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presto(7)
NAME
presto - The Prestoserve pseudodevice driver
SYNOPSIS
#include <sys/presto.h>
#include <sys/prestoioctl.h>
pseudo-device presto
DESCRIPTION
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
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
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.
PRGETSTATUS
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.
PRNGETSTATUS
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.
PRSETSTATE
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.
PRSETMEMSZ
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.
PRRESET
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.
PRFLUSH
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.
PRGETUPRTAB
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.
PRNEXTUPRTAB
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.
PRENABLE
The argument to ioctl is a pointer to a dev_t. This enables Prestoserve
caching on the specified file system.
PRDISABLE
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
Prestoserve license not registered
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.
presto: Block device N currently not present, delay writing of its buffers
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.
presto: using nbytes of NVRAM at addr
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.
presto: initializing nbytes of NVRAM at addr
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.
presto: NVRAM tested status
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.
presto: whichbattery status
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.
presto: cache contains dirty data
This message is displayed at boot time if Prestoserve was not shut down
by using the normal system shutdown procedures.
presto: n dirty buffers found
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.
presto: writing dirty buffers
This message indicates that Prestoserve has begun to write the data in
the dirty buffers to the intended disks.
presto: dirty buffers written
This message indicates that the data in the dirty buffers has been
successfully written to the intended disks.
presto: using software version n-- version n last used
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.
presto: expected sizes n, n -- found n, n
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.
presto: NVRAM increased from nto n bytes at addr
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).
presto: decreased from nto n bytes!
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.
presto: dirty buffers found for host id addr, which is different from this
host's id (addr)
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.
The Prestoserve cache failed memory self-tests.
The cache may contain data which has not been written to disk yet.
When these messages are displayed, Prestoserve allows you to do one of
the following interactively: continue with the boot or halt the
machine.
presto: error n opening dev (n, n), skipping writing of its buffers...
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.
presto: error nopening block device (n, n)
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.
presto: error nioctl'ing dev (n, n)
This message indicates that an ioctl failed for the specified device.
Refer to errno(2) for a complete description of the error.
presto: can NOT do direct I/0 for block device (n, n)
presto: using block device (n, n) in `bounceio' mode
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.
presto: flush dirty buffers
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: Block device n not present or is not
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.
presto: NVRAM probe error
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.
presto: failed to write and read addr from addr
This message indicates that the Prestoserve cache failed the read/write
test at the specified address.
presto: error on dev (n, n)
This message indicates that an I/O error occurred on the specified disk
during a Prestoserve write-back operation.
presto: not enough backup battery power!
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.
presto: disabling...
This message indicates that Prestoserve disabled itself because of
inadequate backup battery power or because a disk error occurred during
a write to disk.
presto: back online!
This message indicates that a disk error or low backup battery power
condition has been corrected and that Prestoserve is enabled again.
ERRORS
The Prestoserve ioctl commands set errno to the specified values for the
following conditions:
[EACCESS]
Prestoserve is not registered for use on this system.
[EPERM]
A caller whose uid is not root tried to use the PRSETSTATE, PRSETMEMSZ,
PRRESET, PRENABLE or PRDISABLE command.
[EBUSY]
An attempt was made to use the PRSETSTATE, PRSETMEMSZ, or PRDISABLE
command, but a fatal disk error or a battery problem exists.
[ENOMEM]
The memory size specified in the PRSETSTATE command exceeds the maximum
size of the cache reported in the presto_status structure.
[ENODEV]
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.
[EINVAL]
An invalid argument was specified with the PRSETMEMSZ or PRSETSTATE
command or that an invalid command was used.
FILES
/dev/pr0
Generic Prestoserve control device
SEE ALSO
errno(2), ioctl(2), presto(8), dxpresto(8X), prestoctl_svc(8),
prestosetup(8), prestotab(4)
Guide to Prestoserve
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