| Bug #94493 | Question about the MLOG_REC_INSERT redo log preallocated | ||
|---|---|---|---|
| Submitted: | 27 Feb 2019 12:21 | Modified: | 28 Feb 2019 2:15 |
| Reporter: | haiqing sun | Email Updates: | |
| Status: | Not a Bug | Impact on me: | |
| Category: | MySQL Server: InnoDB storage engine | Severity: | S3 (Non-critical) |
| Version: | 8.0.15 | OS: | CentOS |
| Assigned to: | CPU Architecture: | Any | |
[27 Feb 2019 15:41]
MySQL Verification Team
Hi, This is actually not a bug. You have missed the following two statements: mach_write_to_2(log_ptr, page_offset(cursor_rec)); log_ptr += 2;
[27 Feb 2019 16:05]
haiqing sun
Yes, but if I am not mistaken that the following two statements are after the mtr initialization finished, and It does not include in 11 bytes.
[27 Feb 2019 16:09]
haiqing sun
mach_write_to_2(log_ptr, page_offset(cursor_rec)); log_ptr += 2; the cursor record offset costs 2 bytes in the record, and the two bytes is after 11 bytes.
[27 Feb 2019 17:41]
MySQL Verification Team
Hi, Took me some time, but here it is ........ This is a code that requires 11 bytes ..... mach_write_to_1(log_ptr, type); log_ptr++; log_ptr += mach_write_compressed(log_ptr, space_id); log_ptr += mach_write_compressed(log_ptr, page_no); This is the initial records. 1 + 5 + 5 bytes = 11 bytes mach_write_compressed() can write up to 5 bytes. Over and done. Not a bug !!!!!!!
[28 Feb 2019 2:15]
haiqing sun
Yes, I see, thank you very much!
[28 Feb 2019 13:54]
MySQL Verification Team
You are welcome.
Description: Hi: I read the code about the MLOG_REC_INSERT record format, and I am confused about pre-allocated memory for the record: { log_ptr = mlog_open(mtr, 11 + 2 + 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN); .... log_ptr = mlog_write_initial_log_record_fast(insert_rec, MLOG_REC_INSERT, log_ptr, mtr); } log_end = &log_ptr[2 + 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN]; /* Write the cursor rec offset as a 2-byte ulint */ mach_write_to_2(log_ptr, page_offset(cursor_rec)); log_ptr += 2; In the code, we allocate 11 bytes for the for the mtr during initialization, but in fact, the total use of bytes is 1(type) + 4(space_id) + 4(page_offset). How to repeat: version: 8.0.15 line: storage/innobase/page/page0cur.cc:953 Suggested fix: I think maybe we only pre-allocate 9 bytes for the mtr during initialization. log_ptr = mlog_open(mtr, 9 + 2 + 5 + 1 + 5 + 5 + MLOG_BUF_MARGIN);