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Add note about nvd / nda selection.

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Noticed by: cem@
This commit is contained in:
Warner Losh 2019-05-19 05:46:24 +00:00
parent e519cee307
commit cd90b46402
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=347966
1 changed files with 33 additions and 1 deletions
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34
share/man/man4/nvme.4
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@ -33,7 +33,7 @@
.\"
.\" $FreeBSD$
.\"
.Dd December 7, 2018
.Dd May 18, 2019
.Dt NVME 4
.Os
.Sh NAME
@ -119,6 +119,38 @@ hw.nvme.force_intx=1
.Ed
.Pp
Note that use of INTx implies disabling of per-CPU I/O queue pairs.
.Pp
The
.Xr nvd 4
driver is used to provide a disk driver to the system by default.
The
.Xr nda 4
driver can also be used instead.
The
.Xr nvd 4
driver performs better with smaller transactions and few TRIM
commands.
It sends all commands directly the drive immediately.
The
.Xr nda 4
driver performs better with larger transactions and also collapses
TRIM commands giving better performance.
It can queue commands to the drive; combine
.Dv BIO_DELETE
commands into a single trip; and
use the CAM I/O scheduler to bias one type of operation over another.
To select the
.Xr nda 4
driver, set the following tunable value in
.Xr loader.conf 5 :
.Bd -literal -offset indent
hw.nvme.use_nvd=0
.Ed
.Pp
This value may also be set in the kernel config file with
.Bd -literal -offset indent
.Cd options NVME_USE_NVD=0
.Ed
.Sh SYSCTL VARIABLES
The following controller-level sysctls are currently implemented:
.Bl -tag -width indent