[CentOS] variable raid1 rebuild speed?

[Les Mikesell]

On 1/13/11 5:58 AM, Kevin Thorpe wrote:
> On 12/01/2011 18:38, Les Mikesell wrote:
>> I have a 750Gb 3-member software raid1 where 2 partitions are always
>> present and the third is regularly rotated and re-synced (SATA disks in
>> hot-swap bays).  The timing of the resync seems to be extremely variable
>> recently, taking anywhere from 3 to 10 hours even if the partition is
>> unmounted and the drives aren't doing anything else and regardless of
>> what I echo into /proc/sys/dev/raid/speed_limit_min or _max.  Is there
>> some way to tell if the drives are going bad or speed it up consistently
>> if they aren't?
> Ummmm.... RAID is not a backup policy. Fine use hotswap and rsync or
> similar but you
> really shouldn't be relying on raid rebuilds for that.

These are backups to begin with.  It is the archive disk for backuppc which 
contains millions of hardlinked files for the de-duplication it uses. It would 
probably take a week or more for rsync to complete a copy (I've tried, but could 
never leave it offline long enough for any file-oriented copy approach to 
finish).  A raid sync should complete in the time it takes for one pass across 
the disk and does not require unmounting except momentarily while failing the 
member out.  I don't see why this is not a good use of raid rebuilds (the 
offsite members are rotated so the one being rebuilt is not the only copy). It 
used to always complete in 2 or 3 hours.   I would just like to know why it 
sometimes takes much longer.

A related issue is that I'd like to use a laptop sized drive for the offsite 
copy and have one that has the same capacity.  However it uses 4k sectors and 
runs about 10x slower even though read speed is the same.  I understand that 
using a newer kernel and offsetting the start of the partition to a 4k boundary 
should fix this but I obviously can't do that with the existing raid. Does 
anyone have experience with the new drives with 4k sectors on Centos, or know if 
it will be addressed in Centos6?

-- 
   Les Mikesell
     lesmikesell at gmail.com