Browse Prior Art Database

Optical Library Cache Reclaim Strategies

IP.com Disclosure Number: IPCOM000112955D
Original Publication Date: 1994-Jun-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 4 page(s) / 108K

Publishing Venue

IBM

Related People

Day III, K: AUTHOR [+2]

Abstract

A unique characteristic of an optical library configured to appear as a single storage device or single file system is that to reclaim certain dirty cache pages within the library controller may require magnitudes more time than other cache pages. This results from the fact that the page being flushed may map to a cartridge that is not currently in a device and hence has to be swapped in. Judicious choosing of page flushing can reduce the number of disk swaps neeed to accomplish the same amount of work under the stress condition of the cache being filled with dirty buffers. A further consideration is that the flushing should avoid serializing on one drive when multiple drives are available. Standard page replacement algorithms need to be altered to account for these phenomena.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 50% of the total text.

Optical Library Cache Reclaim Strategies

      A unique characteristic of an optical library configured to
appear as a single storage device or single file system is that to
reclaim certain dirty cache pages within the library controller may
require magnitudes more time than other cache pages.  This results
from the fact that the page being flushed may map to a cartridge that
is not currently in a device and hence has to be swapped in.
Judicious choosing of page flushing can reduce the number of disk
swaps neeed to accomplish the same amount of work under the stress
condition of the cache being filled with dirty buffers.  A further
consideration is that the flushing should avoid serializing on one
drive when multiple drives are available.  Standard page replacement
algorithms need to be altered to account for these phenomena.  Such a
modified algorithm and ideas for creating variants of it are
presented in this disclosure.

      In an optical library appearing as a single file system, the
user(s) is allowed to concurrently access more cartridges than
physical drives exist.  In these scenarios the library controller
will swap cartridges in and out of the drives to service requests.
Cache space will still contain data that maps to the swapped out
cartridges.  If cached space needs to be reclaimed, a standard LRU
read preference strategy will cause these pages to be flushed,
forcing the cartridge to be swapped back into a drive if the page is
dirty.

      That an advantage can be gained by reordering the flushes can
be seen from the following simplified example.  Assume a system with
only one drive and two users writing large files to separate disks:
disk A and disk B.   At the point where all the cache pages have been
used and are now dirty, another 5 page faults will be required before
a commit is issued.  If the 5 oldest pages on the Least Recently Used
list belong to the volumes B,A,B,B,A in that order and the drive
currently contains disk A then using a pure LRU reclaim, 5 disk
accesses and 4 disk swaps will occur before reaching the commit.  If
instead the page replacement algorithm scans into the LRU list and
reclaims pages from disk A which is already in the drive, only 5 disk
accesses with no disk swaps occur before the commit is received.
Depending on how the commit takes place, an extra disk swap may be
incurred at the start of the commit, but the same number of disk
accesses take place.

      Another phenomenon that must be taken into account when
tailoring a replacement algorithm is to avoid serializing on one
drive.  This can happen if the oldest cache pages are dirty and
belong to one volume.  Then even if the incoming accesses are
scattered across multiple volumes, each user flushes to the same
drive causing a sudden drop in throughput.

      Based on the above observations, two additional principles
needed for an optic library cache replacement algorithm are:

o   Minimize picker movements

...