RAID: identical disks?

[void]

I've read that it's best to set up RAID with identical disks (same brand and
model). I've got a Samsung SP1213N hard drive which I got 21 months ago, and
now I want to set up RAID 1 on my machine. So I'll need to get another
Samsung SP1213N. But I know that sometimes manufacturers will make slight
changes to hardware and still keep the same model number. I don't know if
Samsung has done anything to this particular drive in the 21 months since I
bought mine, so hopefully buying a recent one will be OK to set up a RAID
mirror.

 

[Jim]

Using identical HDs is best, but rarely required. The primary reason for
preferring identical HDs is not compatibility, it's that you'll simply end
up losing space needlessly. Your RAID configuration utility will always
choose the smaller of the HDs when determining the RAID maximum size (of
course). Thus, any space still remaining on the larger HD is essentially
lost.

Some RAID configurations will even let you limit the maximum size even when
the HDs are identical! That's because two HD of size, say, 60GB, may not be
exactly the same in actual bytes, some manufacturers will produce HDs with
slightly more or less than 60GB. I've even seen the SAME model from the
SAME manufacturer a year or two later show a slightly different # of bytes!
So a good RAID controller will allow you some flexibility when configuring
to lower the maximum size of the array by a few bytes, so in the event you
need to replace one of the HDs, you have more replacement options.

The only real hassle in this area I've seen is when your RAID controller is
not so flexible, and then you have to replace one of the HDs, and you can't
find an exact match. If your replacement is slightly less in size, then the
RAID controller will not accept it, it simply can't allow some data to be
lost due to this discrepancy. The solution is to copy the data from the
existing larger drive to the new smaller drive first, then use the new
smaller drive as the source when reinitializing the array. IOW, it's not
really a big deal, just more of a hassle.

Of course, for performance reasons, it's best to match on ATA spec (e.g.,
ATA100), RPMs (e.g., 7200), access time, etc., the better matched the less
degradation in performance. You'll only get out of the RAID'd pair the
lesser of the two HDs in terms of performance, of course.

So bottomline is, there's really not that much to worry about concerning
matched HDs. If you can find a perfectly matched pair, fine, it makes life
a simpler. But it's no big deal. Heck, if you need to replace it in the
future, you can't even be sure you can find a *perfect* replacement. The
RAID controller recognizes that fact, so will let you install ANY HD that
has at least as much capacity as the failed HD. To do otherwise would be
unrealistic.

HTH

Jim

 

[void]

Using identical HDs is best, but rarely required. The primary reason for
preferring identical HDs is not compatibility, it's that you'll simply end
up losing space needlessly. Your RAID configuration utility will always
choose the smaller of the HDs when determining the RAID maximum size (of
course). Thus, any space still remaining on the larger HD is essentially
lost.

Some RAID configurations will even let you limit the maximum size even when
the HDs are identical! That's because two HD of size, say, 60GB, may not be
exactly the same in actual bytes, some manufacturers will produce HDs with
slightly more or less than 60GB. I've even seen the SAME model from the
SAME manufacturer a year or two later show a slightly different # of bytes!

This is what I was wondering about... I've got a 120 GB drive, and want to
buy the exact same model. But the one that I have is 21 months old, so I
wonder if the latest ones will be the same size or slightly different?

It would suck if I bought another one and it was slightly smaller... then I
wouldn't be able to mirror my current one. Of course, I could buy a larger
one, but like you say, the extra space will be wasted. Hmm, what to do?

I've got a Highpoint 370 RAID controller (built in to my Abit KT7-RAID
motherboard... it's old, I know). Do you (or anyone) know if it would be
able to detect if the new hard drive is slightly smaller than my current one?

So a good RAID controller will allow you some flexibility when configuring
to lower the maximum size of the array by a few bytes, so in the event you
need to replace one of the HDs, you have more replacement options.

The only real hassle in this area I've seen is when your RAID controller is
not so flexible, and then you have to replace one of the HDs, and you can't
find an exact match. If your replacement is slightly less in size, then the
RAID controller will not accept it, it simply can't allow some data to be
lost due to this discrepancy. The solution is to copy the data from the
existing larger drive to the new smaller drive first, then use the new
smaller drive as the source when reinitializing the array. IOW, it's not
really a big deal, just more of a hassle.

So that's all I have to do if the new one is smaller than my current one?
Copy the data from the current one to the new one. Since I've never done this
before, could you recommend some programs I could use to do the copying? I'm
running Windows 2000.

