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I have successfully installed DSM 6.2.3 on my home NAS (booting from USB drive). 

 

I have 4 hard drives installed on my NAS: 3 SATA hard drives and 1 SSD. I want to make SSD as my / root file system, so that I can get fast DSM response and the hard drives can sleep longer without wakeup. However, it seems one of my hard drive is mounted as / file system.

 

Is that possible to change that? If so, how to do it?

 

 

Thanks

 

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3 hours ago, fdppi said:

, so that I can get fast DSM response and the hard drives can sleep longer without wakeup. However, it seems one of my hard drive is mounted as / file system.

remember the faq? dsm system partitions are on all disks and its a raid1 over all disks

even if you tweak it to read from the ssd to make it faster it still needs to sync all drives on writing

i dont know how the kernels MD decides what drive its using for reading on boot, might be the 1st found, so connect the ssd to the 1st sata onboard (sata0)

there is also a switch --write-mostly when creating a raid1 that marks slower drives, you would need to apply that marker to drives in the raid1, excluding the ssd

that would bring the reads primarily to the ssd as much as that is possible

i never tried this so be careful,i'm not even sure if its a good idea to to so, lets just say its possible

@flyride - any comment on this one?

 

https://raid.wiki.kernel.org/index.php/Write-mostly

 

setting it on a already existing raid

https://www.tansi.org/hybrid/

(->"How to do it for an existing RAID1")

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All that @IG-88 explained is correct.  DSM installs a /root and /swap to each drive, period, implementing each with their own RAID1 array across all the disks.

 

It is not possible to remove those partitions (there is no way to get that space back).  But it is possible to force DSM to make them hotspares instead of active in each respective RAID1: https://xpenology.com/forum/topic/12391-nvme-optimization-baremetal-to-esxi-report

 

It is not inherently unsafe to do this.  EXCEPT that as you describe your system, there would be no redundancy for DSM, so if something happened to your SSD, DSM configuration would be lost and the system would be unbootable.  I do this configuration on my own NAS, except that it has 2x SSD's that keep redundancy for DSM.

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2 minutes ago, flyride said:

It is not possible to remove those partitions (there is no way to get that space back).  But it is possible to force DSM to make them hotspares instead of active in each respective RAID1: https://xpenology.com/forum/topic/12391-nvme-optimization-baremetal-to-esxi-report

 

what about applying write-mostly after array creation, any experience with that one?

(it would still be normal raid1 with all members active, so no degradation  of the raid1 to a single drive)

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--write-mostly only biases the reads to the fast drive, all writes continue to be dispatched to all member drives.  So I didn't pursue it for my use case, and I suppose it wouldn't quite be what OP was looking for.  But it might have some uses, if someone wanted to play with it and see if DSM gets upset about it.

 

https://raid.wiki.kernel.org/index.php/Write-mostly

 

This will result in the following idiosyncracies:

  • Write performance will be equal to the slowest participant in the RAID-1 array.This can be mitigated with the --write-behind option (which caches writes to the array).
  • Read performance will be somewhere between the fastest device and the slowest device. This is because md WILL occasionally still attempt to read from the slower device...and it will wait for the data to be delivered.
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