presto(7) Miscellaneous Information Manual 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 ear-
lier 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 syn-
chronously 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; how-
ever they can be opened and used as usual. In this case, the presto driver simply passes all I/O requests directly through to the appro-
priate device.
Operation
When Prestoserve is in the PRUP state, it caches all synchronous write requests for enabled file systems to the presto driver in non-
volatile 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.
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 com-
mands 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
This message is displayed if you attempt to use Prestoserve on a system that has not had its license registered. It is necessary to regis-
ter 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 rec-
ognize 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 buf-
fers 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 dif-
ferent 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 configura-
tion 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.
ERRORS
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 initial-
ized for use with Prestoserve, or an error occurred in trying to open/ioctl the device. An invalid argument was specified with the PRSET-
MEMSZ or PRSETSTATE command or that an invalid command was used.
FILES
Generic Prestoserve control device
SEE ALSO
errno(2), ioctl(2), presto(8), dxpresto(8X), prestoctl_svc(8), prestosetup(8), prestotab(4)
Guide to Prestoserve delim off
presto(7)