Disk cache management techniques using non-volatile storage

For use in a computer system having a hard disk, a read cache memory and a non-volatile write cache memory to reduce hard disk access operations, a method for using the non-volatile write cache memory more effectively, comprising the steps of:

• processing each of a plurality of requests to read a block of data from the hard disk by first checking to determine if the requested block is already in non-volatile write cache memory, and performing a read disk access only if the requested block is not available in non-volatile write cache memory;
• processing each request to write a block of data to the hard disk by storing the block in the non-volatile write cache memory;
• repeatedly checking to determine a percentage of blocks in the non-volatile write cache memory that have not been written to the hard disk, and deciding whether to purge data to the hard disk based on the percentage of such blocks as compared with a preselected upper threshold limit and a preselected lower threshold limit;
• setting a purge request flag when the percentage of blocks that have not been written to the hard disk first exceeds the upper threshold limit;
• leaving the purge request flag when the percentage of blocks that have not been written to the hard disk falls below the upper threshold limit;
• clearing the purge request flag when the percentage of blocks that have not been written to the hard disk falls below the lower threshold limit;
• and
• if the step of deciding whether to purge to the hard disk results in a decision to initiate purging, selecting at least one block from the non-volatile write cache memory, waiting until the hard disk is not busy with read operations, and writing the selected one of the blocks to the hard disk;
• whereby the hard disk write accesses needed are reduced in number by the use of multiple decision thresholds and wherein the steps of selecting at least one block, waiting until the hard disk is not busy, and writing to the hard disk are initiated only when the purge request flag is set.

For use in a computer system having a hard disk and a single non-volatile cache memory to reduce disk access operations for both reads and writes, a method for using the non-volatile cache memory more effectively, comprising the steps of:

• processing each one of a plurality of requests to read a block of data from the hard disk by first checking to determine if the requested block is already in the non-volatile cache memory, and performing a read disk access only if the requested block is not available in non-volatile cache memory;
• processing each request to write a block of data to the hard disk by storing the block in the non-volatile cache memory;
• repeatedly checking the non-volatile cache memory to determine a percentage of "dirty" blocks, which have been written to the non-volatile cache memory but have not been written to the hard disk, and deciding whether to purge data to the hard disk based on the percentage of such blocks as compared with a preselected upper threshold limit and a preselected lower threshold limit;
• setting a purge request flag when the percentage of dirty blocks exceeds the upper threshold limit;
• leaving the purge request flag set when the percentage of dirty blocks falls below the upper threshold limit;
• clearing the purge request flag when the percentage of dirty blocks falls below the lower threshold limit;
• and
• if the step of deciding whether to purge to the hard disk results in a decision to initiate purging, selecting at least one dirty block from the non-volatile cache memory, waiting until the hard disk is not busy with read operations, and writing the selected dirty block to the hard disk;
• whereby the number of actual hard disk write accesses needed is reduced by the use of multiple decision thresholds and when the steps of selecting at least one block, waiting until the hard disk is not busy, and writing to the hard disk, are initiated only when the purge request flag is set.

Hard disk controller apparatus for reducing disk accesses needed to satisfy a plurality of read and write requests to a hard disk, the apparatus comprising:

• a read cache memory for storing blocks of data read from the hard disk;
• a non-volatile write cache memory for storing blocks of data to be written to the hard disk;
• means for processing each request to write a block of data to the hard disk by storing the block in the non-volatile write cache memory;
• means for repeatedly checking to determine the a percentage of blocks in the non-volatile write cache memory that have not been written to the hard disk;
• means for deciding whether to purge data to the hard disk based on the percentage of such blocks as compared with a preselected upper threshold limit and a preselected lower threshold limit;
• means for setting a purge request flag when the percentage of blocks that have not been written to the hard disk first exceeds the upper threshold limit, and leaving the purge request flag set when the percentage of blocks that have not been written to the hard disk falls below the upper threshold limit;
• means for clearing the purge request flag when the percentage of blocks that have not been written to the hard disk falls below the lower threshold limit;
• and
• means operative if the step of deciding whether to purge to the hard disk results in a decision to initiate purging, for selecting at least one block from the non-volatile write cache memory, then waiting until the hard disk is not busy with read operations, and writing each of the selected blocks to the hard disk;
• whereby the number of actual hard disk write accesses needed is reduced by the use of multiple decision thresholds and wherein the means for selecting at least one block, waiting until the hard disk is not busy, and writing to the hard disk, are operative only when the purge request flag is set.

Hard disk controller apparatus for reducing disk accesses needed to satisfy a plurality of read and write requests to a hard disk, the apparatus comprising;

• a single non-volatile cache memory for storing blocks of data read from, and to be written to, the hard disk;
• means for processing each request to read a block of data from the hard disk by first checking to determine if the requested block is already in the single non-volatile cache memory, and performing a read disk access only if the requested block is not available in single non-volatile cache memory;
• means for processing each request to write a block of data to the hard disk by storing the block in the single non-volatile cache memory;
• means for repeatedly checking to determine the proportion of blocks in the single non-volatile cache memory that have not been written to the hard disk;
• means for deciding whether to purge data to the hard disk based on the percentage of such blocks as compared with a preselected upper threshold limit and a preselected lower threshold limit;
• means for setting purge request flag when the percentage of blocks that have not been written to the hard disk first exceeds the upper threshold limit, and leaving the purge request flag set when the percentage of blocks that have not been written to the hard disk falls below the upper threshold limit;
• means for clearing the purge request flag when the percentage of blocks that have not been written to the hard disk falls below the lower threshold limit;
• and
• means operative if the step of deciding whether to purge to the hard disk results in a decision to initiate purging, for selecting at least one block from the single non-volatile write cache memory, then waiting until the hard disk is not busy with read operations, and writing the selected blocks to the hard disk;
• whereby the number of actual disk write accesses needed is reduced by the use of multiple decision thresholds and wherein the means for selecting at least one block, waiting until the hard disk is not busy, and writing to the hard disk, are operative only when the purge request flag is set.