I don't understand it - newbie

[evlo]

From what i read about unRaid.

- i can use various capacity of HDDs

- parity take only one drive similar to Raid 5

- data are store on each drive in "row" (one mkv file is not spitted across multiple drives)

- i have maximal usable capacity for data

 

so if i have 9 discs with total capacity of 21TB (6x2TB + 3x3TB)  i have usable capacity of 18,6TB? While data can still be reconstructed in case of 1 HDD failure? And yet still one mkv file is not spitted across multiple drives so i can even take hdd out from array and access data?

 

I can understand how raid 5 works but can't understand/believe how this works without extensive data movement between drives. But still there will always be one drive (or maybe more) with both parity for other drives and actual data.

 

Also maximum is 21 HDDs per server?

[BRiT]

Think RAID-4 with a Dedicated drive for parity except the data is not stripped across the drives. unRAID uses an Even Parity Scheme.

 

Parity is calculated across the drives. Here is a very simplistic view of it. If a drive does not have a Sector to read (ie: the drive is smaller) then a value of 0 is used for it.

 

Sector 0 Parity = Drive-1 Sector 0 XOR Drive-2 Sector 0 XOR Drive-3 Sector 0 ... XOR Drive-N Sector 0

Sector 1 Parity = Drive-1 Sector 1 XOR Drive-2 Sector 1 XOR Drive-3 Sector 1 ... XOR Drive-N Sector 1

...

Sector N Parity = Drive-1 Sector N XOR Drive-2 Sector N XOR Drive-3 Sector N ... XOR Drive-N Sector N

 

[evlo]

I found note on the wiki that parity drive must be at least same size as largest data drive so it makes sense now Thanks.

[Johnm]

Keep in mind, you will not get the performance speed of raid5.

 

also if 2 drives are lost at the same time for some reason, you only loose the data on the failed drives. raid5 you loose ALL data.

 

not all drives have to be spunn up most of the time. just the ones reading and writing (plus parity while writing)

 

the latest test beta supports 24 drives...

 

 

[evlo]

Actually i'm deciding what to chose and one other alternative is zraid from zfs - do you know if all drives need to run when using zraid?

[Johnm]

Actually i'm deciding what to chose and one other alternative is zraid from zfs - do you know if all drives need to run when using zraid?

 

Yes. all drives in the pool spin at once.. so a 20 drive pool will spin 20 drive to play one MP3. thats why unraid is better as a media server.

 

personally i run both high performance storage servers for some stuff and unraid for other stuff.

 

I prefer unraid for media and long term seldom accessed files (like backups, driver downloads, install files)

I use the ZFS and hardware raids for things like virtual drives and heavy file lifting.

 

they are both free to download. test the both out..

 

[mrow]

I agree that for a media server unraid makes the most sense. Most of what you will be doing with a media server is reading data so being able to spin up only one disk is a huge advantage in terms of hardware longevity, heat output and power usage. Not to mention if you have the server in the same room as you, 1 disk spun up is a lot quieter than 9 disks you have all spun up.

 

Someone else mentioned that write speeds are an issue with this type of array but that can mostly be addressed by a cache drive. This is an unprotected disk that data is first written to and then moved to the array at however often you set the mover script to move the data. The cache drive is invisible to you when adding files over your network. So creating a directory for the movie you just downloaded in your movies share will look like it's creating that folder on the array, but it's really creating that full path on the cache drive, for instance "/Movies/1080p/Transformers" so when the mover script goes to move data from your cache drive it will know exactly where to move it. I'm probably making it sound more complicated than it is but my point is that the cache drive negates most of the write speed issues you get with this type of array.

[chickensoup]

I agree that for a media server unraid makes the most sense. Most of what you will be doing with a media server is reading data so being able to spin up only one disk is a huge advantage in terms of hardware longevity, heat output and power usage. Not to mention if you have the server in the same room as you, 1 disk spun up is a lot quieter than 9 disks you have all spun up.

 

Someone else mentioned that write speeds are an issue with this type of array but that can mostly be addressed by a cache drive. This is an unprotected disk that data is first written to and then moved to the array at however often you set the mover script to move the data. The cache drive is invisible to you when adding files over your network. So creating a directory for the movie you just downloaded in your movies share will look like it's creating that folder on the array, but it's really creating that full path on the cache drive, for instance "/Movies/1080p/Transformers" so when the mover script goes to move data from your cache drive it will know exactly where to move it. I'm probably making it sound more complicated than it is but my point is that the cache drive negates most of the write speed issues you get with this type of array.

 

+1

 

A cache drive is a fantastic addition to an unRAID box if you can handle the (minimal) risk of data loss on that drive. At most you shouldn't lose more than a day's work/downloads.

 

also if 2 drives are lost at the same time for some reason, you only loose the data on the failed drives. raid5 you loose ALL data.

 

loose = opposite of tight

lose = opposite of win

win = what unraid does

 

sorry john, pet hate