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Linux内核源代码 为压缩文件 是<<Linux内核>>一书中的源代码

This file contains brief information about the SCSI tape driver.
The driver is currently maintained by Kai M{kisara (email
[e-mail])

Last modified: Sat Aug  5 10:29:07 2000 by [e-mail]


BASICS

The driver is generic, i.e., it does not contain any code tailored
to any specific tape drive. The tape parameters can be specified with
one of the following three methods:

1. Each user can specify the tape parameters he/she wants to use
directly with ioctls. This is administratively a very simple and
flexible method and applicable to single-user workstations. However,
in a multiuser environment the next user finds the tape parameters in
state the previous user left them.

2. The system manager (root) can define default values for some tape
parameters, like block size and density using the MTSETDRVBUFFER ioctl.
These parameters can be programmed to come into effect either when a
new tape is loaded into the drive or if writing begins at the
beginning of the tape. The second method is applicable if the tape
drive performs auto-detection of the tape format well (like some
QIC-drives). The result is that any tape can be read, writing can be
continued using existing format, and the default format is used if
the tape is rewritten from the beginning (or a new tape is written
for the first time). The first method is applicable if the drive
does not perform auto-detection well enough and there is a single
"sensible" mode for the device. An example is a DAT drive that is
used only in variable block mode (I don't know if this is sensible
or not :-).

The user can override the parameters defined by the system
manager. The changes persist until the defaults again come into
effect.

3. Up to four modes can be defined and selected using the minor number
(bits 5 and 6). Mode 0 corresponds to the defaults discussed
above. Additional modes are dormant until they are defined by the
system manager (root). When specification of a new mode is started,
the configuration of mode 0 is used to provide a starting point for
definition of the new mode.

Using the modes allows the system manager to give the users choices
over some of the buffering parameters not directly accessible to the
users (buffered and asynchronous writes). The modes also allow choices
between formats in multi-tape operations (the explicitly overridden
parameters are reset when a new tape is loaded).

If more than one mode is used, all modes should contain definitions
for the same set of parameters.

Many Unices contain internal tables that associate different modes to
supported devices. The Linux SCSI tape driver does not contain such
tables (and will not do that in future). Instead of that, a utility
program can be made that fetches the inquiry data sent by the device,
scans its database, and sets up the modes using the ioctls. Another
alternative is to make a small script that uses mt to set the defaults
tailored to the system.

The driver supports fixed and variable block size (within buffer
limits). Both the auto-rewind (minor equals device number) and
non-rewind devices (minor is 128 + device number) are implemented.

In variable block mode, the byte count in write() determines the size
of the physical block on tape. When reading, the drive reads the next
tape block and returns to the user the data if the read() byte count
is at least the block size. Otherwise, error ENOMEM is returned.

In fixed block mode, the data transfer between the drive and the
driver is in multiples of the block size. The write() byte count must
be a multiple of the block size. This is not required when reading but
may be advisable for portability.

Support is provided for changing the tape partition and partitioning
of the tape with one or two partitions. By default support for
partitioned tape is disabled for each driver and it can be enabled
with the ioctl MTSETDRVBUFFER.

By default the driver writes one filemark when the device is closed after
writing and the last operation has been a write. Two filemarks can be
optionally written. In both cases end of data is signified by
returning zero bytes for two consecutive reads.

The compile options are defined in the file linux/drivers/scsi/st_options.h.


BSD AND SYS V SEMANTICS

The user can choose between these two behaviours of the tape driver by
defining the value of the symbol ST_SYSV. The semantics differ when a
file being read is closed. The BSD semantics leaves the tape where it
currently is whereas the SYS V semantics moves the tape past the next
filemark unless the filemark has just been crossed.

The default is BSD semantics.


BUFFERING

The driver uses tape buffers allocated either at system initialization
or at run-time when needed. One buffer is used for each open tape
device. The size of the buffers is selectable at compile and/or boot
time. The buffers are used to store the data being transferred to/from
the SCSI adapter. The following buffering options are selectable at
compile time and/or at run time (via ioctl):

Buffering of data across write calls in fixed block mode (define
ST_BUFFER_WRITES).

