Re: [PATCH v5 0/5] Initial support for multi-actuator HDDs

["Martin K. Petersen" <martin.petersen@oracle.com>]

Damien,

> With the single LUN approach, the fault domain does not really change
> from a regular device. The typical use in case of bad heads would be
> to replace the drive or reformat it at lower capacity with head
> depop. That could be avoided with dm-linear on top (one DM per
> actuator) though.

I was more thinking along the lines of btrfs making sure to place
dup metadata on different actuators or similar.

> The above point led me to this informational only implementation.
> Without optimization, we get the same as today. No changes in
> performance and use.  Better IOPS is gain for lucky workloads
> (typically random ones). Going forward, more reliable IOPS &
> throughput gains are possible with some additional changes.

Sure, but that means the ranges need to affect both I/O scheduling and
data placement. We need to make sure that the data placement interface
semantics are applicable to other types of media than multi actuator
drives. Filesystems shouldn't have to look at different queue limits if
they sit on top of dm-linear instead of sd. From an I/O scheduling
perspective I concur that device implementation details are pertinent.

-- 
Martin K. Petersen	Oracle Linux Engineering