RAID1 synchronisation - howto OR not necessary?

[Jan Catrysse]

> From: owner-freebsd-questions at freebsd.org 
> [mailto:owner-freebsd-questions at freebsd.org] On Behalf Of Bill Moran
> Sent: Friday, November 23, 2007 3:28 PM
> To: Jan Catrysse
> Cc: freebsd-questions at freebsd.org
> Subject: Re: RAID1 synchronisation - howto OR not necessary?
> 
> "Jan Catrysse" <j.catrysse at proximedia.be> wrote:
> > 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.
> 
> No.
> 
> 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.

Hello again,

I understand what you mean by synchronization not beeing the same as data verification.
What I mean is indeed data verification.

For the Intel ICH8R controller FreeBSD uses ATA(4), no vendor support is available.

It is possible however to control the array using ATACONTROL STATUS / DETACH / ADDSPARE / REBUILD / ...
but no VERY command seem to exist.

As you suggest I will repost the question with another subject. Would it be a big problem if I contact the FreeBSD developper of ATA(4) directly?

I didn't dig in GEOM because I wondered what happens if the primary disk fails when two disks are in a RAID1 config?

Cheers,
Jan