Asynchronous writing. Writing the buffer contents to the tape is
started and the write call returns immediately. The status is checked
at the next tape operation.

Buffered writes and asynchronous writes may in some rare cases cause
problems in multivolume operations if there is not enough space on the
tape after the early-warning mark to flush the driver buffer.

Read ahead for fixed block mode (ST_READ_AHEAD). Filling the buffer is
attempted even if the user does not want to get all of the data at
this read command. Should be disabled for those drives that don't like
a filemark to truncate a read request or that don't like backspacing.

The buffer size is defined (in 1024 byte units) by ST_BUFFER_BLOCKS or
at boot time. If this size is not large enough, the driver tries to
temporarily enlarge the buffer. Buffer allocation uses chunks of
memory having sizes 2^n * (page size). Because of this the actual
buffer size may be larger than the buffer size specified with
ST_BUFFER_BLOCKS.

A small number of buffers are allocated at driver initialisation. The
maximum number of these buffers is defined by ST_MAX_BUFFERS. The
maximum can be changed with kernel or module startup options. One
buffer is allocated for each drive detected when the driver is
initialized up to the maximum.

The driver tries to allocate new buffers at run-time if
necessary. These buffers are freed after use. If the maximum number of
initial buffers is set to zero, all buffer allocation is done at
run-time. The advantage of run-time allocation is that memory is not
wasted for buffers not being used. The disadvantage is that there may
not be memory available at the time when a buffer is needed for the
first time (once a buffer is allocated, it is not released). This risk
should not be big if the tape drive is connected to a PCI adapter that
supports scatter/gather (the allocation is not limited to "DMA memory"
and the buffer can be composed of several fragments).

The threshold for triggering asynchronous write in fixed block mode
is defined by ST_WRITE_THRESHOLD. This may be optimized for each
use pattern. The default triggers asynchronous write after three
default sized writes (10 kB) from tar.

Scatter/gather buffers (buffers that consist of chunks non-contiguous
in the physical memory) are used if contiguous buffers can't be
allocated. To support all SCSI adapters (including those not
supporting scatter/gather), buffer allocation is using the following
three kinds of chunks:
1. The initial segment that is used for all SCSI adapters including
those not supporting scatter/gather. The size of this buffer will be
(PAGE_SIZE << ST_FIRST_ORDER) bytes if the system can give a chunk of
this size (and it is not larger than the buffer size specified by
ST_BUFFER_BLOCKS). If this size is not available, the driver halves
the size and tries again until the size of one page. The default
settings in st_options.h make the driver to try to allocate all of the
buffer as one chunk.
2. The scatter/gather segments to fill the specified buffer size are
allocated so that as many segments as possible are used but the number
of segments does not exceed ST_FIRST_SG.
3. The remaining segments between ST_MAX_SG (or the module parameter
max_sg_segs) and the number of segments used in phases 1 and 2
are used to extend the buffer at run-time if this is necessary. The
number of scatter/gather segments allowed for the SCSI adapter is not
exceeded if it is smaller than the maximum number of scatter/gather
segments specified. If the maximum number allowed for the SCSI adapter
is smaller than the number of segments used in phases 1 and 2,
extending the buffer will always fail.


MODULE PARAMETERS

The buffer size, write threshold, and the maximum number of allocated buffers
are configurable when the driver is loaded as a module. The keywords are:

buffer_kbs=xxx             the buffer size in kilobytes is set to xxx
write_threshold_kbs=xxx    the write threshold in kilobytes set to xxx
max_buffers=xxx            the maximum number of tape buffer set to xxx
max_sg_segs=xxx		   the maximum number of scatter/gather
			   segments

Note that if the buffer size is changed but the write threshold is not
set, the write threshold is set to the new buffer size - 2 kB.


BOOT TIME CONFIGURATION

If the driver is compiled into the kernel, the same parameters can be
also set using, e.g., the LILO command line. The preferred syntax is
to use the same keywords as when loading the driver as module. If
several parameters are set, the keyword-value pairs are separated with
a comma (no spaces allowed). A colon can be used instead of the equal
mark. The definition is prepended by the string st=. Here is an
example:

	st=buffer_kbs:64,max_buffers:2