Technical data

st(7D) Devices SunOS 5.5

Write Data Buffering Tape drives with data compression require a much higher data rate in order to stream

the tape. Write data buffering in the driver improves streaming to the drive without

changing the application and augments the buffering in the tape drive itself. If write data

buffering is enabled, data is buffered in the driver and the request is immediately ack-

nowledged by the driver before it has been written to the tape drive. This enables the

driver to submit the next request as soon as the previous request completes and the appli-

cation to prepare the next request while the current request is in progress. A SCSI tape

drive that allows buffering requiresORing the ﬂag ST_BUFFERED_WRITES with other

ﬂags in the ’options’ ﬁeld in st.conf. Refer to Tape Conﬁguration. By default, this option

is set for the built-in conﬁg entries of the Archive DAT and Exabyte drives.

In order for write buffering to work properly, sufﬁcient space after LEOT must be avail-

able to empty the write buffers. Older tape devices usually do not have sufﬁcient space

after LEOT.

To turn on tape buffering, a property in st.conf called "tape-driver-buffering" should be

added. The value assigned to this property is the maximum number of buffered write

requests allowed. For example, 0 indicates no write request buffering allowed, while 2

indicates buffer up to 2 write requests. If this property is not speciﬁed in st.conf, the

driver defaults to a value of 0. The maximum size of write request that can be buffered is

speciﬁed through a property in st.conf called "tape-driver-buf-max-size". If this property

is not speciﬁed in st.conf, the driver defaults the buffer size to a value of 1 Mbyte.

An example of st.conf, where the maximum number of write requests buffered is 4 and

maximum size of write request buffered is 2 Mbyte, is given below. This applies to all

nodes in this conf ﬁle.

SMI"

tape-driver-buffering = 4; tape-driver-buf-max-size = 0x200000;

name="st" class="scsi"

target=0 lun=0;

name="st" class="scsi"

target=1 lun=0;

name="st" class="scsi"

target=2 lun=0;

...

In the case of a SCSI bus reset, a medium error, or any other fatal transport error on a buf-

fered request, the driver returns an error on subsequent write requests and allows no

more writes. If no further write requests occur, an error is returned on close.

7D-310 modiﬁed 30 Aug 1995