Of course, for performance reasons, it's best to match on ATA spec (e.g.,
ATA100), RPMs (e.g., 7200), access time, etc., the better matched the less
degradation in performance. You'll only get out of the RAID'd pair the
lesser of the two HDs in terms of performance, of course.

So bottomline is, there's really not that much to worry about concerning
matched HDs. If you can find a perfectly matched pair, fine, it makes life
a simpler. But it's no big deal. Heck, if you need to replace it in the
future, you can't even be sure you can find a *perfect* replacement. The
RAID controller recognizes that fact, so will let you install ANY HD that
has at least as much capacity as the failed HD. To do otherwise would be
unrealistic.

HTH

Jim

Thank you Jim for the very detailed response.

 

[Arno Wagner]

I've read that it's best to set up RAID with identical disks (same brand and
model). I've got a Samsung SP1213N hard drive which I got 21 months ago, and
now I want to set up RAID 1 on my machine. So I'll need to get another
Samsung SP1213N. But I know that sometimes manufacturers will make slight
changes to hardware and still keep the same model number. I don't know if
Samsung has done anything to this particular drive in the 21 months since I
bought mine, so hopefully buying a recent one will be OK to set up a RAID
mirror.

No need to have the same disks. Just make sure that if you replace a
failed disk in an existing array, that the new disk has the exact
same sector number or a larger one. (Disk sizes vary by a bit between
different models and manufacturers, even if the same size is claimed.)

Arno

 

[wolfgang schneider]

I've read that it's best to set up RAID with identical disks (same brand and
model). I've got a Samsung SP1213N hard drive which I got 21 months ago, and
now I want to set up RAID 1 on my machine. So I'll need to get another
Samsung SP1213N. But I know that sometimes manufacturers will make slight
changes to hardware and still keep the same model number. I don't know if

yes .

Samsung has done anything to this particular drive in the 21 months since I
bought mine, so hopefully buying a recent one will be OK to set up a RAID
mirror.

it mighn't be 100% perfect ;-)

 

[Paul]

I've read that it's best to set up RAID with identical disks (same brand and
model). I've got a Samsung SP1213N hard drive which I got 21 months ago, and
now I want to set up RAID 1 on my machine. So I'll need to get another
Samsung SP1213N. But I know that sometimes manufacturers will make slight
changes to hardware and still keep the same model number. I don't know if
Samsung has done anything to this particular drive in the 21 months since I
bought mine, so hopefully buying a recent one will be OK to set up a RAID
mirror.

Buy two new drives. Use the old Samsung SP1213N as your backup
drive. A RAID can still malfunction and destroy the contents
of both drives. That is why a backup drive is still a requirement.
When the backup operation is finished, disconnect the backup
drive and put it in a safe place. That will prevent a bad
power supply from destroying all three disk drives.

What is the benefit of a mirror ? It eliminates the need to
do immediate maintenance, in the event of a failure of one
of the two disks. It doesn't eliminate the need to do backups,
when both drives fail or both drives are destroyed by a
calamity (could be software, firmware, hardware, or a physical
event, like a fire or flood).

Paul

 

[Jim]

bytes!

This is what I was wondering about... I've got a 120 GB drive, and want to
buy the exact same model. But the one that I have is 21 months old, so I
wonder if the latest ones will be the same size or slightly different?

It would suck if I bought another one and it was slightly smaller... then I
wouldn't be able to mirror my current one. Of course, I could buy a larger
one, but like you say, the extra space will be wasted. Hmm, what to do?

I've got a Highpoint 370 RAID controller (built in to my Abit KT7-RAID
motherboard... it's old, I know). Do you (or anyone) know if it would be
able to detect if the new hard drive is slightly smaller than my current one?

Again, it really doesn't matter. Let's assume the new one is even 1 byte
smaller, so what? Just hook up new HD, copy older larger HD's data to new
smaller HD, and make the new smaller HD the master. Then setup RAID to use
new smaller HD as source, older larger HD as target! Since your old HD is
now LARGER, the RAID controller won't care. Unless your current HD is using
up EVERY LAST BYTE and can't be downsized a smidgen, I don't see the
problem. You can use a variety of partition products like BootIt NG,
Partition Magic, etc., to CLONE from one HD to the other. The only
complication is that if the older HD is larger, and you're consuming ALL of
it, it obviously has a larger partition than the new HD can accomodate. So
the partitioning software has to support RESIZING, which most all do.

So that's all I have to do if the new one is smaller than my current one?
Copy the data from the current one to the new one. Since I've never done this
before, could you recommend some programs I could use to do the copying? I'm
running Windows 2000.

