Adding a new mirror disk to RAID1 configuration

Adding a new mirror disk to RAID1 configuration

[Luke Reeves]

I have a system with two 40GB drives, and the first drive (hda) is setup 
  under an md device with RAID1 driving it.  I'm trying to now make the 
second disk, hdc, a mirror of the first.  When I try to add it using 
mdadm the second disk becomes a spare and no synchronization is done. 
Is there any way to add the second disk directly as a mirror?  Thanks.

	Luke Reeves
	http://www.neuro-tech.net/

RE: Adding a new mirror disk to RAID1 configuration

[Guy]

My guess is you configured the RAID1 array as having 1 disk.  Now if you add
more disks to it, they are spares.  You should have configured the array as
having 2 disks, with 1 missing.  Then when you add a disk, it will re-build
to it.

Read about raidreconf.  Google it, if needed!  It may allow you to modify a
RAID1 array.  I know it allows you to add a disk to a RAID5 array, but don't
know about RAID1.  I have never used this tool.

Guy

-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Luke Reeves
Sent: Friday, July 09, 2004 12:55 AM
To: linux-raid@vger.kernel.org
Subject: Adding a new mirror disk to RAID1 configuration

I have a system with two 40GB drives, and the first drive (hda) is setup 
  under an md device with RAID1 driving it.  I'm trying to now make the 
second disk, hdc, a mirror of the first.  When I try to add it using 
mdadm the second disk becomes a spare and no synchronization is done. 
Is there any way to add the second disk directly as a mirror?  Thanks.

	Luke Reeves
	http://www.neuro-tech.net/
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Re: Adding a new mirror disk to RAID1 configuration

[Luke Reeves]

I've looked into raidreconf, but apparently it can only do operations on 
RAID levels 0 and 5.

	Luke

Guy wrote:
> My guess is you configured the RAID1 array as having 1 disk.  Now if you add
> more disks to it, they are spares.  You should have configured the array as
> having 2 disks, with 1 missing.  Then when you add a disk, it will re-build
> to it.
> 
> Read about raidreconf.  Google it, if needed!  It may allow you to modify a
> RAID1 array.  I know it allows you to add a disk to a RAID5 array, but don't
> know about RAID1.  I have never used this tool.
> 
> Guy
> 
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Luke Reeves
> Sent: Friday, July 09, 2004 12:55 AM
> To: linux-raid@vger.kernel.org
> Subject: Adding a new mirror disk to RAID1 configuration
> 
> I have a system with two 40GB drives, and the first drive (hda) is setup 
>   under an md device with RAID1 driving it.  I'm trying to now make the 
> second disk, hdc, a mirror of the first.  When I try to add it using 
> mdadm the second disk becomes a spare and no synchronization is done. 
> Is there any way to add the second disk directly as a mirror?  Thanks.
> 
> 	Luke Reeves
> 	http://www.neuro-tech.net/
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html

Re: Adding a new mirror disk to RAID1 configuration

[David Greaves]

no, no no :)

Install the second disk (hdc)
build it as md1 RAID1 with a 'missing' disk this time
copy the data over from md0
get rid of md0
add the first disk as the missing disk
it will sync

don't do it again

David


Luke Reeves wrote:

> I've looked into raidreconf, but apparently it can only do operations 
> on RAID levels 0 and 5.
>
>     Luke
>
> Guy wrote:
>
>> My guess is you configured the RAID1 array as having 1 disk.  Now if 
>> you add
>> more disks to it, they are spares.  You should have configured the 
>> array as
>> having 2 disks, with 1 missing.  Then when you add a disk, it will 
>> re-build
>> to it.
>>
>> Read about raidreconf.  Google it, if needed!  It may allow you to 
>> modify a
>> RAID1 array.  I know it allows you to add a disk to a RAID5 array, 
>> but don't
>> know about RAID1.  I have never used this tool.
>>
>> Guy
>>
>> -----Original Message-----
>> From: linux-raid-owner@vger.kernel.org
>> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Luke Reeves
>> Sent: Friday, July 09, 2004 12:55 AM
>> To: linux-raid@vger.kernel.org
>> Subject: Adding a new mirror disk to RAID1 configuration
>>
>> I have a system with two 40GB drives, and the first drive (hda) is 
>> setup   under an md device with RAID1 driving it.  I'm trying to now 
>> make the second disk, hdc, a mirror of the first.  When I try to add 
>> it using mdadm the second disk becomes a spare and no synchronization 
>> is done. Is there any way to add the second disk directly as a 
>> mirror?  Thanks.
>>
>>     Luke Reeves
>>     http://www.neuro-tech.net/
>> -
>> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
>> the body of a message to majordomo@vger.kernel.org
>> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
> -
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>

