Tribblix: manual page: mhd.4i

MHD(4I) Ioctl Requests MHD(4I)

NAME

mhd - multihost disk control operations

SYNOPSIS

#include <sys/mhd.h>

DESCRIPTION

The mhd ioctl(2) control access rights of a multihost disk, using disk
reservations on the disk device.

The stability level of this interface (see attributes(7)) is evolving.
As a result, the interface is subject to change and you should limit
your use of it.

The mhd ioctls fall into two major categories: (1) ioctls for non-
shared multihost disks and (2) ioctls for shared multihost disks.

One ioctl, MHIOCENFAILFAST, is applicable to both non-shared and shared
multihost disks. It is described after the first two categories.

All the ioctls require root privilege.

For all of the ioctls, the caller should obtain the file descriptor for
the device by calling open(2) with the O_NDELAY flag; without the
O_NDELAY flag, the open may fail due to another host already having a
conflicting reservation on the device. Some of the ioctls below permit
the caller to forcibly clear a conflicting reservation held by another
host, however, in order to call the ioctl, the caller must first obtain
the open file descriptor.

Non-shared multihost disks
Non-shared multihost disks ioctls consist of MHIOCTKOWN, MHIOCRELEASE,
MHIOCSTATUS, and MHIOCQRESERVE. These ioctl requests control the
access rights of non-shared multihost disks. A non-shared multihost
disk is one that supports serialized, mutually exclusive I/O mastery by
the connected hosts. This is in contrast to the shared-disk model, in
which concurrent access is allowed from more than one host (see below).

A non-shared multihost disk can be in one of two states:

+o Exclusive access state, where only one connected host has I/O
access

+o Non-exclusive access state, where all connected hosts have I/O
access. An external hardware reset can cause the disk to enter
the non-exclusive access state.

Each multihost disk driver views the machine on which it's running as
the "local host"; each views all other machines as "remote hosts". For
each I/O or ioctl request, the requesting host is the local host.

Note that the non-shared ioctls are designed to work with SCSI-2 disks.
The SCSI-2 RESERVE/RELEASE command set is the underlying hardware
facility in the device that supports the non-shared ioctls.

The function prototypes for the non-shared ioctls are:

ioctl(fd, MHIOCTKOWN);
ioctl(fd, MHIOCRELEASE);
ioctl(fd, MHIOCSTATUS);
ioctl(fd, MHIOCQRESERVE);

MHIOCTKOWN Forcefully acquires exclusive access rights to the
multihost disk for the local host. Revokes all access
rights to the multihost disk from remote hosts. Causes
the disk to enter the exclusive access state.

Implementation Note: Reservations (exclusive access
rights) broken via random resets should be reinstated by
the driver upon their detection, for example, in the
automatic probe function described below.

MHIOCRELEASE Relinquishes exclusive access rights to the multihost
disk for the local host. On success, causes the disk to
enter the non- exclusive access state.

MHIOCSTATUS Probes a multihost disk to determine whether the local
host has access rights to the disk. Returns 0 if the
local host has access to the disk, 1 if it doesn't, and
-1 with errno set to EIO if the probe failed for some
other reason.

MHIOCQRESERVE Issues, simply and only, a SCSI-2 Reserve command. If
the attempt to reserve fails due to the SCSI error
Reservation Conflict (which implies that some other host
has the device reserved), then the ioctl will return -1
with errno set to EACCES. The MHIOCQRESERVE ioctl does
NOT issue a bus device reset or bus reset prior to
attempting the SCSI-2 reserve command. It also does not
take care of re-instating reservations that disappear
due to bus resets or bus device resets; if that behavior
is desired, then the caller can call MHIOCTKOWN after
the MHIOCQRESERVE has returned success. If the device
does not support the SCSI-2 Reserve command, then the
ioctl returns -1 with errno set to ENOTSUP. The
MHIOCQRESERVE ioctl is intended to be used by high-
availability or clustering software for a "quorum" disk,
hence, the "Q" in the name of the ioctl.

Shared Multihost Disks

Shared multihost disks ioctls control access to shared multihost disks.
The ioctls are merely a veneer on the SCSI-3 Persistent Reservation
facility. Therefore, the underlying semantic model is not described in
detail here, see instead the SCSI-3 standard. The SCSI-3 Persistent
Reservations support the concept of a group of hosts all sharing access
to a disk.

The function prototypes and descriptions for the shared multihost
ioctls are as follows:

ioctl(fd, MHIOCGRP_INKEYS, (mhioc_inkeys_t *)k)

Issues the SCSI-3 command Persistent Reserve In Read Keys to the
device. On input, the field k->li should be initialized by the
caller with k->li.listsize reflecting how big of an array the caller
has allocated for the k->lilist field and with `k->li.listlen == 0'.
On return, the field k->li.listlen is updated to indicate the number
of reservation keys the device currently has: if this value is
larger than k->li.listsize then that indicates that the caller
should have passed a bigger k->li.list array with a bigger
k->li.listsize. The number of array elements actually written by
the callee into k->li.list is the minimum of k->li.listlen and
k->li.listsize. The field k->generation is updated with the
generation information returned by the SCSI-3 Read Keys query. If
the device does not support SCSI-3 Persistent Reservations, then
this ioctl returns -1 with errno set to ENOTSUP.

