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Re: Questions about software RAID level 1 (how long should it take to sync, etc?)

On Mon, 2006-04-03 at 14:53, Mike Garey wrote:
> I'm in the process of setting up a software RAID level 1 on debian
> testing with two 160 gig drives on an Asus P4P800 motherboard with
> Intel Pentium 4 CPU 3.00GHz and 512 megs of RAM.  I've been following
> the instructions in /usr/share/doc/mdadm/rootraiddoc.97.html and have
> just issued the command "mdadm --add /dev/md0 /dev/hda1" to add my
> original debian installation to the RAID array (after already having
> copied the contents of hda1 to hdc1, and then rebooting from hdc1). 
> When I cat /proc/mdstat, I get the following:
> [>....................]  recovery =  4.9% (7907776/158577472)
> finish=1533.5min speed=1636K/sec
> I'm curious about a few things:
> 1) does 25 hours seem like a reasonable amount of time for an initial
> synchronization of two 160 gig drives?
> 2) would using a hardware card (such as a 3ware 7006-2 Parallel ATA
> RAID Controller) decrease the amount of time to sync?
> 3) will it take this long to synchronize the drives after, say, a
> system crash (ie a hard reboot), or does it only take 25 hours for the
> initial sync ?
> 4) If I do decide to switch to hardware RAID, will I be able to use
> the same drives without a large configuration/resyncrhonization
> process?
> 5) if hda1 contained the original root system, and hdc1 is simply a
> copy of it (ie the drives are a mirror), why is it taking 25 hours to
> "synchronize" the drives, aren't they already sync'd?  Shouldn't this
> be a much faster process?
> Any help or suggestions are very much appreciated.  Thanks!


The min and max rebuild rates are in /proc/sys/dev/raid.  The
RAID subsystem will *try* to rebuild at a rate between these
limits.  Depending upon hardware, you might typically achieve
10-20 MBps on a lightly loaded system.  It can easily take 5
hours or more to sync 160GB.

You're only seeing 1.6MBps.  You may have your max rate set
too low or you may have slow drives or maybe (U)DMA disabled.

Although you know the drive contents are similar (not identical
because timestamps and log files will have changed) that is
not something that the RAID system knows or dares assume.  It
will diligently copy one mirror to the other.

--Mike Bird

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