Newbie Question re hardware vs software RAID

I'm looking at installing a SATA RAID 5 system in my home PC to provide a
means of data recovery in case of hard disk failure (I'm thinking this will
be cheaper in the long run than installing a tape drive for a Tb of data).
When the 400Gb drives are released I'll be looking at getting 4 for the
array.

Is there any way of determining from the manyfacturer specs if the cards are
software or hardware based for the parity calculations. A review on Toms
hardware for the RocketRAID 1820 said that it was software based. Another
article elsewhere indicated that if the card had an XOR processor then the
parity calculations were hardware based and there is a RocketRAID 1820A with
an XOR processor. I don't know how valid that indication about the XOR
processor was.

I'm looking at hardware based RAID because I use Norton Ghost to back up my
system partition (which I plan to have of the RAID 5 volume) and Ghost uses
MS DOS which I don't think a software system would support.

Thanks

On Sun, 7 Nov 2004 23:30:12 +1030, "Gilgamesh"
wrote:

I'm looking at installing a SATA RAID 5 system in my home PC to provide a
means of data recovery in case of hard disk failure (I'm thinking this will
be cheaper in the long run than installing a tape drive for a Tb of data).
When the 400Gb drives are released I'll be looking at getting 4 for the
array.

RAID protection is very different than backup protection. RAID
increases availability/uptime. You need regular offline/offsite
backups regardless. If you have that much important data, you're not
going to be able to get away from spending a few thousand USD to back
it up properly regardless of method.

Is there any way of determining from the manyfacturer specs if the cards are
software or hardware based for the parity calculations. A review on Toms
hardware for the RocketRAID 1820 said that it was software based. Another
article elsewhere indicated that if the card had an XOR processor then the
parity calculations were hardware based and there is a RocketRAID 1820A with
an XOR processor. I don't know how valid that indication about the XOR
processor was.

It doesn't really matter. On the low end you are going to encounter
unnecessary headaches regardless of the exact role of the driver.
Proper implementation of RAID 5 is somewhat of an engineering
nightmare & I would stay away from all parity raid levels on the
"personal storage" level anyway.

I'm looking at hardware based RAID because I use Norton Ghost to back up my
system partition (which I plan to have of the RAID 5 volume) and Ghost uses
MS DOS which I don't think a software system would support.

All you should need is the correct msdos driver to get it to work.
Depending on the card, you may need this even with a full firmware
enterprise controller. Full native MSdos support is spotty across all
raid cards. Fully firmware raid does not necessarily protect you from
issues with dos disk utility support.

"Gilgamesh" wrote in message
...
I'm looking at installing a SATA RAID 5 system in my home PC to provide a
means of data recovery in case of hard disk failure (I'm thinking this
will
be cheaper in the long run than installing a tape drive for a Tb of data).
When the 400Gb drives are released I'll be looking at getting 4 for the
array.

Is there any way of determining from the manyfacturer specs if the cards
are
software or hardware based for the parity calculations. A review on Toms
hardware for the RocketRAID 1820 said that it was software based. Another
article elsewhere indicated that if the card had an XOR processor then the
parity calculations were hardware based and there is a RocketRAID 1820A
with
an XOR processor. I don't know how valid that indication about the XOR
processor was.

I'm looking at hardware based RAID because I use Norton Ghost to back up
my
system partition (which I plan to have of the RAID 5 volume) and Ghost
uses
MS DOS which I don't think a software system would support.

Thanks



RAID 5 is a high end solution - to get a good RAID 5 hardware card, you'll
spend more than another drive will cost, anyways. Better to look at RAID 1
or 0+1 - it's faster because very little calculation is required; a good
card can be had for as little as $15. Rebuilding the array after a drive
fails will be much faster, too.

I think another poster already mentioned, though - it's not a backup. In
your shoes, I'd buy a couple of external HDD cases, and put your new 400GB
drives into them - then ghost or backup your data onto them, and keep them
in a safe place off site. Where would your terabyte of data be if, God
forbid, you had a house fire? Or, some trojan on your computer was serving
kiddie porn, and the FBI confiscated your computer? Or some kid managed to
download a virus, and answered "yes" to "format all"? Or, for that matter,
if Windows does something unexpected (THAT never happens, right?) and you
can't get to your data? You'd be HOSED.