Re: Adding a new mirror disk to RAID1 configuration

[Luke Reeves]

Ah, I understand, that should work correctly.  Thanks David.

And yeah, the second box I configured today I actually did do it 
correctly :-)

	Luke

David Greaves wrote:
> no, no no :)
> 
> Install the second disk (hdc)
> build it as md1 RAID1 with a 'missing' disk this time
> copy the data over from md0
> get rid of md0
> add the first disk as the missing disk
> it will sync
> 
> don't do it again
> 
> David

Re: Adding a new mirror disk to RAID1 configuration

[maarten van den Berg]

On Friday 09 July 2004 07:09, Guy wrote:
> My guess is you configured the RAID1 array as having 1 disk.  Now if you
> add more disks to it, they are spares.  You should have configured the
> array as having 2 disks, with 1 missing.  Then when you add a disk, it will
> re-build to it.

What I do is even more ahead-thinking; I define a raid1 set with several 
missing drives as a rule. So I make a three- or even four disk array, with 
just two drives active (or just one). That way, you never run out of slots 
for extra drives. This stems from the fact that I used the raid toolkit to 
make clones of servers for a customer. Add a drive, let it sync, take it out, 
put it in a new system with a blank disk and hey presto, another webserver.
:-)
I talked that customer into deploying three mirrored drives per server anyway, 
seen that modern IDE drives are not as dependable as they used to be and that 
the cost vs annoyment (read: downtime) -factor certainly warranted using 
three drives per system.   They quickly agreed, too.

> Read about raidreconf.  Google it, if needed!  It may allow you to modify a
> RAID1 array.  I know it allows you to add a disk to a RAID5 array, but
> don't know about RAID1.  I have never used this tool.

Neither have I.  I'd gladly use it cause I have an 99.9999% full raid5 400GB 
system here, but the off-chance that it kills all my data, which certainly 
isn't backed up anywhere NEAR complete keeps me from that.
How do you backup 400G ?  Not [easily] with DVD's, that's for sure.
And my DDS3 streamer (12GB native) could theoretically do it, but that would 
take an immense amount of work and planning... even then, it would take a 
whopping 30+ tapes, so that basically kills several entire weekends...

In the meantime, I tend to just backup "important" stuff on DVD-R, have an 
extra spare disk in the array and just cross my fingers while waiting to 
build an entirely new 1TB array somewhere in the fall.
Maybe I should be more daring but I just can't run that risk...  and DLT is 
prohibitively expensive (even the tape media itself are _more_ expensive than 
an equivalent hdd is and I'm not even talking about the drive itself yet.)
Buying an entire second raid set of disks and burying them in a fireproof safe 
would be less expensive, and I AM including the price of the safe itself too.

Maarten

> Guy
>
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Luke Reeves
> Sent: Friday, July 09, 2004 12:55 AM
> To: linux-raid@vger.kernel.org
> Subject: Adding a new mirror disk to RAID1 configuration
>
> I have a system with two 40GB drives, and the first drive (hda) is setup
>   under an md device with RAID1 driving it.  I'm trying to now make the
> second disk, hdc, a mirror of the first.  When I try to add it using
> mdadm the second disk becomes a spare and no synchronization is done.
> Is there any way to add the second disk directly as a mirror?  Thanks.
>
> 	Luke Reeves
> 	http://www.neuro-tech.net/
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
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-- 
When I answered where I wanted to go today, they just hung up -- Unknown

RE: Adding a new mirror disk to RAID1 configuration

[Guy]

I agree with you about tapes!  For 5+ years now I have been expecting
someone to build a jukebox that uses disk drives, not tapes.  Today you can
buy jukesboxes that store tapes, optical media, cds, dvds, ...  But not hard
disks.  And like you said, a hard disk cost less than a tape with the same
capacity.  I also think the shelf life of a hard disk is longer than a tape.
And, seek time!  Hard disks are so much faster!

