On 1/24/23 14:59, Damien Le Moal wrote:
There is only one priority class that ATA understands: RT (the level is
irrelevant and ignored). All RT class IOs are mapped to high priority NCQ
commands. All other classes map to normal priority (no priority bit set)
commands.
And sure, we could map the level of RT class IOs to a CDL index, as we do
for the CDL class, but what would be the point ? The user should use the
CDL class in that case.
Furthermore, there is one additional thing that we do not yet support but
will later: CDL descriptor 0 can be used to set a target time limit for
high priority NCQ commands. Without this new feature introduced with CDL,
the drive is free to schedule high priority NCQ commands as it wants, and
that is hard coded in FW. So you can endup with very aggressive scheduling
leading to significant overall IOPS drop and long tail latency for low
priority commands. See page 11 and 20 of this presentation for an example:
https://www.snia.org/sites/default/files/SDC/2021/pdfs/SNIA-SDC21-LeMoal-Be-On-Time-command-duration-limits-Feature-Support-in%20Linux.pdf
For a drive that supports both CDL and NCQ priority, with CDL feature
turned off, CDL descriptor 0 defines the time limit hint for high priority
NCQ commands. Again, CDL and NCQ high priority are mutually exclusive.
So for clarity, I really would prefer separating CDL and RT classes as we
did. We could integrate CDL support reusing the RT class + level for CDL
index, but I think this may be very confusing for users, especially
considering that the CLDs on a drive can be defined in any order the user
wants, resulting in indexes/levels that does do not have any particular
order, making it impossible for the host to correctly schedule commands.
Hi Damien,
Thanks again for the detailed reply. Your replies are very informative
and help me understand the context better.
However, I'm still less than enthusiast about the introduction of the
I/O priority class IOPRIO_CLASS_DL. To me command duration limits (CDL)
is a mechanism that is supported by one storage standard (SCSI) and of
which it is not sure that it will be integrated in other storage
standards (NVMe, ...). Isn't the purpose of the block layer to provide
an interface that is independent of the specifics of a single storage
standard? This is why I'm in favor of letting the ATA core translate one
of the existing I/O priority classes into a CDL instead of introducing a
new I/O priority class (IOPRIO_CLASS_DL) in the block layer.
Others may have a different opinion.
Thanks,
Bart.
