RAID1 synchronisation - howto OR not necessary?

Bill Moran wmoran at
Fri Nov 23 06:27:40 PST 2007

"Jan Catrysse" <j.catrysse at> wrote:
> > From: Bill Moran [mailto:wmoran at] 
> > 
> > "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.


There is a _world_ of difference between "verify" and "synchronize".
Periodically verifying the health of a RAID array is good practice.
Re-synchronizing it periodically is stupid.  If you have to do that,
then you wasted money on a RAID card.

> This
> happens in the BIOS for the more expensive cards (> 600€) and with a
> utility/driver for the low budget cards...

Depends on what you're talking about.

Yes, expensive cards do both health checking and resyncing in the BIOS
without the need of operator intervention.

Low-end hot-swappable cards will automatically do the resynchronizing
if they detect a HDD change, but often don't do periodic health checking.

Low-end cards do neither.  However, you have to power the machine down to
replace a failed drive, so you're also accepting the burden of waiting for
the BIOS to resync.  It's part of the cost trade-off.

> 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.

Nice documentation ... Do you verify every 24 hours or once a week?

In any event, the availability of such a utility for FreeBSD depends on
the driver and the (possible) availability of third-party (or even
vendor-supplied) utilities.  For example, LSI provides the megaraid
utility.  It's designed for Linux but works on FreeBSD and allows total
control over the RAID card, including verifications.  Reading the man
page for the driver being used may turn up something.

I don't know if one exists for your specific card, but keep in mind that
the driver you're using may also work with high-end RAID systems that don't
need it, so the absence of one is possible.

I suggest you reformat/repost your question with a subject line more along
the lines of "Looking for a control utility for ICH8R RAID"  It's quite
possible that the people who know a lot about that hardware missed the
original conversation thread.

If there is no such utility, I suggest looking into something like samhain,
which will continually validate that the files on your system are
uncorrupted.  This has the added advantage of warning you if someone has
cracked your system and installed a trojan.

Also consider the benefit of spending the extra $$ on a high-end RAID
card.  There are very good reasons that people are willing to pay more
for them.  Personally, I wouldn't use a low-end RAID card ... GEOM would
be just as good if not better, IMHO.

Bill Moran

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