YES! As I said, this is really a non-problem unless you have some extreme
situation that escapes me.

To CLONE (which is really what you want, you want an EXACT COPY), you could
use BootIt NG.

1) Visit http://www.bootitng.com and download BootIt NG. Unzip the file and
execute BOOTITNG.EXE to create the bootable floppy or CD, your choice
(accept the defaults). Now shutdown the PC.

2) Assuming old (larger) HD is master on primary IDE controller (IDE1), move
old HD to slave on IDE1, or master on secondary IDE controller (IDE2).
Install new (smaller) HD to master on IDE1 (replacing old HD). Be sure to
get HD jumpers setup correctly! Use cable select (CS) if unsure.

3) Now boot the BootIT NG floppy/CD. When the Welcome to Setup screen
appears, select Cancel, follow the prompts, and you will be taken to the
BootIt NG desktop, now select the Partition Manager.

4) Your new HD will be displayed by default (HD0). To see the contents of
the old HD, select HD1 (in the upper left corner). To clone HD1 to HD0,
select the first partition on HD1, hit Copy, switch to HD0, select the
freespace, and hit Paste. Obviously if the old partition(s) doesn't fit,
you'll need to Resize until it does. Select the source partition, hit
Resize, choose a new size, then hit OK. Repeat for each partition on HD1,
in order. NOTE: It's always possible that a Resize may not be allowed *if*
data exists at the end of the partition, despite having lots of freespace.
In that case, defrag your partition(s) first!

5) At this point, although all the partitions on the old HD (HD1) have been
copied to the new HD (HD0), the new HD is NOT yet bootable, we'll correct
that now. Select HD0 from the upper left corner. Hit View MBR, and in the
dialog, you will notice four entries in the MBR (Master Boot Record). Each
entry w/ a non-zero address represents one of your cloned partitions.
Select the bootable partition (usually the first MBR entry), hit "Set
Active", and the partition will indicate Active status. Now hit "Std MBR",
this will initialize the boot loader in the MBR. Finally, hit Apply to save
the changes.

6) Hit Close, remove the BootIt NG floppy/CD, and hit Reboot. Your system
will now reboot as before but using the new HD. Before rebooting Windows,
you may wish to shutdown and remove the old HD (see notes below).

NOTE: As always, anytime you're mucking w/ partitions, it's best to make a
backup, and use a UPS!

Once your systems runs successfully on the new HD, you can configure your
RAID controller for mirroring using the new HD as your source, the old HD as
the target. Of course, the initialization process will destroy the data on
the old HD, but that's to be expected.

Jim

 

[void]

Again, it really doesn't matter. Let's assume the new one is even 1 byte
smaller, so what? Just hook up new HD, copy older larger HD's data to new
smaller HD, and make the new smaller HD the master. Then setup RAID to use
new smaller HD as source, older larger HD as target! Since your old HD is
now LARGER, the RAID controller won't care. Unless your current HD is using
up EVERY LAST BYTE and can't be downsized a smidgen, I don't see the
problem. You can use a variety of partition products like BootIt NG,
Partition Magic, etc., to CLONE from one HD to the other. The only
complication is that if the older HD is larger, and you're consuming ALL of
it, it obviously has a larger partition than the new HD can accomodate. So
the partitioning software has to support RESIZING, which most all do.

OK, one more question. When I get the new hard drive, how can I tell if it is
larger or smaller than my current one?

Thanks again for the detailed response!

 

[void]

No need to have the same disks. Just make sure that if you replace a
failed disk in an existing array, that the new disk has the exact
same sector number or a larger one.

Is it the number of sectors that matters, or just the total size of the disk?

 

[Jim]

OK, one more question. When I get the new hard drive, how can I tell if it is
larger or smaller than my current one?

Thanks again for the detailed response!

You can't be 100% absolutely guaranteed that HDs of the same make and model
are the same, on a very rare occasion, even these will differ if the
manufacturer changes their firmware, for example. But 99.9 of the time, two
identical make and model will be identical in actual storage, and most of
the time, even across makes and models. The only way you know with
absolutely certainty is to buy and try.

If two HDs are of different size (e.g., 60GB vs 80GB), then obviously we
know which is larger or smaller. The differences here that are in question
are ONLY when the two HDs are marked as BOTH 60GB, or BOTH 80GB, etc., and
even then, the differences, where applicable, are extremely small, not
enough to label each as a different size on the label, but enough to matter
to RAID (which will complain about even one byte difference).

