presto - The Prestoserve pseudodevice driver
#include <sys/presto.h>
#include <sys/prestoioctl.h>
pseudo-device presto
The Prestoserve pseudodevice driver presto caches synchronous
writes in nonvolatile memory. Prestoserve causes
synchronous writes to be performed at memory speeds,
rather than at disk speeds. Synchronous writes that result
in Prestoserve cache hits do not perform the earlier physical
disk writes, because only the last write is actually
performed by Prestoserve. Therefore, 50% to 65% of all
the physical disk write operations are avoided, because
every sequential NFS write to a file also causes the inode
and the indirect block to be synchronously written.
The presto driver is layered on other disk drivers, and it
intercepts the other drivers' I/O requests by replacing
the entry points of the original driver in the bdevsw and
cdevsw tables. The presto driver caches the intercepted
synchronous write requests in the Prestoserve cache's nonvolatile
memory. When Prestoserve needs to perform actual
I/O, it calls the original driver's entry points to perform
the I/O. A modified form of Least Recently Used
(LRU) replacement determines when the Prestoserve cache
data needs to be written to the intended disks.
An accelerated disk device (one that has the presto pseudodevice
driver layered on top of its driver), uses the
same major and minor devices that it used before it was
accelerated.
The Prestoserve nonvolatile memory must be found at boot
time in order for Prestoserve to perform its write-caching
function. In addition, Prestoserve must pass diagnostic
tests, and there must be sufficient backup battery power
to guarantee a reasonable amount of cache data stability
(measured in days or weeks) in the event of a power or
hardware failure.
If the Prestoserve nonvolatile memory is not found or if
there is not enough backup battery power, then the disks
are not accelerated; however they can be opened and used
as usual. In this case, the presto driver simply passes
all I/O requests directly through to the appropriate
device.
Operation [Toc] [Back]
When Prestoserve is in the PRUP state, it caches all synchronous
write requests for enabled file systems to the
presto driver in nonvolatile memory and writes the
Prestoserve cache data asynchronously to the intended
disks.
When Prestoserve is in the PRDOWN state, there is no data
in the Prestoserve cache, no data is put into the
Prestoserve cache, and all disk requests are passed
directly to the real disk driver.
When Prestoserve is in the PRERROR state, the data in the
Prestoserve cache can not be written to the intended disks
because of a disk, system, or hardware error.
When the system is shut down normally by using the reboot
system call from the shutdown, halt, or reboot command,
the Prestoserve cache data is written to the intended
disks, and Prestoserve enters the PRDOWN state. This
allows you to fix any system or disk error or to upgrade
or change your system without losing the data in the
Prestoserve cache or corrupting your disks.
If your system was shut down without following normal
shutdown procedures, and you reboot the system, any data
in the Prestoserve cache is written to the intended disks,
if possible. If the data is successfully written to the
intended disks (and if the nonvolatile memory and backup
battery passed the diagnostic tests), Prestoserve enters
the PRDOWN state. If an error occurs, Prestoserve enters
the PRERROR state.
Note
Data can exist in the Prestoserve cache after you reboot
the system only in the event of a previous power failure,
disk device error, or kernel crash that resulted from a
software or hardware problem.
If an error from a disk device occurs or if the backup
battery power is insufficient, Prestoserve writes the
cache data to the intended disks, if possible, and enters
the PRERROR state. When Prestoserve is in the PRERROR
state, new data that is written to a block not found in
the Prestoserve cache is passed directly to the real disk
driver. If new data is written to a block that is found in
the cache, Prestoserve replaces the existing block and
attempts to write the block to the real disk driver to
determine if the error condition on that block still
exists. If the write is successful and if all the
Prestoserve cache data can be written to the intended
disks, Prestoserve leaves the PRERROR state.
The Prestoserve cache never discards data without being
explicitly told to do so by using a PRRESET ioctl command.
This can be done by using the presto -R command. This
command should only be used when there is a fatal disk
error and when the data is not important.
Prestoserve ioctl Commands [Toc] [Back]
The presto pseudodevice driver does not intercept ioctl
commands; they go directly to the actual disk driver.
The following ioctl commands can be performed on the
Prestoserve control device /dev/pr0. Some ioctl commands
affect all of Prestoserve operation, while others only
affect a particular accelerated file system. The argument
to ioctl is a pointer to a presto_status structure, which
contains battery status information, Prestoserve state,
current and maximum nonvolatile memory sizes, and various
Prestoserve statistics. The argument to ioctl is a
pointer to a presto_status structure, which contains
extended chargeable battery status information,
Prestoserve state, current and maximum nonvolatile memory
sizes, and various Prestoserve statistics. The argument
to ioctl is a pointer to an int, which can be either PRUP
to enable Prestoserve or PRDOWN to disable Prestoserve.
