Newbie here...

• I want to know what is unraid and how it differs from raid.
• Which is better raid or unraid
• i understand how 1 parity drive can be used to recover data if one drive fails but if two drive fails will the data on those 2 drives are lost can we recover it using parity drive
• how 2 parity drive works and how 2 drives can be recovered when 2 drives fail
• and can you link some articles and videos and anything related to unraid that will help me understand more about unraid like i said i am a newbie

 

1 hour ago, Shehryar said:

• I want to know what is unraid and how it differs from raid.
• Which is better raid or unraid

Basically, Unraid uses a even-parity calculation across all the data drives to be able to reconstruct the failure of a single data drive.  See here for how it works:

 

        https://wiki.unraid.net/index.php/UnRAID_Manual_6#Parity-Protected_Array

 

Raid is faster in data writing and retrieval operations.  However if two drives fail in a RAID configuration, you lose all of the data in the array.  Also all drives must be the same size. 

 

With Unraid, writing is much slower and the read process is limited to the read speed of the data drive which contains the actual file data.  Also if two drives fail, you only lose the actual data stored on the drives which fail.  The data on the other drives can be read on any Linux system.  The capacity of the data drives can be different as long as the parity is as large or larger than the largest capacity data drive.  It is also quite easy to add capacity to any Unraid server by simply adding another data drive.  

 

1 hour ago, Shehryar said:

i understand how 1 parity drive can be used to recover data if one drive fails but if two drive fails will the data on those 2 drives are lost can we recover it using parity drive

In some cases, it is possible to recover some of the data from the failed drives depending on the nature of the failure.  But it is best not to count on it. 

 

1 hour ago, Shehryar said:

how 2 parity drive works and how 2 drives can be recovered when 2 drives fail

The second parity drive uses a Reed-Solomon encoding scheme (which is basically a very complex matrix operation) to allow reconstruction (in use with the first parity drive) to be able to rebuilt two failed drives at the same time.  If you wish to know more about Reed-Solomon, you can google it!  (You will need the equivalent of a college degree in Mathematics...) 

 

One import thing to realize is that parity protection is NOT a substitute for a reliable multiple-copy backup scheme with one of those copies being in a remote location.   Just think of having your server in a house fire, flood, hurricane or being stolen.  It is also not a replacement for a due-diligent administrator who monitors his servers and sets up all of the notification services so that he receives the earliest possible notice when any problem arises!  

@Frank1940 awesome summary, thank you!

 

Moving this topic to 'Pre-sales Support'.

so we can recover data on the 2 drives if 2 drives fails if we have 2 parity drives in Unraid

Yes

It has been mentioned that the read/writes are slow. How slow?

Are there some examples of when it is slow, or what causes it to be slow?

20 minutes ago, Walt750 said:

It has been mentioned that the read/writes are slow. How slow?

Are there some examples of when it is slow, or what causes it to be slow?

Each disk is an independent filesystem that can be read by itself on any linux. Each file exists completely on a single disk, so reading a file is at the speed of that single disk.

 

Writes are somewhat slower than single disk speed because parity is updated at the same time a disk is written. See here for more details about the 2 methods for updating parity and their tradeoffs:

 

 

Picking in here with a question, so I suppose enabling Turbo Write is a good practice when first setting up unRAID and you have to copy a lot of data from another system/disks?

And then to preserve power consumption and disk wear turn it back off afterwards as in most cases less data is written to the array?

Some folks, when setting up an unRAID server, will load the data to than array without assigning a Parity drive.  Of course, the data on the array will have no protection in the event of a disk failure but often the data is still on the original source drives so there is a backup.  After the data is moved, the Parity drive is added and parity will have to be built.

 

Moving massive amounts of data to an array is always a time consuming process.  The actual transfer rate has many factors involved.  Some of the most significant have to do with the characteristics of the hard drives themselves-- rotational speed, track-to-track movement times and data density are three prime ones.  The transfer rate of a single 25GB BluRay ISO file will be much faster than moving 10GB of 10KB data files due the overhead of the file management routines and latency caused by the head movement from the files tables to the data area.

 

Once, you have a initial loading done, you may want to consider using a SSD cache drive (or SSD cache pool) to speed file transfers to the server.  The Mover can be programed to move the files from cache at convenient time of day.  (Default is 3:00AM as I recall.)

Edited January 20, 20206 yr by Frank1940

That's indeed also an option, to first copy the data over and then add the parity drive to the array. In my case I'm already foreseen to use a SSD as cache drive.

4 hours ago, Stiibun said:

In my case I'm already foreseen to use a SSD as cache drive.

Quick point (in case you did not already realize it)---  In the data moving stage, the SSD is basically useless as it will quickly fill up and then you will be writing directly to the hard drive array.

Yes Frank already realized that, hence why I'll only the cache drive after I moved my data. Or I could install it, and exclude the shares where the data will go. Enough different routes to take to migrate an existing system to unRAID 😁