Recently I had a case when a customer deleted the InnoDB main table space – ibdata1 – and redo logs – ib_logfile*.
MySQL keeps InnoDB files open all the time. The following recovery technique is based on this fact and it allowed to salvage the database.
Actually, the files were deleted long time ago – 6 months or so. As long as file is open physically it still exits in the file system and reachable to processes which have opened it.
Thus, from user perspective nothing has changed after the deletion. By the way, this is a good reason to monitor existence of these files!
But after the restart InnoDB will detect that there is neither system table space nor log files, so it will create empty ones. The InnoDB dictionary will be empty and InnoDB won’t be able to use a bunch of existing ibd files. This situation will be a job for ibdconnect, but as long as MySQL isn’t restarted it is possible to recover database fast. Let me illustrate how.
Let’s simulate the accident. For that I will delete /var/lib/mysql/ib* files, while sysbench generates read/write activity:
root@localhost:~# sysbench --num-threads=16 --max-requests=0 --test=oltp --oltp-table-size=1000000 --max-time=3600 --mysql-user=root run sysbench 0.4.12: multi-threaded system evaluation benchmark No DB drivers specified, using mysql Running the test with following options: Number of threads: 16 Doing OLTP test. Running mixed OLTP test Using Special distribution (12 iterations, 1 pct of values are returned in 75 pct cases) Using "BEGIN" for starting transactions Using auto_inc on the id column Threads started!
root@localhost:/var/lib/mysql# rm ib* root@localhost:/var/lib/mysql#
Now the files have gone, but MySQL is still running. They don’t exist in /var/lib/mysql, but can be reachable in /proc file system:
root@localhost:/var/lib/mysql# ls -la /proc/14101/fd/ | grep -e ibdata -e ib_ lrwx------ 1 root root 64 Aug 7 23:29 3 -> /var/lib/mysql/ibdata1 (deleted) lrwx------ 1 root root 64 Aug 7 23:29 8 -> /var/lib/mysql/ib_logfile0 (deleted) lrwx------ 1 root root 64 Aug 7 23:29 9 -> /var/lib/mysql/ib_logfile1 (deleted)
where 14101 is the PID of mysqld process.
However, we can’t copy them back because at any given point of time, there are modified pages in the buffer pool. Those pages are not written on disk and will be lost if the changes are not permanently written. This can lead to corruption and data loss.
For the same reason we can’t make MySQL backup by just copying the files.
So, we have to make sure all modifications are written to the disk.
For that we have to stop any further writes and wait until InnoDB flushes all pages.
To stop write activity we can either stop application or lock tables:
mysql> flush tables with read lock; Query OK, 0 rows affected (0.37 sec)
Now let’s wait until all dirty pages are flushed on disk. For that we will monitor checkpoint age. The checkpoint age is a difference between current log sequence number and last checkpoint in “SHOW ENGINE INNODB STATUS” output. If checkpoint age is zero, all pages are flushed:
--- LOG --- Log sequence number 363096003 Log flushed up to 363096003 Last checkpoint at 363096003 Max checkpoint age 7782360
To speed up flushing we can set dirty pages percentage to zero:
mysql> set global innodb_max_dirty_pages_pct=0; Query OK, 0 rows affected (0.01 sec)
It is also important to ensure all other background processes have finished their work.
One of them is the insert buffer thread. Its size should be not more than 1(it’s never less than 1):
------------------------------------- INSERT BUFFER AND ADAPTIVE HASH INDEX ------------------------------------- Ibuf: size 1, free list len 398, seg size 400,
Another thread writing in background is the purge thread.
It should purge all transactions up to very last
------------ TRANSACTIONS ------------ Trx id counter 0 16644 Purge done for trx's n:o < 0 16644 undo n:o < 0 0
But if the last transaction wasn’t something that requires a purge operation (SELECT for example) Trx id counter will be bigger.
In that case at least ensure InnoDB isn’t doing any writes:
FILE I/O -------- I/O thread 0 state: waiting for i/o request (insert buffer thread) I/O thread 1 state: waiting for i/o request (log thread) I/O thread 2 state: waiting for i/o request (read thread) I/O thread 3 state: waiting for i/o request (write thread) Pending normal aio reads: 0, aio writes: 0, ibuf aio reads: 0, log i/o's: 0, sync i/o's: 0 Pending flushes (fsync) log: 0; buffer pool: 0 332 OS file reads, 47 OS file writes, 32 OS fsyncs 0.00 reads/s, 0 avg bytes/read, 0.00 writes/s, 0.00 fsyncs/s
When all modified pages are flushed it is safe now to copy InnoDB files back:
root@localhost:/var/lib/mysql# cp /proc/14101/fd/3 /var/lib/mysql/ibdata1 root@localhost:/var/lib/mysql# cp /proc/14101/fd/8 /var/lib/mysql/ib_logfile0 root@localhost:/var/lib/mysql# cp /proc/14101/fd/9 /var/lib/mysql/ib_logfile1
Let’s fix the owner:
root@localhost:/var/lib/mysql# chown -R mysql ib* root@localhost:/var/lib/mysql#
And restart MySQL:
root@localhost:/var/lib/mysql# /etc/init.d/mysql restart
After the restart all InnoDB tables are reachable:
mysql> select count(*) from sbtest; +----------+ | count(*) | +----------+ | 1000000 | +----------+ 1 row in set (0.19 sec)Conclusions
- Add to your monitoring system checks that InnoDB files ibdata and ib_logfile* do exist
- Do not restart MySQL until the further recovery strategy is clear