When a system reboots, Prestoserve is in the PRDOWN state
and must be explicitly enabled by an ioctl. You enable
Prestoserve by using the presto -u command. You can also
automatically enable Prestoserve by specifying the appropriate
run-time variables in the /etc/rc.config file and
specifying file systems in the /etc/prestotab file. The
prestosetup command provides you with an interactive
facility to set up Prestoserve. When Prestoserve goes from
the PRDOWN state to the PRUP state, the Prestoserve I/O
statistics are reset. When Prestoserve goes from the PRUP
state to the PRDOWN state, all the Prestoserve buffers are
written to the intended disks, and the buffers are invalidated.
The argument to ioctl is a pointer to an int,
which is the size in bytes of the Prestoserve nonvolatile
memory to be used. This size cannot be larger than the
maximum size reported in the presto_status structure. The
argument to ioctl is ignored. Like the PRSETSTATE ioctl,
PRRESET sets the Prestoserve state to PRDOWN, but it also
reinitializes all of nonvolatile Prestoserve memory. If
Prestoserve was in the PRERROR state and some Prestoserve
buffers could not be written to the intended disks because
of disk I/O errors, the data in the buffers is lost. This
is the only method you can use to force Prestoserve to
discard data that cannot be written to disk, and it can be
accomplished by using the presto -R command. The argument
to ioctl is ignored. All the data in the Prestoserve
buffers is written to the intended disks, but the buffers
are not invalidated. This command can be used by a daemon
that flushes the cache periodically to minimize the risk
to data in the event of a catastrophic failure. The cache
data can be flushed to the intended disks by using the
presto -F command. The argument to ioctl is a pointer to
a struct uprtab. On input, the upt_bmajordev and upt_unit
fields specify the block device major number and unit number
of the device whose struct uprtab should be returned.
The upt_bmajordev and upt_unit fields are set to NODEV,
which is defined in the header file <sys/param.h>, if the
requested device does not exist or if it is not accelerated.
The struct uprtab contains a upt_enabled field that
is a bit vector indexed by a partition number and that
indicates whether the partition has Prestoserve caching
enabled. The argument to ioctl is a pointer to a struct
uprtab. This ioctl returns the struct uprtab for the
accelerated device with the smallest (block device major
number, unit number) pair that is greater than the
upt_bmajordev and upt_unit fields of the struct uprtab
argument. This allows each accelerated device's struct
uprtab to be retrieved sequentially by specifying the previous
device's (block device major number, unit number)
pair. To get the first accelerated device's struct uprtab
set the upt_bmajordev and upt_unit fields to NODEV. Use
the same struct uprtab that was returned on the previous
call for the next call. When the upt_bmajordev and
upt_unit fields of the struct uprtab argument are greater
than or equal to the last accelerated device's major block
device number, the struct uprtab that is returned has the
upt_bmajordev and upt_unit fields set to NODEV. The argument
to ioctl is a pointer to a dev_t. This enables
Prestoserve caching on the specified file system. The
argument to ioctl is a pointer to a dev_t. If all cached
data for the specified file system can be successfully
written to disk, Prestoserve caching is disabled for this
file system.
Diagnostic and Error Messages [Toc] [Back]
This message is displayed if you attempt to use
Prestoserve on a system that has not had its license registered.
