On 18-12-2013 07.08 -0500, Phil Turmel wrote:

I'd let the sync continue until it fails or completes.  And if it
completes, exercise the array to see if it stays flaky.  If it does not
complete, start swapping parts in the system.

---end quoted text---

I'm responding to an old thread, but current problem. I started a RAID1 rebuild in mid-December, and it's still running - now with 2712 read errors - and counting. (I enclosed smartctl output).

# cat /proc/mdstat
Personalities : [raid1] 
md0 : active raid1 sda1[0] sdb1[1]
      521984 blocks [2/2] [UU]
      
md1 : active raid1 sda2[2] sdb2[1]
      976237824 blocks [2/1] [_U]
      [==============>......]  recovery = 70.9% (692700480/976237824) finish=68.5min speed=68956K/sec


md0 is a boot partition, and md1 is the operating system. 
I configured LVM on md1, and allocated 68GB (of 1TB): 

# vgdisplay /dev/VolGroup00
  VG Name               VolGroup00
  VG Size               931.00 GB
  Alloc PE / Size       2176 / 68.00 GB
  Free  PE / Size       27616 / 863.00 GB
 
Currently, only ~5GB of the 1TB disk is used.

At this point, what is my best option for limiting downtime of the server (i.e. avoiding a rebuild)? 
I added a drive (/dev/sdc) with identical geometry, and consider using dd, i.e.

# dd if=/dev/sdb of=/dev/sdc bs=4096 conv=sync,noerror

This may not be the most efficient method of transferring data, since .5% of the disk is used. But obviously, I'm not in a hurry.

Please excuse my ignorance, but after it's cloned, is it possible to add /dev/sdc2 to md1 while it's syncing (to /dev/sda2)? Or do I need to wait until /dev/sdb fails to replace it with /dev/sdc? 

Thanks!
Julie