Jim

 

[Arno Wagner]

No need to have the same disks. Just make sure that if you replace a
failed disk in an existing array, that the new disk has the exact
same sector number or a larger one.

[/QUOTE]

Is it the number of sectors that matters, or just the total size of
the disk?

That is the same. But the number of sectors is stated with absolute
accuracy, while the total size may be accurate (if stated in Bytes)
or may be approximate (e.g. 160GB). Usually it is only approximate
and the disk is slightly larger (may be several 100MBs). Often
(Samsung, Seagate, Maxtor,...) the number of sectors is stated
as LBA <some number> or <some number> LBA. You can also get it
from the datasheet of the disk. The total size is
512 Bytes * <number of sectors)
and is given with SI prefixes.

Arno

 

[J. Clarke]

I've read that it's best to set up RAID with identical disks (same brand
and
model). I've got a Samsung SP1213N hard drive which I got 21 months ago,
and
now I want to set up RAID 1 on my machine. So I'll need to get another
Samsung SP1213N. But I know that sometimes manufacturers will make slight
changes to hardware and still keep the same model number. I don't know if
Samsung has done anything to this particular drive in the 21 months since
I bought mine, so hopefully buying a recent one will be OK to set up a
RAID mirror.

In practice, as long as the secondary is of the same capacity or higher than
the primary, and of about the same performance, a RAID1 will work fine. In
fact there is a school of thought that the two disks should be _different_
brands and/or models on the theory that if they are they same they might be
so closely matched that they both might fail at the same time or close
enough to it that you don't have time to rebuild the mirror.

Having seen four Japanese-made light bulbs installed at the same time fail
within four hours of each other a year and a half later, I am not going to
denigrate this notion, although I don't consider such failure to be
exceedingly likely.

 

[Guest]

Yeah so that's what my question is. If I go ahead and buy another
drive that's marked as the same size, how do I know if the new drive is
smaller than my current one (in which case I will need to copy the data
from my current drive to the new one) or if the new drive is larger
than my current one (in which case I won't need to copy the data)? Do
I just go ahead and try to set up RAID-1, with the current drive as the
source and the new drive as the target, and if the new drive is
smaller, the RAID controller will detect that and refuse to proceed?

Thanks again.


Jim wrote:

You can't be 100% absolutely guaranteed that HDs of the same make and
model
are the same, on a very rare occasion, even these will differ if the
manufacturer changes their firmware, for example. But 99.9 of the
time, two
identical make and model will be identical in actual storage, and most
of
the time, even across makes and models. The only way you know with
absolutely certainty is to buy and try.

If two HDs are of different size (e.g., 60GB vs 80GB), then obviously
we
know which is larger or smaller. The differences here that are in
question
are ONLY when the two HDs are marked as BOTH 60GB, or BOTH 80GB, etc.,
and
even then, the differences, where applicable, are extremely small, not
enough to label each as a different size on the label, but enough to
matter
to RAID (which will complain about even one byte difference).

 

[Jim]

lol, yeah, that's one way Why worry about it UNTIL it happens? If it
happens, you'll find out soon enough, right? Why worry about what *might*
happen, when the chances are very good it won't, and if it does, it's an
easy fix. It's not like something is going to BREAK or cause a catastrophe
here, it's merely an inconvenience, one for which you now have a solution,
should it happen.

Jim

 

[Rod Speed]

In practice, as long as the secondary is of the same capacity or
higher than the primary, and of about the same performance, a RAID1
will work fine. In fact there is a school of thought that the two
disks should be _different_ brands and/or models on the theory that
if they are they same they might be so closely matched that they both
might fail at the same time or close enough to it that you don't have
time to rebuild the mirror.

Having seen four Japanese-made light bulbs installed at the same time
fail within four hours of each other a year and a half later, I am
not going to denigrate this notion, although I don't consider such
failure to be exceedingly likely.

Yeah, its one approach, particularly if something goes
out of spec like the cooling or power supply etc.

 

[Arno Wagner]

(e-mail address removed) wrote:

In practice, as long as the secondary is of the same capacity or
higher than the primary, and of about the same performance, a RAID1
will work fine.

Why that? Do you assume array creation only with the primay disk
connected? Or is some RAID software too stupid to recognize which
disk is the smaller one?

In fact there is a school of thought that the two disks should be
_different_ brands and/or models on the theory that if they are they
same they might be so closely matched that they both might fail at
the same time or close enough to it that you don't have time to
rebuild the mirror.

This sure makes sense if both disks have a shared weakness, e.g.
extreme sensitivity to heat, power-spikes, shock or the like.
I find it overly paranoid for everyday use, but especially in
a machine you cannot easily get to this makes sense. There you
might even consider using 3 or more different disks in RAID1
array.

