RAID1 synchronisation - howto OR not necessary?

Jan Catrysse j.catrysse at
Fri Nov 23 01:21:54 PST 2007

> From: Bill Moran [mailto:wmoran at] 
> Sent: Thursday, November 22, 2007 9:57 PM
> To: Jan Catrysse
> Cc: freebsd-questions at
> Subject: Re: RAID1 synchronisation - howto OR not necessary?
> "Jan Catrysse" <J.Catrysse at> wrote:
> >
> > Dear subscribers,
> > 
> > I am currently running a production server:
> > FreeBSD 6.2 STABLE
> > Onboard Intel ICH8R Raid 1 with 2x SATA300 500GB HDD Using ATA for 
> > Raid1
> > 
> > On Windows systems it is an absolute must to do a Raid 
> Synchronisation 
> > every once and a while to maintain data consistency.
> Wow.  Any RAID controller with that requirement is junk.  
> Where did you get the information from that you had to 
> synchronize?  Does it say so in the manual?  Does it actually 
> say "every once and a while" or does it give a specific schedule.
> I suspect that you have wrong information ... but if your 
> information is correct, I'd get a refund on that RAID controller.
> > I am some what disturbed that that kind of command / tool seems not 
> > available on FreeBSD.
> There is not, because such a thing should not be necessary.
> > Are there other methods to do the synchronisation?
> > 1) Maybe atacontrol detach & atacontrol addspare & 
> atacontrol rebuild?
> > 2) Maybe data consistency is always maintained automatically by the 
> > driver?
> > 3) Maybe I have to panic and urgently make backups?
> If resyncing your raid on a regular basis is truly a 
> requirement, I'd do
> #3 at least, followed by getting my money back.

Hi Bill,

Thank you for your input.

I assumed this was common knowledge, but I can be wrong?
I've checked some other RAID controllers in the company and all of them have the need to be verified/synchronized once and a while. This happens in the BIOS for the more expensive cards (> 600€) and with a utility/driver for the low budget cards...

In my case I am using a real inexpensive onboard Intel ICH8R controller but I presume even a low cost controller needs to do the job done. Furthermore it's only a simple mirror... Intel provides a small windows utility to synchronize.

This is what I found in a 3Ware manual:
Verification can provide early warning of a disk drive problem or failure.
...verification once every 24 hours...
Not verifying the unit periodically can lead to an unstable array unit and may cause data loss.
It is strongly recommended that you schedule a verify at least 1 time per week.
For a RAID 1 or RAID 10 unit, a verify compares the data of one mirror with the other. For RAID 5, RAID 6, and RAID 50, a verify calculates parity and compares it to what is written on the disk drive.


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