Nataraj wrote: > Does software raid 1 compare checksums or otherwise verify that the same > bits are coming from both disks during reads? What I'm interested in, > is whether bit errors that were somehow undetected by the hardware would > be detected by the raid 1 software. > > Thanks, > Nataraj I've been thinking about this as well. Fact is that with CentOS-5 kernels (but not with CentOS-4, as this functionality became available in kernel 2.6.17) you could (or rather _should_ regularly) echo check > /sys/block/mdX/md/sync_action to check agreement between the two (or more) copies. When this finishes, /sys/block/mdX/md/mismatch_cnt shows you the number of mismatches. You can fix these with echo repair > /sys/block/mdX/md/sync_action This applies to at least RAID1 and RAID5. At this point the question arises: how does the "repair job" know which copy is the correct one? I have no answer to this question. BTW, there is - even with current kernels - no speed gain in using RAID1 - see http://kernelnewbies.org/KernelProjects/Raid1ReadBalancing . HTH a bit, Kay