Allow different starting block for CACHE


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I've been able to reproduce on multiple machines with multiple samsung ssd's that the limitation of the starting block at 64 causes issues with the 840 and 850 (possibly 860) samsung SSD's.  They have a non-standard NAND Erase block size of 1536k instead of 1024k.  It causes poor performance with uneven write speeds to the drives. 

 

Please allow us to set the starting block size for the BTRFS pool drives when building.  

 

Thanks!

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@limetech just for reference, I was able to reproduce the same issue with multiple Samsung SSD's and there is a thread where others had the same issue back in 2017 when using samsung's in their cache pools.  

 

Since removing my samsung from the pool and swapping to XFS; my machine is working like a boss with no issues whatsoever...  details showing the samsung write issues were in this other thread:

 

If you start reading this post: 

and read down a few more posts, you'll see where I did tests and have graphs etc showing the write performance differences to the Samsung device when going from the starting block of 64 up to 2048.  

 

 

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3 minutes ago, dnoyeb said:

Since removing my samsung from the pool and swapping to XFS; my machine is working like a boss with no issues whatsoever.

 

Thanks for the info.  Haven't read through all that yet, but would you speculate:

 

a) samsung with btrfs cache disk => issue exists

b) samsung with xfs cache disk => issue does not exist

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I completely removed the samsung and went with a sandisk in the end since I saw issues writing to the samsung via unassigned devices with the wrong starting block.

 

I will try and pull the sandisk and swap to the samsung this weekend as a test to see....  

 

One other item of note is I have 256gb of ram; some speculated that was introducing another factor into things.

 

 

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I have 2x 960 EVO's in Raid-0 cache...  Just checked my starting blocks and they were both at zero...  Possibly because I followed these steps before formatting them: 

 

https://wiki.archlinux.org/index.php/Solid_State_Drive

Quote

Trim an entire device

If you want to trim your entire SSD at once, e.g. for a new install, or you want to sell your SSD, you can use the blkdiscard command, which will instantly discard all blocks on a device.

Warning: all data on the device will be lost!

# blkdiscard /dev/sdX

 

 

Which I did with these commands:

blkdiscard /dev/nvme0n1
blkdiscard /dev/nvme1n1

Before formatting them with the webgui...

 

EDIT:

Come to think of it, I think I had unusually low performance the first time I tried it... I then tried the above while troubleshooting the issue and may have accidentally fixed it because of it...   Also might this have something to do with the difference in size between GPT/MBR and having no partition table?

Edited by Warrentheo
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  • 1 month later...
On 3/21/2018 at 2:14 PM, limetech said:

 

Thanks for the info.  Haven't read through all that yet, but would you speculate:

 

a) samsung with btrfs cache disk => issue exists

 b) samsung with xfs cache disk => issue does not exist

 

I also have this issue with a Samsung cache disk.  It was btrfs, so to try to resolve it, I reformatted it as xfs, but I'm still experiencing the problem.

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  • 1 month later...
  • 9 months later...
  • 6 months later...

trying to follow this but confused.

I have 4 samsung 860 evo 1TB ssd in raid 10 as cache and have great read and write speed accessing them over a 10G network. Average 450-500MB/s write and 800MB/s read. Also when i had only 2 drives in raid 1 all was great. Did not do anything special and just had unraid create the cache in default btrfs settings. What am i missing ? If it was a wrong default formatting option for samsungs wouldnt i also be affected ? Is there another factor at play here why only some are affected ?

 

 

ps starting blocks all at 64

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