Back to RAID1...
I would not want my array to be in a constant bad state.  If I wanted to be
able to clone drives as you do, I would configure for 3 drives and have 3
drives so the array was in a good state.  The 3rd drive would be the clone.
Pull it when needed, then replace it.

Or, if I ever upgrade from kernel 2.4, I would use
mdadm --grow /dev/md0 --raid-disks 3
Add the drive to clone to, then back to normal.
mdadm --grow /dev/md0 --raid-disks 2
(Thanks Neil!)
Oops, not sure you can decrease the number!

Feature request!
Someone add a shrink option.
mdadm --shrink /dev/md0 --raid-disks 2
Or does --grow allow you to decrease the number?
The name would imply no.

Guy


-----Original Message-----
From: linux-raid-owner@vger.kernel.org
[mailto:linux-raid-owner@vger.kernel.org] On Behalf Of maarten van den Berg
Sent: Friday, July 09, 2004 2:36 PM
To: linux-raid@vger.kernel.org
Subject: Re: Adding a new mirror disk to RAID1 configuration

On Friday 09 July 2004 07:09, Guy wrote:
> My guess is you configured the RAID1 array as having 1 disk.  Now if you
> add more disks to it, they are spares.  You should have configured the
> array as having 2 disks, with 1 missing.  Then when you add a disk, it
will
> re-build to it.

What I do is even more ahead-thinking; I define a raid1 set with several 
missing drives as a rule. So I make a three- or even four disk array, with 
just two drives active (or just one). That way, you never run out of slots 
for extra drives. This stems from the fact that I used the raid toolkit to 
make clones of servers for a customer. Add a drive, let it sync, take it
out, 
put it in a new system with a blank disk and hey presto, another webserver.
:-)
I talked that customer into deploying three mirrored drives per server
anyway, 
seen that modern IDE drives are not as dependable as they used to be and
that 
the cost vs annoyment (read: downtime) -factor certainly warranted using 
three drives per system.   They quickly agreed, too.

> Read about raidreconf.  Google it, if needed!  It may allow you to modify
a
> RAID1 array.  I know it allows you to add a disk to a RAID5 array, but
> don't know about RAID1.  I have never used this tool.

Neither have I.  I'd gladly use it cause I have an 99.9999% full raid5 400GB

system here, but the off-chance that it kills all my data, which certainly 
isn't backed up anywhere NEAR complete keeps me from that.
How do you backup 400G ?  Not [easily] with DVD's, that's for sure.
And my DDS3 streamer (12GB native) could theoretically do it, but that would

take an immense amount of work and planning... even then, it would take a 
whopping 30+ tapes, so that basically kills several entire weekends...

In the meantime, I tend to just backup "important" stuff on DVD-R, have an 
extra spare disk in the array and just cross my fingers while waiting to 
build an entirely new 1TB array somewhere in the fall.
Maybe I should be more daring but I just can't run that risk...  and DLT is 
prohibitively expensive (even the tape media itself are _more_ expensive
than 
an equivalent hdd is and I'm not even talking about the drive itself yet.)
Buying an entire second raid set of disks and burying them in a fireproof
safe 
would be less expensive, and I AM including the price of the safe itself
too.

