB/second.
Our previous storage array averaged about 4-6 hours to rebuild a RAID 5 12+1 array with 146GB 10k RPM disks, on an array that was in excess of 90% idle. Rebuilding a 400GB SATA-I array often took upwards of 48 hours.
nate
For a "real life" example, we have a 3 year old 12x 1TB SATA box using an Adaptec RAID controller, doing RAID 6 that takes about 3 days to rebuild the array each time a drive fails. Which, to date, has happened 10 times... (Fortunately, this is only a BackupPC box.)
FWIW, we've not experienced a second drive failure during the rebuild process, yet. But we have had drives fail within a few weeks of each other, so it's probably going to happen one of these days..
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