Good Luck!
ECM

On Mon, 8 Nov 2004 22:11:35 -0800, "ECM"
wrote:

RAID 5 is a high end solution - to get a good RAID 5 hardware card, you'll
spend more than another drive will cost, anyways. Better to look at RAID 1
or 0+1
1+0 is generally better than 0+1 from a fault recoverability
standpoint. It is very fast so 0+1 is advisable in fewer
circumstances.

- it's faster because very little calculation is required;
No parity calculation is done, actually.

a good
card can be had for as little as $15. Rebuilding the array after a drive
fails will be much faster, too.
Like what? I'd be interested to know. Considering the state of
enterprise controllers I'm quite fearful of the $15 category.

Curious George wrote in message . ..
On Mon, 8 Nov 2004 22:11:35 -0800, "ECM"
wrote:

RAID 5 is a high end solution - to get a good RAID 5 hardware card, you'll
spend more than another drive will cost, anyways. Better to look at RAID 1
or 0+1
1+0 is generally better than 0+1 from a fault recoverability
standpoint. It is very fast so 0+1 is advisable in fewer
circumstances.

- it's faster because very little calculation is required;
No parity calculation is done, actually.

a good
card can be had for as little as $15. Rebuilding the array after a drive
fails will be much faster, too.
Like what? I'd be interested to know. Considering the state of
enterprise controllers I'm quite fearful of the $15 category.

I wasn't really speaking of enterprise level equipment - the OP was
asking about home use, I believe.

And yes, you're right - 1+0 is faster to recover, especially in large
arrays. I'm not sure whether the lower end cards support 1+0, however
- most advertise 0+1. The GigaRAID controller built in to Gigabyte's
MB's, for instance, doesn't give you the option of 1+0 - it sets up
the RAID 0 arrays and then mirrors one with the other; there are no
other options.

Either way, however, would be preferable to a RAID 5 solution
(especially a cheap one). And I believe both of us mentioned that RAID
in general is not a substitute for backups..... I always remind myself
that there's at least three parts of any RAID array that can fail -
two drives and a controller.

ECM

On 11 Nov 2004 08:37:58 -0800, (ECM) wrote:

Curious George wrote in message . ..
On Mon, 8 Nov 2004 22:11:35 -0800, "ECM"
wrote:

RAID 5 is a high end solution - to get a good RAID 5 hardware card, you'll
spend more than another drive will cost, anyways. Better to look at RAID 1
or 0+1
1+0 is generally better than 0+1 from a fault recoverability
standpoint. It is very fast so 0+1 is advisable in fewer
circumstances.

- it's faster because very little calculation is required;
No parity calculation is done, actually.

a good
card can be had for as little as $15. Rebuilding the array after a drive
fails will be much faster, too.
Like what? I'd be interested to know. Considering the state of
enterprise controllers I'm quite fearful of the $15 category.

I wasn't really speaking of enterprise level equipment - the OP was
asking about home use, I believe.

I know. What I was saying was my experience with enterprise
controllers causes me to question the viability of extremely cheap
consumer level cards. I'm not trying to shoot you down, I would still
like to know if you would recommend one such card.

And yes, you're right - 1+0 is faster to recover, especially in large
arrays.

It's not that it restores faster, it's that it can sustain a greater
number of failures. When you loose a drive in a 0+1 the array becomes
essentially a raid 0 striped set. With RAID 1+0 you can loose a
maximum of half your drives and still operate (provided you are lucky
and the 'right' ones fail). Because they both use striped & mirrored
data they _should_ restore at the same/similar rate. But it doesn't
really matter so much as running degraded/recovery mode does not
affect performance to the extent the parity levels can.

I'm not sure whether the lower end cards support 1+0, however
- most advertise 0+1.

because the low end is marketed to the enthusiast segment who are more
concerned with performance (even mythical) over reliability and they
use small non-mission-critical arrays.

The GigaRAID controller built in to Gigabyte's
MB's, for instance, doesn't give you the option of 1+0 - it sets up
the RAID 0 arrays and then mirrors one with the other; there are no
other options.

on-board raid is not something to be taken seriously as it lacks so
many important features. From what I see the same is true of low-end
cards. Lacking such features can invalidate the theoretical
reliability gains from raid. The devil is in the details.

