Access-based eviction of blocks from solid state drive cache memory

1. A method comprising:

maintaining, in a first memory, a plurality of segments, each segment including a plurality of blocks, and maintaining, in a second memory, a block temperature for each of a plurality of blocks, wherein the first memory is a read-cache memory where one segment is written at a time, wherein each block is readable from the first memory without reading the corresponding complete segment, each block temperature being based on a frequency of access to the respective block, wherein a segment temperature for each segment is based on block temperatures for the corresponding plurality of blocks within the segment;

selecting a segment from a plurality of segments with a lowest segment temperature for eviction based on block temperatures from the second memory;

identifying blocks in the selected segment with a block temperature greater than a threshold temperature;

evicting the selected segment from the first memory; and

writing to the first memory a new segment having the identified blocks.

2. The method as recited in claim 1 , wherein the block temperature increases when the block is accessed, wherein the block temperature decreases periodically.

3. The method as recited in claim 1 , wherein the segment temperature is equal to a total number of bytes in blocks with temperatures greater than or equal to a temperature threshold.

4. The method as recited in claim 1 , wherein the first memory is a solid-state drive (SSD).

5. The method as recited in claim 1 , wherein the second memory is one of a random access memory (RAM) or a dynamic random-access memory (DRAM).

6. The method as recited in claim 1 , wherein each block is associated with a volume, wherein each segment is operable to include blocks from one volume and each segment is operable to include blocks from a plurality of volumes.

7. The method as recited in claim 1 , wherein a read-cache memory is a memory acting as a cache for read requests and not acting as a cache for write requests.

8. The method as recited in claim 1 , wherein the identified blocks are copied forward by being re-cached in the first memory, wherein blocks in the segment that were not identified are not copied forward.

9. The method as recited in claim 1 , wherein selecting a segment further includes:

calculating the segment temperature for segments cached in the first memory; and

selecting the segment with the lowest calculated temperature.

10. The method as recited in claim 1 , wherein a non-volatile random-access memory (NVRAM) memory is used as write cache, wherein the segments are formed in the NVRAM and transferred to hard disk storage.

11. A system comprising:

a first memory for storing a plurality of segments, each segment including a plurality of blocks, wherein the first memory is a read-cache memory where one segment is written at a time, wherein each block is readable from the first memory without reading the corresponding complete segment;

a second memory for storing a block temperature for each of the plurality of blocks, each block temperature being based on a frequency of access to the respective block, wherein a segment temperature for each segment is based on block temperatures for the corresponding plurality of blocks within the segment; and

a processor operable to select a segment from a plurality of segments with a lowest segment temperature for eviction based on block temperatures from the second memory, wherein the processor identifies identifying blocks in the selected segment with a block temperature greater than a threshold temperature, wherein the selected segment is evicted from the first memory and a new segment having the identified blocks is written to the first memory.

12. The system as recited in claim 11 , further including:

hard disk storage for keeping all blocks in non-volatile memory.

13. The system as recited in claim 12 , further including:

a non-volatile random-access memory (NVRAM) memory used as a write-cache memory, wherein the segments are formed in the NVRAM and transferred to the hard disk storage.

14. The system as recited in claim 11 , wherein blocks are of variable length.

15. A non-transitory computer-readable storage medium storing a computer program, the computer-readable storage medium comprising:

program instructions for maintaining, in a first memory, a plurality of segments, each segment including a plurality of blocks, and maintaining, in a second memory, a block temperature for each of a plurality of blocks, wherein the first memory is a read-cache memory where one segment is written at a time, wherein each block is readable from the first memory without reading the corresponding complete segment, each block temperature being based on a frequency of access to the respective block, wherein a segment temperature for each segment is based on block temperatures for the corresponding plurality of blocks within the segment;

program instructions for selecting a segment from a plurality of segments with a lowest segment temperature for eviction based on block temperatures from the second memory;

program instructions for identifying blocks in the selected segment with a block temperature greater than a threshold temperature;

program instructions for evicting the selected segment from the first memory; and

program instructions for writing to the first memory a new segment having the identified blocks.

16. The storage medium as recited in claim 15 , wherein the first memory is a solid-state drive (SSD), wherein the second memory is one of a random access memory (RAM) or a dynamic random-access memory (DRAM).