Question about how UnRAID "Most-Free" works (with parity)

[HTWingNut]

I'm just using the trial at the moment to toy around with UnRAID to understand how it works. I have four 2TB disks set up as 3 data disks and one parity disk. I set it up with "most-free" allocation method on my test share. It's set up on 1GbE.

 

I started to transfer a few hundred 1GB files to check transfer speed. It runs at only about 60-70 MB/sec. When I look at the transfer speeds I see this:

 

 

The transfer speed from the host PC over SMB shows 60-70MB/sec. But I consistently see writes across all three data disks similar to what' sin this image. Shouldn't it only write one at a time? I didn't think UnRAID split files up, but distributed individual ones across the disks? I understand Parity would require reading from all disks, but not writing.

 

Can someone please help me understand how this works?

Thanks.

[trurl]

Most Free is the least efficient allocation method, because it causes Unraid to switch disks just because one disk temporarily has more free space than others. Each file will be written to only one disk, but when writing multiple files, Most Free is writing each file to whichever disk has the most free, depending on other settings for the share such as Split Level.

[trurl]

I always recommend default Highwater allocation. It is a good compromise that will write all disks eventually, but without constantly switching disks.

[HTWingNut]

5 minutes ago, trurl said:

Most Free is the least efficient allocation method, because it causes Unraid to switch disks just because one disk temporarily has more free space than others. Each file will be written to only one disk, but when writing multiple files, Most Free is writing each file to whichever disk has the most free, depending on other settings for the share such as Split Level.

 

Thanks.

But I'm writing only one file at a time, sequentially.

 

EDIT: And transfer speed says ~ 60-70MB/sec but no single data disk ever hits that performance, only the parity disk.

 

Edited March 18, 2022 by HTWingNut

[trurl]

Do you mean you are writing a single file, waiting for that write to complete, before starting the next?

 

 

[HTWingNut]

1 minute ago, trurl said:

Do you mean you are writing a single file, waiting for that write to complete, before starting the next?

 

 

YES

[trurl]

I/O is buffered in RAM. Your PC might think the transfer is complete, but some of the data is still in RAM on Unraid and probably not written out yet.

[itimpi]

What do you have Settings -> Disk Settings -> Tunable (md_write_method): set to?  

[trurl]

10 minutes ago, HTWingNut said:

transfer speed from the host PC over SMB shows 60-70MB/sec

 

6 minutes ago, HTWingNut said:

And transfer speed says ~ 60-70MB/sec but no single disk ever hits that performance, only the parity disk

Those displayed numbers are just snapshots and shouldn't be considered accurate realtime transfer speeds. Probably better to consider average speeds from the PC end of things.

 

2 minutes ago, itimpi said:

What do you have Settings -> Disk Settings -> Tunable (md_write_method): set to?  

https://wiki.unraid.net/Manual/Storage_Management#Array_Write_Modes

[trurl]

You would probably get a better idea of transfer speed if you set the share to Highwater and transfer many files together.

[HTWingNut]

6 minutes ago, trurl said:

I/O is buffered in RAM. Your PC might think the transfer is complete, but some of the data is still in RAM on Unraid and probably not written out yet.

Ah, so it caches to RAM by default then? I have 8GB RAM, and Dashboard showing consistently 42% used.

 

5 minutes ago, itimpi said:

What do you have Settings -> Disk Settings -> Tunable (md_write_method): set to?  

Tunable (md_write_method) is set to "Auto"

[trurl]

1 minute ago, HTWingNut said:

it caches to RAM by default

https://www.linuxatemyram.com/

[HTWingNut]

2 minutes ago, trurl said:

 

Those displayed numbers are just snapshots and shouldn't be considered accurate realtime transfer speeds. Probably better to consider average speeds from the PC end of things.

 

 

Thanks. That's the thing. The PC transfer speed is showing about 60MB/sec, roughly equal to the parity write speed. Maybe just coincidence, not sure. But single UnRAID data disk write speed is about half of PC write speed.

 

Either way, not a big deal. Just curious what was going on.

 

 

[trurl]

Try Highwater and it will write to disk1 for a while and you can see what speed that shows. Probably what you are seeing with Most Free is just an artifact of all the other things already discussed about Most Free, RAM buffering, etc.

[HTWingNut]

3 minutes ago, trurl said:

Try Highwater and it will write to disk1 for a while and you can see what speed that shows. Probably what you are seeing with Most Free is just an artifact of all the other things already discussed about Most Free, RAM buffering, etc.

Thanks.