While my own recent personal unRAID server is churning along nicely I had an idea for another application. This project would start (really) small and I'm wondering what the advantages/disadvantages of using 2 drives vs 3 drives would be? (1 parity, 1 data vs. 1 parity, 2 data)

 

Assuming there are no old drives to work with - starting fresh - would it be better to go with say 2 1TB drives or 3 640-750GB drives?

 

I don't see the storage needs for this project to really increase in the immediate future so whatever the initial build is, is likely how it will stay at least for a year. What IS important is stability. Are there any benefits to doing 3 drives vs 2? Is using 2 drives with unRaid essentially like Raid 1 with the exception that the drive would have to be rebuilt from the parity drive should it fail?  

 

Edit: I had meant Raid 1.

RAID 0 striped data across 2 drives making what appears to be one drive double the size.

 

RAID 1 mirrors the data drive to a second drive. It's more like RAID 1, but not exactly the same.

 

I would not purchase smaller drives just so I could have 3 drives instead of 2. I believe the $/GB right now favour the 1.5T drives, making them the most cost effective ones to purchase. But then, there are some real deals every now and then on 1T drives too.

 

Peter

 

NO. Raid 0 is striping the two drives together with NO protection from failure. If either drive fails, ALL the data is lost.

 

Raid 1 mirroring is like using 1 Data drive and 1 Parity drive, where if 1 drive fails, the data is still available to be rebuilt from the other drive.

 

There are two types of drives - those that have failed and those that have not failed YET. All drives will fail. Personally, I prefer to have less drives involved.

 

Wow, that was a Friday blunder. I meant to make the comparison between unRaid w/ 2 drives and Raid 1.

 

So the inclination then is to get 2 drives rather than three. That makes most sense to me, I just wanted to be sure there wouldn't be any hidden benefits to having more than two drives.

 

It seems that that 2 drives in unRaid is quite similar to raid 1 with the differences being that with unRaid, redundancy is through the parity drive whereas Raid 1 is an exact image so should one drive fail, you can keep working. With unraid (unless it was the parity drive that failed) would you need to replace the failed drive and rebuild the array before being able to use it?

With unraid (unless it was the parity drive that failed) would you need to replace the failed drive and rebuild the array before being able to use it?

Wrong... with parity and a single disk failure you would still be able to read and write to the array exactly as before.  It is exactly equivalent to raid-1.  This true regardless of which disk fails.  The data disk can fail and you will still be able to read and write to the array.  The same if the parity disk were to fail.

 

In fact, since unRAID uses even parity, with only one data disk, the parity disk will be an exact mirror copy.  (If a bit is set to a "1" on the data disk, the same bit MUST be set to a "1" on the parity disk to keep an even number of  "1"s for that bit position.  The same for any bit set to "0" since again to keep even parity the corresponding bit on parity must be set to "0" to keep an even number of "1"s in that bit position.

 

Obviously, if both disks fail you can not recover, but if either fail, stop the array, power down, unplug the existing failed drive, insert the replacement, power up, press "Start" to  reconstruct the data onto the replacement.  (don't press the button labeled as "restore" as it immediately invalidates parity, throwing away the data needed to reconstruct a failed drive)

 

Joe L.

To add to Joe L.'s statement, your 'array' (in this case, your single surviving disk) is still available for reads and writes during and immediately after the drive failure.  So if it takes you a few days to get a replacement disk mailed to you, your data is still available for use during that interim (just like RAID 1).  Your data is unprotected during that time, of course.

In fact, since unRAID uses even parity, with only one data disk, the parity disk will be an exact mirror copy.

Joe L.

 

The data may be on the drive but it's not an exact copy. You can not just connect it to the data drive position and have it work as a data disk. You might be abele to run some recovery tools on it and get the data to show.

 

Peter

In fact, since unRAID uses even parity, with only one data disk, the parity disk will be an exact mirror copy.

Joe L.

 

The data may be on the drive but it's not an exact copy. You can not just connect it to the data drive position and have it work as a data disk. You might be abele to run some recovery tools on it and get the data to show.

 

Peter

Actually, it might be possible.  It all depends on if the parity disk has a partition table.  If not, just adding the missing partition table would be all that is needed.  The data on the partition would be an exact copy. (where there only one data drive in combination with parity)

 

I've never partitioned my parity disk, but when I run

fdisk -lu /dev/sdf

 

I get:

fdisk -lu /dev/sdf

 

Disk /dev/sdf: 1000.2 GB, 1000204886016 bytes

1 heads, 63 sectors/track, 31008336 cylinders, total 1953525168 sectors

Units = sectors of 1 * 512 = 512 bytes

Disk identifier: 0x00000000

 

  Device Boot      Start        End      Blocks  Id  System

/dev/sdf1              63  1953525167  976762552+  83  Linux

 

Guess what... It does have a partition table defined.  Therefore, it appears you wont have any issue connecting the parity drive as a data drive and reading it in a two disk array.

 

Try fdisk -lu /dev/sdX on your parity drive. See what it says.

 

Joe L.

Interesting, thanks for the input. It looks as though unRaid is more robust than I guessed in a 2 disk config. I've just done some quick testing by copying data over and verifying checksums. Looks like unRaid doesn't mind this board / controller combo. This week I'll be purchasing a pair of drives - likely Seagate 7200.12 1TB or 1.5TB disks and run some more tests.

 

Thanks again!