Dismiss
InnovationQ will be updated on Sunday, Oct. 22, from 10am ET - noon. You may experience brief service interruptions during that time.
Browse Prior Art Database

Preventing Overflow in a Wraparound Buffer

IP.com Disclosure Number: IPCOM000100347D
Original Publication Date: 1990-Apr-01
Included in the Prior Art Database: 2005-Mar-15
Document File: 2 page(s) / 99K

Publishing Venue

IBM

Related People

McCauley, JN: AUTHOR [+2]

Abstract

The IBM 3990 Model 3 uses a battery-powered buffer as temporary storage for write data accepted from a channel program and not yet written to the non-volatile DASD media. This data is used to update DASD if the primary source of the data, the cache storage, is lost. The buffer is a 4-megabyte wraparound buffer. The oldest data must be written to DASD before the space can be reused. During normal operation an area of 'free space' is maintained to accommodate most write operations by destaging the oldest data. When the 'free space' for new data falls below predefined thresholds selected operation write directly to DASD so that the free space in NVS is used in the most efficient manner and operations are not delayed to make NVS space available.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 52% of the total text.

Preventing Overflow in a Wraparound Buffer

       The IBM 3990 Model 3 uses a battery-powered buffer as
temporary storage for write data accepted from a channel program and
not yet written to the non-volatile DASD media. This data is used to
update DASD if the primary source of the data, the cache storage, is
lost.  The buffer is a 4-megabyte wraparound buffer.  The oldest data
must be written to DASD before the space can be reused.  During
normal operation an area of 'free space' is maintained to accommodate
most write operations by destaging the oldest data.  When the 'free
space' for new data falls below predefined thresholds selected
operation write directly to DASD so that the free space in NVS is
used in the most efficient manner and operations are not delayed to
make NVS space available.

      The IBM 3990 Model 3 offers DASD Fast Write to provide caching
performance for write operations.  DASD Fast Write improves
performance by writing data to the electronic buffers, thus saving
the time normally required to position the arm and orient the disk
when accessing DASD.  Fast Writes are only allowed when an image of
the track being written is in cache or when the operation is
formatting the track starting with the first user record.  When a
track is initially accessed it is put in cache and remains in cache
until space is required by more current data.

      DASD Fast Write uses a 4-Megabyte buffer with battery back-up
to store data that has been accepted from the channel and not yet
transferred to DASD.  This buffer, called the Non-Volatile Storage or
NVS, provides back-up for the cache.  The cache is a large (up to 256
megabytes), volatile electronic buffer which contains track images of
the most recently used tracks including the modified data stored in
NVS.  Normal operations are with the cache while the NVS is only used
to synchronize DASD if the data in cache is lost due to a cache
failure or power loss.  DASD fast write operations write the data
from the channel to the cache and concurrently to the NVS.

      The NVS is a wrap-around buffer and only the write data for
write hits, tracks that currently exist in cache and full track
format writes are stored in NVS.  Space is normally made available
for new data by writing the oldest data to DASD so that a block of
free space exists between the most recent write data and the oldest
data.  It is desirable to retain as much data in the buffer as
possible so that writes to the same area may be transferred to a DASD
in a single operation.  Leaving a limi...