Disk drive executing retry operation after adjusting jog value in response to gain control

1. A disk drive comprising:

a disk comprising a plurality of tracks, wherein each track comprises a plurality of data sectors;

a head actuated radially over the disk for generating a read signal; and

control circuitry operable to:

first read a data sector while adjusting an amplitude of the read signal in response to a gain control setting; and

when an error occurs while attempting to first read the data sector:

generate a first jog value in response to the gain control setting relative to a nominal gain control value;

servo the head over the data sector in response to the first jog value;

second read the data sector; and

when the first read is successful, adjust the nominal gain control value.

2. The disk drive as recited in claim 1 , wherein when an error occurs when attempting to second read the data sector, the control circuitry is further operable to:

generate a second jog value having a polarity opposite the first jog value;

servo the head over the data sector in response to the second jog value; and

third read the data sector.

3. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to adjust the first jog value when the first jog value exceeds a threshold.

4. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to generate the jog value by averaging the gain control setting while reading the data sector to generate a gain control average.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to generate the jog value by computing a gain control delta relative to a difference between the gain control average and the nominal gain control value.

6. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to generate the jog value as a function of the gain control delta.

7. The disk drive as recited in claim 5 , wherein the control circuitry is further operable to generate the jog value by indexing a lookup table with the gain control delta.

8. The disk drive as recited in claim 5 , wherein when the first read is successful, the control circuitry is further operable to adjust the nominal gain control value in response to the gain control average.

9. A method of operating a disk drive, the disk drive comprising a disk having a plurality of tracks, wherein each track comprises a plurality of data sectors, a head actuated radially over the disk for generating a read signal, the method comprising:

first reading a data sector while adjusting an amplitude of the read signal in response to a gain control setting; and

when an error occurs while attempting to first read the data sector:

generating a first jog value in response to the gain control setting relative to a nominal gain control value;

serving the head over the data sector in response to the first jog value;

second reading the data sector; and

when the first read is successful, adjusting the nominal gain control value.

10. The method as recited in claim 9 , wherein when an error occurs when attempting to second read the data sector, further comprising:

generating a second jog value having a polarity opposite the first jog value;

serving the head over the data sector in response to the second jog value; and

third reading the data sector.

11. The method as recited in claim 9 , further comprising adjusting the first jog value when the first jog value exceeds a threshold.

12. The method as recited in claim 9 , further comprising generating the jog value by averaging the gain control setting while reading the data sector to generate a gain control average.

13. The method as recited in claim 12 , further comprising generating the jog value by computing a gain control delta relative to a difference between the gain control average and the nominal gain control value.

14. The method as recited in claim 13 , further comprising generating the jog value as a function of the gain control delta.

15. The method as recited in claim 13 , further comprising generating the jog value by indexing a lookup table with the gain control delta.

16. The method as recited in claim 13 , wherein when the first read is successful, further comprising adjusting the nominal gain control value in response to the gain control average.

17. A disk drive comprising:

a disk comprising a plurality of tracks, wherein each track comprises a plurality of data sectors;

a head actuated radially over the disk for generating a read signal; and

control circuitry operable to:

first read a data sector while adjusting an amplitude of the read signal in response to a gain control setting; and

when an error occurs while attempting to first read the data sector:

generate a first jog value by averaging the gain control setting while reading the data sector to generate a gain control average and computing a gain control delta relative to a difference between the gain control average and a nominal gain control value;

servo the head over the data sector in response to the first jog value; and

second read the data sector.

18. A method of operating a disk drive, the disk drive comprising a disk having a plurality of tracks, wherein each track comprises a plurality of data sectors, a head actuated radially over the disk for generating a read signal, the method comprising:

first reading a data sector while adjusting an amplitude of the read signal in response to a gain control setting; and

when an error occurs while attempting to first read the data sector:

generating a first jog value by averaging the gain control setting while reading the data sector to generate a gain control average and computing a gain control delta relative to a difference between the gain control average and a nominal gain control value;

serving the head over the data sector in response to the first jog value; and

second reading the data sector.