Maarten

> Guy
>
> -----Original Message-----
> From: linux-raid-owner@vger.kernel.org
> [mailto:linux-raid-owner@vger.kernel.org] On Behalf Of Luke Reeves
> Sent: Friday, July 09, 2004 12:55 AM
> To: linux-raid@vger.kernel.org
> Subject: Adding a new mirror disk to RAID1 configuration
>
> I have a system with two 40GB drives, and the first drive (hda) is setup
>   under an md device with RAID1 driving it.  I'm trying to now make the
> second disk, hdc, a mirror of the first.  When I try to add it using
> mdadm the second disk becomes a spare and no synchronization is done.
> Is there any way to add the second disk directly as a mirror?  Thanks.
>
> 	Luke Reeves
> 	http://www.neuro-tech.net/
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
> -
> To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> the body of a message to majordomo@vger.kernel.org
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When I answered where I wanted to go today, they just hung up -- Unknown

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Re: Adding a new mirror disk to RAID1 configuration

[Paul Clements]

Guy wrote:

> Feature request!
> Someone add a shrink option.
> mdadm --shrink /dev/md0 --raid-disks 2
> Or does --grow allow you to decrease the number?

Yes, grow can shrink the array, despite its name.

The grow command can also change the size of the array, e.g.:

mdadm --grow /dev/md0 --size 10000000

--
Paul

Re: Adding a new mirror disk to RAID1 configuration

[maarten van den Berg]

On Friday 09 July 2004 21:29, Guy wrote:
> I agree with you about tapes!  For 5+ years now I have been expecting
> someone to build a jukebox that uses disk drives, not tapes.  Today you can
> buy jukesboxes that store tapes, optical media, cds, dvds, ...  But not
> hard disks.  And like you said, a hard disk cost less than a tape with the
> same capacity.  I also think the shelf life of a hard disk is longer than a
> tape. And, seek time!  Hard disks are so much faster!

(...Not to disagree with myself, but)
Well there are drawbacks of course, but that is mostly relevant to businesses 
anyway.  Like that it is harder to insert a disk than a tape, and that disks 
cannot stand rough treatment much.  There is also the problem of the head of 
a disk sticking to the platter when left really long without use. The keyword 
here is "less [ moving parts | intelligence ], thus less to break".  That is 
true in a business setting, sure.  Virtually no amount of DLT robots would 
match the cost of a lost businessday combined with several consultants 
scrambling to get the data back in place (for fortune-500 companies).
But then again, tape also needs to be re-winded every now and again so to 
avoid it sticking together and / or to decrease the effect of adjacent layers 
messing with each others (there is a name for that effect but I forgot) so 
there is not (much) difference to disks in that regard.

Also of note is that the high speed of disks can be a drawback instead of an 
asset when it comes to backups.  A virus or a rogue (or stupid...) user can 
render a harddisks' data useless in minutes, whereas erasing a tape still 
takes hours (except with a bulk eraser but virii cannot do that).  This 
leaves you much more time to react to attacks and other bad stuff. 

But for most home users, these issues are largely irrelevant as the cost of 
rebuilding  / reacquiring the files (multiplied by the chances of something 
bad happening) is lower than what the backup thereof would cost.
Add to that the fact that for most home users the time that a backup needs to 
stay reliable is less important than for businesses (who still highly values 
his BBS collection of 320x240-pix gifs nowadays ??) 
So, disks are a reasonable alternative.

They do sell disk "jukeboxes" though, albeit not by that name.  You can buy 
NAS appliances with scsi hotswap units, that's as close to a jukebox as 
you're gonna get.  The pricetag -again- was prohibitive up till now, but we 
will most certainly see that change in 1-2 years when SATA will be default. 

> Back to RAID1...
> I would not want my array to be in a constant bad state.  If I wanted to be
> able to clone drives as you do, I would configure for 3 drives and have 3
> drives so the array was in a good state.  The 3rd drive would be the clone.
> Pull it when needed, then replace it.

Not being a coder, but what would be the possible bad consequences of a 
continuously degraded array ??  I can't imagine a degraded raid 5 array being 
any worse than a raid 0 array with the same amount of disks (leaving the 
issue of parity-calculations alone for now). It's not that there exists a 
"best-before" date on raid arrays, degraded or not. A raid array in degraded 
mode will happily survive several centuries IF the remaining disks do not 
fail.    And mdadm will (IIRC) still notify you whenever the array get 'more 
degraded' than you "designed" it to be. (Right ?)

