Disk drive adjusting read-ahead to optimize cache memory allocation
View Patent ↗A disk drive is disclosed which receives a read command from a host computer, the read command comprising a command size representing a number of blocks of read data to read from the disk. A number M of cache segments are allocated from a cache buffer, wherein each cache segment comprises N blocks. The number M of allocated cache segments is computed by summing the command size with a predetermined default number of read-ahead blocks to generate a summation, and integer dividing the summation by N leaving a residue number of default read-ahead blocks. In one embodiment, the residue number of default read-ahead blocks are not read, in another embodiment the residue number of default read-ahead blocks are read if the residue number exceeds a predetermined threshold, and in yet another embodiment the number of read-ahead blocks is extended so that the summation divides evenly by N.
1. A disk drive comprising:
(a) a disk comprising a plurality of tracks, each track comprising a plurality of blocks;
(b) a head actuated radially over the disk;
(c) a semiconductor memory comprising a cache buffer for caching data written to the disk and data read from the disk; and
(d) a disk controller for:
receiving a read command from a host computer, the read command comprising a command size representing a number of blocks of read data to read from the disk;
allocating M cache segments from the cache buffer, wherein:
each of the M cache segment comprises N blocks; and
the number M of allocated cache segments is computed by:
summing the command size with a predetermined default number of read-ahead blocks to generate a summation; and
integer dividing the summation by N which results in a residue number of default read-ahead blocks;
reading the read data from the disk and storing the read data in part of the allocated cache segments; and
adjusting a read-ahead operation in response to the residue number of default read-ahead blocks to read read-ahead data from the disk following the read data and storing the read-ahead data in a remainder of the allocated cache segments.
2. The disk drive as recited in claim 1 , wherein the read-ahead operation is terminated prior to reading the residue number of default read-ahead blocks.
3. The disk drive as recited in claim 1 , wherein if the residue number of default read-ahead blocks exceeds a threshold, the disk controller for:
(a) allocating an additional cache segment;
(b) reading the residue number of default read-ahead blocks from the disk; and
(c) storing the residue number of default read-ahead blocks in the additional cache segment.
4. The disk drive as recited in claim 1 , wherein if the residue number of default read-ahead blocks is non-zero, the disk controller for:
(a) allocating an additional cache segment;
(b) reading the residue number of default read-ahead blocks from the disk;
(c) reading an extended number of read-ahead blocks from the disk; and
(d) storing the residue number of default read-ahead blocks and the extended number of read-ahead blocks in the additional cache segment.
5. The disk drive as recited in claim 1 , wherein the number of allocated cache segments is computed by summing a predetermined number of pre-read blocks with the command size and the predetermined default number of read-ahead blocks to generate the summation.
6. The disk drive as recited in claim 1 , wherein:
(a) the cache buffer comprises a plurality of cache segments each comprising P blocks where P<N; and
(b) the disk controller for allocating the cache segments comprising P blocks for write commands.
7. The disk drive as recited in claim 6 , wherein:
(a) the cache buffer comprises a plurality of segment pools;
(b) each segment pool comprises a plurality of cache segments; and
(c) each cache segment comprises 2 k number of blocks where k is a predetermined integer for each segment pool.
8. A method of reading data through a head actuated radially over a disk in a disk drive, the disk comprising a plurality of tracks, each track comprising a plurality of blocks, the disk drive comprising a cache buffer for caching read data, the method comprising the steps of:
(a) receiving a read command from a host computer, the read command comprising a command size representing a number of blocks of read data to read from the disk;
(b) allocating M cache segments of the cache buffer, wherein:
each of the M cache segments comprises N blocks; and
the number M of allocated cache segments is computed by:
summing the command size with a predetermined default number of read-ahead blocks to generate a summation; and
integer dividing the summation by N which results in a residue number of default read-ahead blocks;
(c) reading the read data from the disk and storing the read data in part of the allocated cache segments; and
(d) adjusting a read-ahead operation in response to the residue number of default read-ahead blocks to read read-ahead data from the disk following the read data and storing the read-ahead data in a remainder of the allocated cache segments.
9. The method of reading data as recited in claim 8 , further comprising the step of terminating the read-ahead operation prior to reading the residue number of default read-ahead blocks.
10. The method of reading data as recited in claim 8 , wherein if the residue number of default read-ahead blocks exceeds a threshold, further comprising the steps of:
(a) allocating an additional cache segment;
(b) reading the residue number of default read-ahead blocks from the disk; and
(c) storing the residue number of default read-ahead blocks in the additional cache segment.
11. The method of reading data as recited in claim 8 , wherein if the residue number of default read-ahead blocks is non-zero, further comprising the steps of:
(a) allocating an additional cache segment;
(b) reading the residue number of default read-ahead blocks from the disk;
(c) reading an extended number of read-ahead blocks from the disk; and
(d) storing the residue number of default read-ahead blocks and the extended number of read-ahead blocks in the additional cache segment.
12. The method of reading data as recited in claim 8 , wherein the number of allocated cache segments is computed by summing a predetermined number of pre-read blocks with the command size and the predetermined default number of read-ahead blocks to generate the summation.
13. The method of reading data as recited in claim 8 , wherein the cache buffer comprises a plurality of cache segments each comprising P blocks where P<N, further comprising the step of allocating the cache segments comprising P blocks for write commands.
14. The method of reading data as recited in claim 13 , wherein:
(a) the cache buffer comprises a plurality of segment pools;
(b) each segment pool comprises a plurality of cache segments; and
(c) each cache segment comprises 2 k number of blocks where k is a predetermined integer for each segment pool.