Btrfs RAID-10 performance

[KSB]

On 2020.09.15. 11:22, Linda A. Walsh wrote:
> On 2020/09/10 07:40, Miloslav Hůla wrote:
>> I cannot verify it, but I think that even JBOD is propagated as a 
>> virtual device. If you create JBOD from 3 different disks, low level 
>> parameters may differ.
> ----
>     JBOD allows each disk to be seen by the OS, as is.  You wouldn't
> create JBOD disk from 3 different disks -- JBOD would give you 3 separate
> JBOD disks for the 3 separate disks.
> 
>     So for your 16  disks, you are using 1 long RAID0?  You realize
> 1 disk goes out, the entire array needs to be reconstructed.  Also
> all of your spindles can be tied up by long read/writes -- optimal speed
> would come from a read 16 stripes wide spread over the 16 disks.
> 
>     What would be better, IMO, is going with a RAID-10 like your subject
> says, using 8-pairs of mirrors and strip those.  Set your stripe unit
> for 64K to allow the disks to operate independently.  You don't want
> a long 16-disk stripe, as that's far from optimal for your mailbox load.
> What you want is the ability to have multiple I/O ops going at the same
> time -- independently.  I think as it stands now, you are far more likely
> to get contention as different mailboxes are accessed with contention
> happening within the span, vs. letting each 2 disk mirror potentially doing
> a different task -- which would likely have the effect of raising your
> I/O ops/s.
>     Running raid10 on top of raid0 seems really wasteful
> 
> 

You create individual raid0 from each individual disk, write buffers 
off, of course. That is how it's going on sh***y controllers. For some 
controllers, firmware upgrade will add JBOD, for some you need to flash 
IT firmware, for some you can switch to HBA mode.
But anyway - use HBA or GOOD RAID controller.

--
KSB