It is necessary to register a valid license in
order to use Prestoserve. During a system reboot, dirty
buffers were found for a block device with the major number
N. The dirty buffers could not be written to the
device because the device was not registered with
Prestoserve. Prestoserve will remain in the ERROR state
until the device with major number N registers itself with
Prestoserve, at which time the dirty buffers will be
flushed back to the device. This message is displayed at
boot time and indicates that Prestoserve recognized its
control information portion of the cache. It is a normal
Prestoserve startup message. This message is displayed at
boot time and indicates that Prestoserve did not recognize
the cache as being in either a clean (containing no data)
or a dirty (containing data) state. The message is usually
displayed when the cache is used for the first time, after
the cache has been cleared by using a diagnostic command,
or after backup battery failure. This message is displayed
at boot time, and it indicates that the cache
tested as either read/write ok or readonly ok. The message
is a normal Prestoserve startup message. The status for
the primary battery or a secondary battery, if applicable,
is reported as either OK, LOW, or DISABLED. This message
is displayed at boot time and when there is a change in
the state of the backup battery power level. This message
is displayed at boot time if Prestoserve was not shut down
by using the normal system shutdown procedures. This message
indicates that dirty buffers were found after the
system rebooted. The data is written to the intended disks
as soon as possible, usually when the first I/O request
occurs for any accelerated device. This message indicates
that Prestoserve has begun to write the data in the dirty
buffers to the intended disks. This message indicates
that the data in the dirty buffers has been successfully
written to the intended disks. This message is displayed
at boot time and indicates that the kernel is now being
run with a version of the Prestoserve software that is
different from the version used previously. Usually, this
message is displayed when you first boot the system after
performing a software upgrade. This message indicates
that the block size and fragment size in the Prestoserve
control information portion of the cache are different
from the information that was expected. This message
should only be displayed when you first boot the system
after performing a software upgrade. This message indicates
that only a portion of the Prestoserve cache was
being used when the system was shut down, but now
Prestoserve is using the entire cache. The presto -s command,
which changes the size of the Prestoserve cache, is
described in presto(8). This message indicates that a
hardware or software problem exists because the size of
the Prestoserve cache at reboot is less than the size of
the cache when the system shut down. This message is displayed
at boot time and indicates that Prestoserve was not
shut down normally and that the cache contents were previously
in a different system (for example, either the cache
was moved or the system ID, which is usually the on-board
Ethernet hardware address, has changed). Prestoserve
allows you to do one of the following interactively: discard
the data, write the data to disk, or halt the
machine. When these messages are displayed, Prestoserve
allows you to do one of the following interactively:
continue with the boot or halt the machine. This message
indicates that dirty Prestoserve buffers were found after
the system rebooted, and the data in the dirty buffers
could not be written to the specified device because the
device failed to open. You should verify that the device
is online and that the kernel successfully found the
device at boot time. Refer to errno(2) for a complete
description of the error. This message indicates that the
specified device failed to open. You should check your
disk configuration and make sure that the drive is on
line. Refer to errno(2) for a complete description of the
error. This message indicates that an ioctl failed for
the specified device. Refer to errno(2) for a complete
description of the error. These messages indicate that
the disk controller for the specified device could not
directly address the cache when Prestoserve was enabled on
the file system. This message is displayed when
Prestoserve is in the PRERROR state and receives a request
to write the data in a dirty buffer to the intended disks.
Presto-ized in this kernel! This message indicates that
Prestoserve was not shut down cleanly, and the system was
previously running a kernel with an accelerated device
that the current kernel does not accelerate. You should
boot a kernel that accelerates all the devices that were
previously accelerated. This message is displayed at system
startup if a Prestoserve cache read/write error
occurred, and it indicates that the cache could not be
accessed. It indicates a hardware or software error. This
message indicates that the Prestoserve cache failed the
read/write test at the specified address. This message
indicates that an I/O error occurred on the specified disk
during a Prestoserve write-back operation. This message
indicates that there is inadequate backup battery power.
Prestoserve attempts to write all Prestoserve cache data
to the intended disks and then enters the PRERROR state.
This message indicates that Prestoserve disabled itself
because of inadequate backup battery power or because a
disk error occurred during a write to disk. This message
indicates that a disk error or low backup battery power
condition has been corrected and that Prestoserve is
enabled again.
The Prestoserve ioctl commands set errno to the specified
values for the following conditions: Prestoserve is not
registered for use on this system. A caller whose uid is
not root tried to use the PRSETSTATE, PRSETMEMSZ, PRRESET,
PRENABLE or PRDISABLE command. An attempt was made to use
the PRSETSTATE, PRSETMEMSZ, or PRDISABLE command, but a
fatal disk error or a battery problem exists. The memory
size specified in the PRSETSTATE command exceeds the maximum
size of the cache reported in the presto_status structure.
Prestoserve was not successfully started at boot
time, or the PRENABLE or PRDISABLE command was used and
the device specified by dev_t is not a device initialized
for use with Prestoserve, or an error occurred in trying
to open/ioctl the device. An invalid argument was specified
with the PRSETMEMSZ or PRSETSTATE command or that an
invalid command was used.
Generic Prestoserve control device
errno(2), ioctl(2), presto(8), dxpresto(8X),
prestoctl_svc(8), prestosetup(8), prestotab(4)
Guide to Prestoserve
presto(7)
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