Multiple Cache drives (priority?)

[syrys]

Hey Guys,

 

Im about to start my first unraid server, im just trying to figure out if unraid actually supports multiple cache drives with sort of prioritising?

 

I understand unraid like most other parity raid system, is slow on writes. So if i put a 250GB SSD as cache, it will write to the ssd first then it will write from the ssd to the drives later. But, is it recommended to put multiple cache drives (SSDs and normal spinny drives), if so, is there any way to tell it to prioritise SSDs first then the spinny drives then the actual raid?

 

Sorry if the question is worded badly. If you are confused let me know, i can try draw a diagram or something

 

Also, what are unraid 6.1.x with single parity write speeds like (without cache) for normal drives? what about the write speeds on the dual parity on the unraid 6.2.x?

 

Cheers.

[JorgeB]

Hey Guys,

 

Im about to start my first unraid server, im just trying to figure out if unraid actually supports multiple cache drives with sort of prioritising?

 

I understand unraid like most other parity raid system, is slow on writes. So if i put a 250GB SSD as cache, it will write to the ssd first then it will write from the ssd to the drives later. But, is it recommended to put multiple cache drives (SSDs and normal spinny drives), if so, is there any way to tell it to prioritise SSDs first then the spinny drives then the actual raid?

 

No.

 

 

 

Also, what are unraid 6.1.x with single parity write speeds like (without cache) for normal drives? what about the write speeds on the dual parity on the unraid 6.2.x?

 

Cheers.

 

Most users get between 40 and 60MB/s. It will be the same with dual parity, unless your CPU is something like and Atom or very old single core, even so it will only be a few percent slower.

[syrys]

No.

 

So, if you have multiple cache drives on your raid (lets say 2x 120gb ssds and 1x 1tb WD Blue), when you are copying some files which of the following is correct:

 

1. unraid will randomly pick a cache drive and write to that, and when it fills, picks another cache drive and write to that untill all cache drives are filled then it writes to the main raid.

2. unraid write to all cache drives at the same time (to increase write speed). So all the cache drives will be active/spinning (like a typical raid 0?), and you essentially have the write speeds of all non-full cache drives added up.

3. Write to each cache drive, but one at a time (randomly or in some order). ie. writes packet 1 to drive 1, packet 2 to drive 3, packet 3 to drive 1 again etc.

4. you cannot have multiple cache drives, noob.

 

[JorgeB]

Assuming using default RAID1 config, all files will be written to at least 2 disks at the same time, so the slowest one will limit write speed.

 

If you're using more than 2 disks don't know which ones will be chosen, but it will probably based on available space.

[testdasi]

Most users get between 40 and 60MB/s.

A bit of clarification: that figure is probably with reconstruct-write (aka Turbo write) turned off.

 

6.2.0 beta has it as an selectable option so can change with GUI.

6.1.9 I think you can edit some files to turn it on.

 

My experience with turbo write is about 110 - 130 MB/s on array.

 

[trurl]

No.

 

So, if you have multiple cache drives on your raid (lets say 2x 120gb ssds and 1x 1tb WD Blue), when you are copying some files which of the following is correct:

 

1. unraid will randomly pick a cache drive and write to that, and when it fills, picks another cache drive and write to that untill all cache drives are filled then it writes to the main raid.

2. unraid write to all cache drives at the same time (to increase write speed). So all the cache drives will be active/spinning (like a typical raid 0?), and you essentially have the write speeds of all non-full cache drives added up.

3. Write to each cache drive, but one at a time (randomly or in some order). ie. writes packet 1 to drive 1, packet 2 to drive 3, packet 3 to drive 1 again etc.

4. you cannot have multiple cache drives, noob.

None of the above. The only way to have multiple cache drives is with a btrfs cache pool which by default is a btrfs raid1 mirror. btrfs raid1 is a little different than traditional raid1 if you have more than 2 drives in the pool, but basically it makes sure there is a redundant copy of anything on some other disk in the pool.

 

For all read/write operations, the pool is considered to be a single volume.