A number of modules (RAID, logical volumes) have logical
"stripes" that require splitting an I/O into several
child I/O. For example, on a RAID-0 with 128KB strip size,
an I/O that spans a 128KB boundary will require sending
one I/O for the portion that comes before the boundary to
one member disk, and another I/O for the portion that comes
after the boundary to another member disk. Logical volumes
are similar - data is allocated in clusters, so an I/O that
spans a cluster boundary may need to be split since the
clusters may not be contiguous on disk.
Putting the splitting logic in the common bdev layer ensures
bdev module authors don't have to always do this themselves.
This is especially helpful for cases like splitting an I/O
described by many iovs - we can simplify this a lot by
handling it in the common bdev layer.
Note that currently we will only submit one child I/O
at a time. This could be improved later to submit multiple
child I/O in parallel, but the complexity in the iov splitting
code also increases a lot.
Note: Some Intel NVMe SSDs have a similar characteristic.
We will not use this bdev stripe feature for NVMe though -
we want to primarily use the splitting functionality inside
of the NVMe driver itself to ensure it remains fully
functional. Many SPDK users use the NVMe driver without
the bdev layer.
Signed-off-by: Jim Harris <james.r.harris@intel.com>
Change-Id: Ife804ecc56f6b2b55345a0d0ae9fda9e68632b3b
Reviewed-on: https://review.gerrithub.io/423024
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
4bd9762165