Even if a particular controller does not support a nested raid level
you can still do it by using a combination of firmware and software
raid (i.e. OS stripe of 2 mirrored logical drives or OS mirror of 2
striped sets or OS stripe of 2 raid 5 logical drives, etc.) -but you
won't be able to boot of it.

Either way, however, would be preferable to a RAID 5 solution
(especially a cheap one). And I believe both of us mentioned that RAID
in general is not a substitute for backups..... I always remind myself
that there's at least three parts of any RAID array that can fail -
two drives and a controller.

ECM

good points.

In message Curious George
wrote:

And yes, you're right - 1+0 is faster to recover, especially in large
arrays.

It's not that it restores faster, it's that it can sustain a greater
number of failures. When you loose a drive in a 0+1 the array becomes
essentially a raid 0 striped set. With RAID 1+0 you can loose a
maximum of half your drives and still operate (provided you are lucky
and the 'right' ones fail). Because they both use striped & mirrored
data they _should_ restore at the same/similar rate. But it doesn't
really matter so much as running degraded/recovery mode does not
affect performance to the extent the parity levels can.

You should take this into account when you're buying drives too.

Buy half your drives from one batch, and half from another batch (if you
have that control -- Staying with the same model if you can, of course.

If there is a physical defect which affects an entire batch, it won't
take down your entire array, it will probably take down the affected
batch only and you can replace the drives.


--
I've given up on sigs. I just couldn't think of anything clever to say.

On Sat, 13 Nov 2004 05:08:28 GMT, Curious George wrote:


And yes, you're right - 1+0 is faster to recover, especially in large
arrays.

It's not that it restores faster, it's that it can sustain a greater
number of failures. When you loose a drive in a 0+1 the array becomes
essentially a raid 0 striped set. With RAID 1+0 you can loose a
maximum of half your drives and still operate (provided you are lucky
and the 'right' ones fail).

Sorry, I realize that is a bit of an overstatement. That can be true
But it is actually more complicated than that. The truth is that
there is a marked increase in the likelihood that a second disk
failure/multiple failures will bring down a 0+1 array. Both 0+1 & 1+0
are supposed to be able to sustain more than one failure in certain
circumstances.

Also when a failed disk is replaced 1+0 only has to re-mirror one
drive but 0+1 has to re-mirror the entire failed set. RAID 1+0 is
supposed to recover much faster but in reality, restore speed is also
determined by the priority that process is assigned as well as other
peculiarities of the controller, array, etc.. I'm not sure that this
is much of a reason to choose one level over another.

In message Curious George
wrote:

Sorry, I realize that is a bit of an overstatement. That can be true
But it is actually more complicated than that. The truth is that
there is a marked increase in the likelihood that a second disk
failure/multiple failures will bring down a 0+1 array. Both 0+1 & 1+0
are supposed to be able to sustain more than one failure in certain
circumstances.

If the right drives fail either type can handle up to 50% of the drives
failing simultaneously. The difference is that when one drive has
failed and you're considering which remaining drives can fail without
taking down the entire array, a 1+0 array has more choices of drives
which can die while still continuing to run.


--
I've given up on sigs. I just couldn't think of anything clever to say.

On Fri, 12 Nov 2004 22:31:13 -0700, DevilsPGD
wrote:

You should take this into account when you're buying drives too.

Buy half your drives from one batch, and half from another batch (if you
have that control -- Staying with the same model if you can, of course.

This may be viable. One potential issue is when you buy in a single
batch you are more likely to get the same model and firmware
revisions. Some firmware may have raid issues or raid related
conflicts with differing levels. With ata you generally can't upgrade
that.

If there is a physical defect which affects an entire batch, it won't
take down your entire array, it will probably take down the affected
batch only and you can replace the drives.

Interesting. In many cases though you can run into problem with
multiple failures that a 50/50 split can't save you from.

Another viable option is thorough testing before array creation and
having spares on-site. You can use the spare(s) to create a disk
rotation schedule to combat the problem of many drives dying at a
similar time down the line because they are identical age with
identical usage. You also don't have to loose any sleep while you
wait for the warranty replacements and can survive additional
failure(s).