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Raid with multiple sizes

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June 9, 2014 10:15:58 AM

Okay so I have just bought a new 1TB HDD. That means now I have 3 1TB drives and 1 2TB drive. I was initially thinking about using the 3 1tb drives in raid 5 (using mdadm) and just finding some other use for the 2tb drive. It occurred to me, though, that I could partition the 2tb drive into two 1tb partition and create a "5 device array" using mdadm. My question is how would the performance of this 5tb array compare to the 3tb array that uses all separate drives? Would it be faster or slower? Maybe the answer is different for worst case vs best case vs average case? Any insight would be great.

Also is there any other more clever scheme that I could use to get more storage/performance from these four drives?

Edit: I have just realized that if the 2tb drive were to fail it would destroy a RAID 5 array. In light of this new info I would have to use raid 6 if I were to make a 5tb array. This would still give me an additional tb of storage in my primary array though so I guess the question is now all the drives in RAID 6 vs the three drives in RAID 5. Which would have better performance?

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a c 300 G Storage
June 9, 2014 10:24:00 AM

Second of all, most hardware RAID would just make the other 1 GB on that drive completely unusable. Can't create a separate volume on it.

Second, IMHO, don't RAID unless either A) you just want to play around with it, or B) it addresses a specific need that can't be addressed otherwise. For one thing, data recovery is a byotch. May I ask why you intend to RAID the drives?

And, finally, the obligatory comment. RAID is never an alternative to external backups; RAID arrays should be backed up, too.
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June 9, 2014 10:32:20 AM

There are a few reasons that I want to RAID.

1. Higher performance for a zippier computer.

2. Everything is in one large space. I don't have to remember what is stored where.

3. The main reason is just for learning and fun. I am always wanting to learn about different technologies and methods available.

I will also note that this is my home desktop and I do not have any critical data on it. While I wouldn't be happy losing my movies and music, it wouldn't be the end of the world.
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June 9, 2014 10:38:02 AM

WyomingKnott said:
Second of all, most hardware RAID would just make the other 1 GB on that drive completely unusable. Can't create a separate volume on it.


I plan on using mdadm aka linux software RAID which does have this capability.

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a b G Storage
June 9, 2014 11:25:25 AM

Hi

Raid 5 is more reliable than raid 0 but is not usually faster than a single drive
While stripe raid 0 is faster, but less safe

Why not use raid bios option to form raid 5 with 3 drives and keep 2TB drive for second drive ?
, giving you 2 x 2TB drives

I have never used raid at home as I never needed to
but at work used servers from HP / compaq & Dell

Make sure the drives are separated and cooled by fans and have anti vibration mountings

One server was from a company with little (no) server experience
3 hard disks were stacked up with no spaces and when the fan failed one drive cooked , fortunately it was raid 5 so self repairing once fan and drive replaced

Regards
Mike Barnes
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June 9, 2014 11:45:39 AM

Yes I will most likely do exactly that (3 drives in RAID 5 and one 2tb drive separate). I was just wondering how performance would be affected if I added the 2tb drive as two 1tb volumes and made the array RAID 6.

Also I am definitely not using hardware RAID as it is very expensive and I am not using motherboard RAID as it is just software RAID designed for windows where I use linux. I will be using mdadm which is linux software RAID.
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a b G Storage
June 9, 2014 11:52:34 AM

Accepting your assertion that two partitions on the same drive can be added to a RAID set (and I'm not convinced of that), performance would be abysmal. The heads on the 2TB drive would be continually thrashing between the two partitions. We all know that head movement is an order of magnitude slower than the data access speed of a hard drive.

Don't even think of doing this; it's a terrible idea.
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June 9, 2014 11:58:22 AM

Yes thank you that sounds quite accurate. I should have that about the head movement problem. I will still test this, of course, just to see what happens but I am pretty sure you are right.
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a b G Storage
June 9, 2014 3:12:55 PM

dmobbjr said:
Yes thank you that sounds quite accurate. I should have that about the head movement problem. I will still test this, of course, just to see what happens but I am pretty sure you are right.


There's nothing to test. a single hard drive cannot be used as 2 separate devices in RAID 5.
A RAID5 volume requires the same range of LBA's on all drives.
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June 9, 2014 3:55:19 PM

TyrOd said:
dmobbjr said:
Yes thank you that sounds quite accurate. I should have that about the head movement problem. I will still test this, of course, just to see what happens but I am pretty sure you are right.


There's nothing to test. a single hard drive cannot be used as 2 separate devices in RAID 5.
A RAID5 volume requires the same range of LBA's on all drives.


I seem to have just created a RAID 0 array from two halves of the same disk. Note the disks used for md1

dylan@dylan-TA890FXE ~ $ cat /proc/mdstat
Personalities : [raid0] [linear] [multipath] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid0 sdd2[1] sdd1[0]
244197376 blocks super 1.2 512k chunks

md0 : active raid0 sdb1[0] sdc1[1]
1953522944 blocks super 1.2 128k chunks

unused devices: <none>



By the way, is there a proper way to format cli output?
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a b G Storage
June 9, 2014 4:09:33 PM

dmobbjr said:
TyrOd said:
dmobbjr said:
Yes thank you that sounds quite accurate. I should have that about the head movement problem. I will still test this, of course, just to see what happens but I am pretty sure you are right.


There's nothing to test. a single hard drive cannot be used as 2 separate devices in RAID 5.
A RAID5 volume requires the same range of LBA's on all drives.


I seem to have just created a RAID 0 array from two halves of the same disk. Note the disks used for md1

dylan@dylan-TA890FXE ~ $ cat /proc/mdstat
Personalities : [raid0] [linear] [multipath] [raid1] [raid6] [raid5] [raid4] [raid10]
md1 : active raid0 sdd2[1] sdd1[0]
244197376 blocks super 1.2 512k chunks

md0 : active raid0 sdb1[0] sdc1[1]
1953522944 blocks super 1.2 128k chunks

unused devices: <none>



By the way, is there a proper way to format cli output?


No, no you haven't.

Regardless of the file structure manipulation and virtual LBA configuration the "ID" in RAID means "independent/inexpensive disk/drive.
1 Head Rack Assembly, 1 motor spindle means 1 drive/disk.
Calling a striped logical layer RAID is obtuse. It doesn't function electronically, mechanically, or mathematically like RAID.
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a c 300 G Storage
June 9, 2014 6:29:04 PM

dmobbjr said:
There are a few reasons that I want to RAID.

1. Higher performance for a zippier computer.

2. Everything is in one large space. I don't have to remember what is stored where.

3. The main reason is just for learning and fun. I am always wanting to learn about different technologies and methods available.

I will also note that this is my home desktop and I do not have any critical data on it. While I wouldn't be happy losing my movies and music, it wouldn't be the end of the world.


Ahh, number 3 says it all for me. It's why I had a RAID array in my home office back in the days when Ultra SCSI was the best external connection! Loud blasted thing. Nowadays I can get better performance with a nice, quiet Thunderbolt enclosure.

@TyrOd - you are correct, both theoretically and practically, but please be polite about it on the forums.

@dmobbjr - To expand a little, the entire point of RAID is to spread, mirror, or checksum data across multiple spindles. Having two parts of the same RAID on the same drive might be an interesting exercise, but it only degrades the performance and safety of the RAID, so it's only useful for experiments.

And it does, technically, make it something other than a RAID - at least, no raid level yet known to humankind. No harm in it if you're just fiddling around.
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