Re: Software RAID 6 initial sync very slow

thomas62186218@aol.com wrote:

Hi all,

I am debating between hardware or software RAID for a high-performance 
Linux storage server. One area of concern in my initial mdadm tests 
concerns sync speed. My setup is as follows:

Linux Fedora 7 x64
12 x 1TB Hitachi 1TB SATAII drives connected via LSI SAS3801E PCIe HBA
Dual 2.5GHz quad-core Xeon
8GB RAM

My first test was to create a RAID 6 md device, 128K chunk, with all 
12 drives. The sync is currently underway at a very slow 30MB/sec, 
with mdadm estimating this as a 9hr process. With an Adaptec 5805 RAID 
card tested, I performed the RAID 6 initial sync with the same 12 
drives in just under 5 hours. Meanwhile, our total CPU usage during 
mdadm syncing is at 5%, so there's plenty of CPU not being used.

Why is the sync so slow? This concerns me not only because initial 
sync time is important, but because this may be indicative of slow 
sync times in the event of a drive failure.

Two things will limit this, chunk size and max speed set in the 
/sys/block/mdNN/md/sync_speed_max setting. What you are seeing does 
sound slow.

-- 
Bill Davidsen [off-list ref]
  "Woe unto the statesman who makes war without a reason that will still
  be valid when the war is over..." Otto von Bismark