I have 3 Samsung 500gb SSD formatted in btrfs.  Here is info from first cache drive:

 

Data, RAID1: total=190.00GiB, used=182.99GiB
System, RAID1: total=32.00MiB, used=48.00KiB
Metadata, RAID1: total=2.00GiB, used=528.52MiB
GlobalReserve, single: total=203.19MiB, used=0.00B
 

So what is happening here?  I see space as 750gb.  How is this working?  Is it protected and if so how?

 

3 minutes ago, rmilyard said:

I see space as 750gb.

http://carfax.org.uk/btrfs-usage/

So still not sure what is happening. I see only getting 750gb but how does the parity works?

There's no parity on the cache pool, you'll have 750GB usable and data will be mirrored, i.e., it will exist always in 2 different devices.

 

 

Just now, johnnie.black said:

There's no parity con the cache pool, you'll have 750GB usable and data will be mirrored, i.e., it will exist always in 2 different devices.

 

So I am little confused.  Since each drive is 500gb how can it be mirrored?  

Data is mirrored, not drives, (500 + 500 + 500) /2 = 750

 

 

Just now, johnnie.black said:

Dat is mirrored, not drives, (500 + 500 + 500) /2 = 750

 

So not sure this would be best way to setup cache using 3 SSDs?  

2 minutes ago, rmilyard said:

So not sure this would be best way to setup cache using 3 SSDs?

Depends on if you want maximum space or redundancy, you can see the available profiles and how to change them here:

https://lime-technology.com/forums/topic/46802-faq-for-unraid-v6/?do=findComment&comment=480421

 

55 minutes ago, johnnie.black said:

Data is mirrored, not drives, (500 + 500 + 500) /2 = 750

So for example, 250GB would be mirrored on disk 1 and 2, an additional 250GB mirrored on disk 1 and 3, and the last 250GB mirrored on disk 2 and 3. Don't know if this is exactly what happens but probably similar in principle.

Just now, trurl said:

So for example, 250GB would be mirrored on disk 1 and 2, an additional 250GB mirrored on disk 1 and 3, and the last 250GB mirrored on disk 2 and 3. Don't know if this is exactly what happens but probably similar in principle.

It would alternate, devices with the most free space are written to first, so for example, it would write first to d1 + d2, next block would go to d2+d3, then d3 + d1 and so on.

1 minute ago, trurl said:

So for example, 250GB would be mirrored on disk 1 and 2, an additional 250GB mirrored on disk 1 and 3, and the last 250GB mirrored on disk 2 and 3. Don't know if this is exactly what happens but probably similar in principle.

 

So if mirrored on to 2 different drives you don't have any fault tolerance?  I am starting to think since just using cache drives for dockers and downloads etc then since backing up appdata etc via CA Backup it would be best to setup for speed?

2 minutes ago, rmilyard said:

So if mirrored on to 2 different drives you don't have any fault tolerance?

If you mirror to 2 devices there is fault tolerance.

2 minutes ago, johnnie.black said:

If you mirror to 2 devices there is fault tolerance.

 

Just don't see how works.  If one drive is 500gb it would take one more drive of 500gb to mirror it right?  But if this is split between 2 drive so 250gb each then if one of the mirrored drives fails then all gone?  Or is the data mirrored really on all 3 drives?  Guess my mind not working right trying to figure out.

 

To be honest I guess doesn't matter much since if anything fails easy enough since CA Backup to just restore it all.

18 minutes ago, rmilyard said:

If one drive is 500gb it would take one more drive of 500gb to mirror it right?

If only using two drives the mirror needs to be the same size, or not all space would be used.

 

21 hours ago, rmilyard said:

I have 3 Samsung 500gb SSD

these are all the same size correct?

Edited March 28, 20188 yr by johnnie.black

17 minutes ago, rmilyard said:

 

Just don't see how works.  If one drive is 500gb it would take one more drive of 500gb to mirror it right?  But if this is split between 2 drive so 250gb each then if one of the mirrored drives fails then all gone?  Or is the data mirrored really on all 3 drives?  Guess my mind not working right trying to figure out.

 

Even though my example is not exactly what happens in the details, study it for a minute and pay close attention to the disk numbers I give in each 250GB mirror.

 

28 minutes ago, trurl said:

So for example, 250GB would be mirrored on disk 1 and 2, an additional 250GB mirrored on disk 1 and 3, and the last 250GB mirrored on disk 2 and 3.

 

20 minutes ago, rmilyard said:

Just don't see how works.  If one drive is 500gb it would take one more drive of 500gb to mirror it right?

Read to fast this and though you were referring to a two device mirror.

 

I already mentioned how it works with 3 devices, but to expand a little, mirror is done on data blocks, not files, i.e., if you copy a large file to a 3 device mirror the file won't be copied to just 2 devices, its blocks will be split on the 3 existing devices, so say the file is 150GB, in the end it will use 100GB on each device, but you can always lose any one of the 3 devices without losing data, lose 2 and all data is lost (you could possibly recover partial or very small files only).

4 minutes ago, johnnie.black said:

say the file is 150GB, in the end it will use 100GB on each device

on each of 3 devices, so 300 GB in total, which is twice 150GB.

 

Or you could just pretend the block size is 250GB then my example might be easier to understand.

 

The point here is that any of the data is always on 2 disks, so the data is mirrored. It isn't necessary to mirror a whole disk as long as there is a copy of the data on 2 disks.