> Or, if I ever upgrade from kernel 2.4, I would use
> mdadm --grow /dev/md0 --raid-disks 3
> Add the drive to clone to, then back to normal.
> mdadm --grow /dev/md0 --raid-disks 2

Sure, but that functionality didn't exist when I first did this. And anyway, I 
have great respect and confidence in Neil's code, but messing with the 
topology of an already built array is probably inherently more dangerous than 
just relying on the time-honoured sync and resync stages of degrading and 
(re-)filling arrays.  
But like I said, what do I know, I'm no coder nor expert on this. 

> (Thanks Neil!)
> Oops, not sure you can decrease the number!
>
> Feature request!
> Someone add a shrink option.
> mdadm --shrink /dev/md0 --raid-disks 2

I'd wager that Neil would agree with me in saying "What does it hurt to have 
too many devices defined ?" Those empty slots don't hurt nobody, except 
(maybe) a tiny little computing overhead... Overkill never killed anybody ;-)

> Or does --grow allow you to decrease the number?
> The name would imply no.

Well, ancient raidstop WAS a symlink to raidstart, so... you never know  ;-)

cheers,
Maarten

-- 
When I answered where I wanted to go today, they just hung up -- Unknown

Re: Adding a new mirror disk to RAID1 configuration

[Mark Hahn]

> anyway.  Like that it is harder to insert a disk than a tape, and that disks 
> cannot stand rough treatment much.  There is also the problem of the head of 

whereas tapes lose data if you let their ambient get warm or humid;
have you ever let a tape sit for 5 years, and tried to read it?
I certainly wouldn't fee sanguine about trying that.  not to mention
the fact that tape drives are incredibly finicky/flakey/rare.

> a disk sticking to the platter when left really long without use. The keyword 

not at all clear how much of a threat this is.  obviously, there have 
been models with design problems in the past, but I don't believe 
this is a general property of disks.  after all, disks sit on vendor 
shelves for significant periods of time.

> here is "less [ moving parts | intelligence ], thus less to break".  That is 
> true in a business setting, sure.  Virtually no amount of DLT robots would 
> match the cost of a lost businessday combined with several consultants 
> scrambling to get the data back in place (for fortune-500 companies).

critical data must be hot-replicated.  

> asset when it comes to backups.  A virus or a rogue (or stupid...) user can 
> render a harddisks' data useless in minutes, whereas erasing a tape still 
> takes hours (except with a bulk eraser but virii cannot do that).  This 
> leaves you much more time to react to attacks and other bad stuff. 

ah, a great new security idea: slow down all IO!  seriously, real data
management means tracking versions, not just slamming the latest version
over top of last night's.

> Not being a coder, but what would be the possible bad consequences of a 
> continuously degraded array ??

some reads require you to run the block-parity algorithm to reconstruct
the data.  some writes, too.  the worst is that your data is now vulnerable.

> I can't imagine a degraded raid 5 array being 
> any worse than a raid 0 array with the same amount of disks (leaving the 
> issue of parity-calculations alone for now).

you can't ignore the parity, since with a disk gone, some of your data
has to be inferred using parity.

> It's not that there exists a 
> "best-before" date on raid arrays, degraded or not.

it's purely a risk-assessment question.  if you build an 8+1 raid5,
and lose one disk, then the next disk failure will kill your data.
the liklihood of one of the 8 remaining disks failing is 8x the liklihood
of a single disk failing (probably higher, since the initial failure was
probably not completely independent.)

the main issue is that your liklihood of losing data takes a steep dive
when you're in degraded mode.  you were smugly robust, and now are holding
your breath, vulnerable...

Re: Adding a new mirror disk to RAID1 configuration

[Maarten van den Berg]

On Saturday 10 July 2004 02:03, Mark Hahn wrote:


> > here is "less [ moving parts | intelligence ], thus less to break".  That
> > is true in a business setting, sure.  Virtually no amount of DLT robots
> > would match the cost of a lost businessday combined with several
> > consultants scrambling to get the data back in place (for fortune-500
> > companies).
>
> critical data must be hot-replicated.