Having seen four Japanese-made light bulbs installed at the same
time fail within four hours of each other a year and a half later, I
am not going to denigrate this notion, although I don't consider
such failure to be exceedingly likely.

Pretty unlikely without a common factor. Maybe overvoltage or
mechanical shock?

Arno

 

[kony]

Yeah so that's what my question is. If I go ahead and buy another
drive that's marked as the same size, how do I know if the new drive is
smaller than my current one (in which case I will need to copy the data
from my current drive to the new one) or if the new drive is larger
than my current one (in which case I won't need to copy the data)? Do
I just go ahead and try to set up RAID-1, with the current drive as the
source and the new drive as the target, and if the new drive is
smaller, the RAID controller will detect that and refuse to proceed?

You could do that, yes it would "refuse" to proceed.

If the drive is already a couple years old, there might not
be much if any cost difference between the remaining-working
old drive and a slightly larger new drive (for example, old
drive 60GB and new 80GB), if you wanted to completely avoid
the issue just buy an 80GB replacement and only use 60GB of
it. So you waste 20GB... not a big deal if the cost
difference wasn't much, though if that first drive fails and
you already had a larger 2nd drive, THEN when you go to
replace the first you would have the option of making whole
array larger if it's worth the hassle of copying off the
data to recreate it in the larger size.

 

[Curious George]

What is the benefit of a mirror ? It eliminates the need to
do immediate maintenance, in the event of a failure of one
of the two disks.

In addition, it can allow hardware replacement without taking a system
or volume down.

& when properly implemented it can greatly reduce, even virtually
eliminate the risk of lost data due to bad sectors. In some cases
there are also extra verification of disk IO well beyond regular disk
error checking.

It doesn't eliminate the need to do backups,
when both drives fail or both drives are destroyed by a
calamity (could be software, firmware, hardware, or a physical
event, like a fire or flood).

don't forget the more common calamities like operator error, malware,
malicious users, and some business compliance & insurance
requirements.

 

[Curious George]

In practice, as long as the secondary is of the same capacity or higher than
the primary, and of about the same performance, a RAID1 will work fine.

When disks are mismatched the best case scenario is performance,
space, firmware optimizations are limited by the lesser drive. In a
worst case scenario it causes compatibility problems. Fortunately on
modern hardware esp with software or firmware assisted software raid
this worst case scenario is virtually a non-issue.

In
fact there is a school of thought that the two disks should be _different_
brands and/or models on the theory that if they are they same they might be
so closely matched that they both might fail at the same time or close
enough to it that you don't have time to rebuild the mirror.

Having seen four Japanese-made light bulbs installed at the same time fail
within four hours of each other a year and a half later, I am not going to
denigrate this notion, although I don't consider such failure to be
exceedingly likely.

There is most definitely a U-shaped or bathtub curve to hardware
failure over time. However both drives in a 2 drive array dying
natural deaths within hours of another is quite unlikely.

Rather than mixing models some ppl buy parts from different suppliers
to hedge their bets or simply count on premature failure to introduce
media of different ages or simply proactively decommission arrays at
the end of expected service life rather than wait for the catastrophic
event.

 

[J. Clarke]

When disks are mismatched the best case scenario is performance,
space, firmware optimizations are limited by the lesser drive.

Yep, but a WD and a Seagate of the same vintage will generally be pretty
closely matched in performance, space, and firmware optimizations, and if
there is a difference in capacity between one bran of 250 GB drive and
another, it's not so huge a difference that one would consider it
"limiting" in any but the most pedantic sense.

In a
worst case scenario it causes compatibility problems. Fortunately on
modern hardware esp with software or firmware assisted software raid
this worst case scenario is virtually a non-issue.

While that is indeed a worst case, have you ever seen it actually happen?
Most RAID today is software, not hardware, and quite honestly Windows and
Linux and Novell don't _care_ whether a RAID is composed of different
brands and models as long as the capacity and performance are about the
same.

There is most definitely a U-shaped or bathtub curve to hardware
failure over time. However both drives in a 2 drive array dying
natural deaths within hours of another is quite unlikely.

I believe that I said "exceedingly unlikely" myself. It can happen though.

Rather than mixing models some ppl buy parts from different suppliers
to hedge their bets or simply count on premature failure to introduce
media of different ages or simply proactively decommission arrays at
the end of expected service life rather than wait for the catastrophic
event.

Where does one find an "expected service life" rating?