Mixing different drives

[arendal]

Hi, what exatctly happens when you mix drives of different speed and capasity?

 

If I have understood this software correct a single unRAID "drive or array" is made out of several other drives to give you a single drive letter os share on the network.

 

Does that mean that it is the slowest disk that determins the speed?

 

When you combine different 1 2 og 3TB drives in the array the parity drive must be 3TB, right?

But, if you have many dives lets say 8 drives that has a combined capasity of 20TB but none of them is bigger than 3TB, it is still sufficient with a 3TB parity drive?

[maxinc]

Hi, what exatctly happens when you mix drives of different speed and capasity?

 

If I have understood this software correct a single unRAID "drive or array" is made out of several other drives to give you a single drive letter os share on the network.

 

Basically yes, it will create a single pool of storage equal to the combined size of all data drives (excl. parity drive) to which you can assign one or multiple user shares as you see fit.

 

Does that mean that it is the slowest disk that determins the speed?

 

That is only true during routine parity checks / rebuilds (once a month or when adding / removing drives) where all drives need to read data simultaneously for the computation. Since files are not split among drives, the read and write speeds are determined by the drive where data being read / written sits, although writes are much slower than reads since they require parity to be written on the parity drive too.

 

When you combine different 1 2 og 3TB drives in the array the parity drive must be 3TB, right?

But, if you have many dives lets say 8 drives that has a combined capasity of 20TB but none of them is bigger than 3TB, it is still sufficient with a 3TB parity drive?

 

The parity drive needs to be equal or larger than the largest data drive in the array. for  1T + 1T + 2T + 3T data drives you need at least 3T parity (but can be larger such as 4T). If you add a 4T data drive into the array, you need to change the parity drive to at least 4T drive.

[trurl]

...

If I have understood this software correct a single unRAID "drive or array" is made out of several other drives to give you a single drive letter os share on the network.

...

This part seems a little confused.

 

From another system unRAID appears to be one or more shares. You can map any share to a drive letter if needed but it is not required. I seldom have any need to map a drive letter. I typically find that just referring to files and folders with a UNC path is sufficient for most purposes.

 

The names and number of shares are completely configurable in unRAID. Any single share in unRAID can be on one or more disks, and this is also completely configurable.

 

The disks in unRAID are in some sense completely independent, and any individual file will ultimately be stored on only one physical disk. Read speed is only affected by the speed of the individual disk being read.  If there is a problem on one disk it doesn't affect any files on other disks.

 

Parity allows a single disk to be reconstructed if it has problems. It must be as large as the largest disk. Writing to the array requires reading, recalculating, and writing parity as well so it is slower than a simple single disk write would be.

 

 

[arendal]

Hi and thank you for your replies.

 

The reason I ask is that like so many others I have a growing collection of movies in my little NAS an I need more storage space.

 

Instead of buying another NAS I want do build my own file server.

 

I have several 500GB to 2TB drives lying around and also a few 3TB USB3 disks.

 

I want to combine as many "small" drives as possible to the array and have one drive letter / share on my nework.

And, if possible use the external USB 3.0 drive for parity.

 

If I understan you correctly the share will be seen on the network as 1 share, but when I mangage the Unraid I will see the drives as independent drives?

That also means that I can replace the smallest drives one by one to build a bigger share without farmatting or loosing data?

 

This almost seems to good to be true, what is the downside?

[lainie]

... And, if possible use the external USB 3.0 drive for parity. ...

If you mean to use the drive in its external USB case for parity... that is not currently possible.

 

Many people will take an external USB drive, remove the drive from its enclosure (giving access to the SATA hard drive inside), attach the SATA drive through a SATA cable & use it as a regular internal drive. This is often done when it is cheaper to purchase an eternal drive than it is to purchase a similar sized internal drive... but this usually voids any warranty on the drive (though with all the drive manufacturers shortening their warranties it is becoming less of an issue).

 

I've seen a recent feature request made by an unRAID user to have an external USB drive be useable as a cache drive (selected from the webGUI & available at startup), but that is not currently possible either.

 

...

I want to combine as many "small" drives as possible to the array and have one drive letter / share on my nework.

...

 

If I understan you correctly the share will be seen on the network as 1 share, but when I mangage the Unraid I will see the drives as independent drives? ...

 

Yes, unRAID can combine many small drives to show as one large share. You setup the shares you want & decide how many shares to create to suit your needs. You can access the disks through the shares you create or as independent disks. When setting up the shares through the unRAID webGUI, you will see the individual drives & can decide which disks are included or excluded in any share you create.

[trurl]

This almost seems to good to be true, what is the downside?

Speed. Some other more traditional RAIDs will give you increased speed for reading and/or writing by accessing multiple disks in parallel (striping).

 

unRAID read speed is the same as the read speed of the individual drive the file is stored on.

 

unRAID write speed is slower because it actually performs 2 reads and 2 writes. It has to read the parity and the disk to be written so it can calculate the new parity, and then it has to write the destination disk with the new data and the parity disk with the new parity.

 

The slower write speed is not much of a downside for media files since they are not often written. Faster writing can be had by using a cache drive that will temporarily store writes at normal writing speeds and then later move them to the parity protected array in the background. See the wiki and forum for more about cache.

[Helmonder]

This almost seems to good to be true, what is the downside?

Speed. Some other more traditional RAIDs will give you increased speed for reading and/or writing by accessing multiple disks in parallel (striping).

 

unRAID read speed is the same as the read speed of the individual drive the file is stored on.

 

unRAID write speed is slower because it actually performs 2 reads and 2 writes. It has to read the parity and the disk to be written so it can calculate the new parity, and then it has to write the destination disk with the new data and the parity disk with the new parity.

 

The slower write speed is not much of a downside for media files since they are not often written. Faster writing can be had by using a cache drive that will temporarily store writes at normal writing speeds and then later move them to the parity protected array in the background. See the wiki and forum for more about cache.

 

Try it out though.... Unraid is not suitable to be run as something like a high bandwidth storage backend but the speed is more then enough for my home setup... HD movies stream like a MF and there is delay at all in using My Documents from my windows system..

 

The speed thing is mitigated (but not completely) by using Cache_Dirs (giving you read access of your directory structure from memory, no need to spin up disks) and possibly a cache drive ( I started using that but found that I do not -really- need it.. Am now using it as a base for all the cool plugins ;-)

[trurl]

...a cache drive ( I started using that but found that I do not -really- need it.. Am now using it as a base for all the cool plugins ;-)

I also use it as a download destination, but otherwise plugins and other apps is the only thing I use my cache disk for. In fact it seems like the cache disk is more important for apps than it is for data.

[Helmonder]

...a cache drive ( I started using that but found that I do not -really- need it.. Am now using it as a base for all the cool plugins ;-)

I also use it as a download destination, but otherwise plugins and other apps is the only thing I use my cache disk for. In fact it seems like the cache disk is more important for apps than it is for data.

 

The cache drive as an option is a really nice thing and should remain, for some situations it is usefull.

 

Now what we need is a way to have a disk mounted but not part of the array .. Only way to do this with unraid stock is using the cache drive..

[dgaschk]

Search for SNAP in the User User Customizations forum..

[Helmonder]

Search for SNAP in the User User Customizations forum..

 

I am aware of SNAP. have even used it, but a core functionality like providing a share I would prefer to be in unraid itself ..

[dgaschk]

You can manually enter mount commands. See the wiki.