...in addition to, yes. But that same critical data must be backed up 
somewhere far away, safe, too.  And that is not hot-replicateable.  
Any stuff that is hot-replicated WILL suffer the same fate as the original 
stuff in case of a user thinko, virus or intrusion.  Hot replication guards 
against media failure<period>  It does not guard against anything else. 

> > asset when it comes to backups.  A virus or a rogue (or stupid...) user
> > can render a harddisks' data useless in minutes, whereas erasing a tape
> > still takes hours (except with a bulk eraser but virii cannot do that). 
> > This leaves you much more time to react to attacks and other bad stuff.
>
> ah, a great new security idea: slow down all IO!  seriously, real data
> management means tracking versions, not just slamming the latest version
> over top of last night's.

Laugh all you want but I've seen it happen all the time. The CEO starts 
deleting a bunch of stuff and realizes his error too late. Thanks for tape 
backups, because even with a fifty-way raid disk mirror set that data would 
be gone.  The fact that tapemedia are offline seriously increases the 
security thereof. The same goes for CD / DVD media. If it's not online, it's 
not eraseable.  In the case of a tape robot, it is online (more or less) but 
it will take a week erasing all. Not so with harddisks.  Are you still saying 
you do not see any advantages in that slow speed, security-wise ???

> > Not being a coder, but what would be the possible bad consequences of a
> > continuously degraded array ??
>
> some reads require you to run the block-parity algorithm to reconstruct
> the data.  some writes, too.  the worst is that your data is now
> vulnerable.

Please read back further in a thread before you start contributing without 
knowing what it was about..  The issue was about raid 1, and whether it was 
bad to have missing drives as_a_matter_of_fact. To be totally clear about 
this once and for all: This is the question we're talking about:  
A RAID1 set with four disks defined, two missing
OR 
A RAID1 set with two disks defined, none missing

> > I can't imagine a degraded raid 5 array being
> > any worse than a raid 0 array with the same amount of disks (leaving the
> > issue of parity-calculations alone for now).
>
> you can't ignore the parity, since with a disk gone, some of your data
> has to be inferred using parity.

As I said above, you are missing the point here.  And I left out the parity 
just because it bears no relevance at all to the question, which is stated 
above.  Or to make it even clearer yet: What is better, a degraded two-disk 
raid1 or a single drive ?

> > It's not that there exists a
> > "best-before" date on raid arrays, degraded or not.
>
> it's purely a risk-assessment question.  if you build an 8+1 raid5,
> and lose one disk, then the next disk failure will kill your data.
> the liklihood of one of the 8 remaining disks failing is 8x the liklihood
> of a single disk failing (probably higher, since the initial failure was
> probably not completely independent.)

PLease read back.  You're confusing this with another thread perhaps.
I'll repeat:  I stated that you can build a raid 1 array consisting of 4 
drives; two online, and two missing.  So, it IS degraded, but it has two 
drives, which is exactly identical to a traditional raid-1 two-disk setup.
Hence, no data is at risk any more than in the second case. 

Now in this special scenario, we were just wondering if the state of being 
degraded held any drawbacks. Note again that it is the EXACT same physical 
layout: we have two drives and they are a mirror of each other. 

Maarten

Re: Adding a new mirror disk to RAID1 configuration

[Neil Brown]

On Friday July 9, luke@neuro-tech.net wrote:
> I have a system with two 40GB drives, and the first drive (hda) is setup 
>   under an md device with RAID1 driving it.  I'm trying to now make the 
> second disk, hdc, a mirror of the first.  When I try to add it using 
> mdadm the second disk becomes a spare and no synchronization is done. 
> Is there any way to add the second disk directly as a mirror?  Thanks.

If you have a very recent 2.6 kernel and very recent mdadm, then you
can

   mdadm --grow /dev/md0 --raid-disks 2

to tell it that there should really be 2 raid disks, and then the
spare will start synchronising.

Otherwise you will need to recreate the array telling that you really
wanted it to have to raid disks (one missing or failed) in the first
place.

This is assuming that it currently thinks it has one raid-disk.
A
  cat /proc/mdstat
or
  mdadm -D /dev/md0

would help.

NeilBrown