ioctl(fd, MHIOCGRP_INRESV, (mhioc_inresvs_t *)r)

Issues the SCSI-3 command Persistent Reserve In Read Reservations to
the device. Remarks similar to MHIOCGRP_INKEYS apply to the array
manipulation. If the device does not support SCSI-3 Persistent
Reservations, then this ioctl returns -1 with errno set to ENOTSUP.

ioctl(fd, MHIOCGRP_REGISTER, (mhioc_register_t *)r)

Issues the SCSI-3 command Persistent Reserve Out Register. The
fields of structure r are all inputs; none of the fields are
modified by the ioctl. The field r->aptpl should be set to true to
specify that registrations and reservations should persist across
device power failures, or to false to specify that registrations and
reservations should be cleared upon device power failure; true is
the recommended setting. The field r->oldkey is the key that the
caller believes the device may already have for this host initiator;
if the caller believes that that this host initiator is not already
registered with this device, it should pass the special key of all
zeros. To achieve the effect of unregistering with the device, the
caller should pass its current key for the r->oldkey field and an
r->newkey field containing the special key of all zeros. If the
device returns the SCSI error code Reservation Conflict, this ioctl
returns -1 with errno set to EACCES.

ioctl(fd, MHIOCGRP_RESERVE, (mhioc_resv_desc_t *)r)

Issues the SCSI-3 command Persistent Reserve Out Reserve. The
fields of structure r are all inputs; none of the fields are
modified by the ioctl. If the device returns the SCSI error code
Reservation Conflict, this ioctl returns -1 with errno set to
EACCES.

ioctl(fd, MHIOCGRP_PREEMPTANDABORT, (mhioc_preemptandabort_t *)r)

Issues the SCSI-3 command Persistent Reserve Out Preempt-And-Abort.
The fields of structure r are all inputs; none of the fields are
modified by the ioctl. The key of the victim host is specified by
the field r->victim_key. The field r->resvdesc supplies the
preempter's key and the reservation that it is requesting as part of
the SCSI-3 Preempt-And-Abort command. If the device returns the
SCSI error code Reservation Conflict, this ioctl returns -1 with
errno set to EACCES.

ioctl(fd, MHIOCGRP_PREEMPT, (mhioc_preemptandabort_t *)r)

Similar to MHIOCGRP_PREEMPTANDABORT, but instead issues the SCSI-3
command Persistent Reserve Out Preempt. (Note: This command is not
implemented).

ioctl(fd, MHIOCGRP_CLEAR, (mhioc_resv_key_t *)r)
Issues the SCSI-3 command Persistent Reserve Out Clear. The input
parameter r is the reservation key of the caller, which should have
been already registered with the device, by an earlier call to
MHIOCGRP_REGISTER.

For each device, the non-shared ioctls should not be mixed with the
Persistent Reserve Out shared ioctls, and vice-versa, otherwise, the
underlying device is likely to return errors, because SCSI does not
permit SCSI-2 reservations to be mixed with SCSI-3 reservations on a
single device. It is, however, legitimate to call the Persistent
Reserve In ioctls, because these are query only. Issuing the
MHIOCGRP_INKEYS ioctl is the recommended way for a caller to determine
if the device supports SCSI-3 Persistent Reservations (the ioctl will
return -1 with errno set to ENOTSUP if the device does not).

MHIOCENFAILFAST Ioctl

The MHIOCENFAILFAST ioctl is applicable for both non-shared and shared
disks, and may be used with either the non-shared or shared ioctls.

ioctl(fd, MHIOENFAILFAST, (unsigned int *)millisecs)

Enables or disables the failfast option in the multihost disk driver
and enables or disables automatic probing of a multihost disk,
described below. The argument is an unsigned integer specifying the
number of milliseconds to wait between executions of the automatic
probe function. An argument of zero disables the failfast option
and disables automatic probing. If the MHIOCENFAILFAST ioctl is
never called, the effect is defined to be that both the failfast
option and automatic probing are disabled.

Automatic Probing

The MHIOCENFAILFAST ioctl sets up a timeout in the driver to
periodically schedule automatic probes of the disk. The automatic
probe function works in this manner: The driver is scheduled to probe
the multihost disk every n milliseconds, rounded up to the next
integral multiple of the system clock's resolution. If

1. the local host no longer has access rights to the multihost
disk, and

2. access rights were expected to be held by the local host,

the driver immediately panics the machine to comply with the failfast
model.

If the driver makes this discovery outside the timeout function,
especially during a read or write operation, it is imperative that it
panic the system then as well.

RETURN VALUES

Each request returns -1 on failure and sets errno to indicate the
error.

EPERM Caller is not root.

EACCES Access rights were denied.

EIO The multihost disk or controller was unable to
successfully complete the requested operation.

EOPNOTSUP The multihost disk does not support the operation.
For example, it does not support the SCSI-2
Reserve/Release command set, or the SCSI-3
Persistent Reservation command set.

STABILITY

Uncommitted