.\" Copyright (c) 2012-2021 The DragonFly Project.  All rights reserved.
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.\" This code is derived from software contributed to The DragonFly Project
.\" by Matthew Dillon <dillon@backplane.com>
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.Dd October 30, 2021
.Dt XDISK 4
.Os
.Sh NAME
.Nm xdisk
.Nd Remote disk block device dmsg service
.Sh SYNOPSIS
To compile this driver into the kernel,
place the following line in your
kernel configuration file:
.Bd -ragged -offset indent
.Cd "device xdisk"
.Ed
.Pp
Alternatively, to load the driver as a
module at boot time, place the following line in
.Xr loader.conf 5 :
.Bd -literal -offset indent
xdisk_load="YES"
.Ed
.Pp
Note that
.Dx
does not compile this driver into the kernel by default.
.Pp
Since the userland hammer2 service daemon must be running, you cannot use
xdisk block devices as boot devices.
.Sh DESCRIPTION
The
.Nm
driver exports local block devices and imports remote block devices.
The devices show up under
.Pa /dev/xa* ,
.Pa /dev/serno ,
.Pa /dev/by-label ,
and
.Pa /dev/*-by-uuid .
It is recommended that all drive references go via
.Pa /dev/serno ,
.Pa /dev/by-label ,
or
.Pa /dev/*-by-uuid .
The
.Pa /dev/xa*
paths can change at a whim depending on the order of operations.
.Pp
If a device goes away or the connection is lost, the device entry
remains until the ref-count drops to 0.
Any active I/O will hard-block.
Resumption of the connection resumes operations where they left off.
No I/O requests are lost.
The driver is robust across reconnection, crashes, and reboots, but
possibly not across power failures if the target drive loses buffered
write data for completed I/Os.
.Pp
Over a 1Gbe network, synchronous D1 latency still allows for around
30-40 MBytes/sec.
Filesystem mounts can usually max-out the line rate
(in excess of 100 MBytes/sec) due to filesystem read-ahead and write
buffering.
.Pp
The
.Xr hammer2 8
service daemon is used as an intermediary and must be running on all
machines in question.
.Sh SEE ALSO
.Xr ahci 4 ,
.Xr nvme 4 ,
.Xr hammer2 8
.Sh HISTORY
The
.Nm
driver has been in the source tree a while
but is now automatically built and part of base as of
.Dx 6.1 .
.Sh AUTHORS
.An -nosplit
The
.Nm
driver was written by
.An Matthew Dillon Aq Mt dillon@backplane.com