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Re: MySQL Cluster NDB Backup performance
Posted by: Mikael Ronström
Date: May 27, 2021 11:38AM

The speed of backups is controlled by MinDiskWriteSpeed and MaxDiskWriteSpeed

The comment on single LDM thread for backups is no longer valid, the backup
is now parallelised.

I don't know why you get bursts every 60 seconds, probably has some relation
to some buffering somewhere in OS or file system.

This is a cut from the manual:

BackupDiskWriteSpeedPct

Version (or later) NDB 8.0.13
Type or units percent
Default 50
Range 0 - 90
Restart Type
N (NDB 8.0.13)

During normal operation, data nodes attempt to maximize the disk write speed used for local checkpoints and backups while remaining within the bounds set by MinDiskWriteSpeed and MaxDiskWriteSpeed. Disk write throttling gives each LDM thread an equal share of the total budget. This allows parallel LCPs to take place without exceeding the disk I/O budget. Because a backup is executed by only one LDM thread, this effectively caused a budget cut, resulting in longer backup completion times, and—if the rate of change is sufficiently high—in failure to complete the backup when the backup log buffer fill rate is higher than the achievable write rate.

This problem can be addressed by using the BackupDiskWriteSpeedPct configuration parameter, which takes a value in the range 0-90 (inclusive) which is interpreted as the percentage of the node's maximum write rate budget that is reserved prior to sharing out the remainder of the budget among LDM threads for LCPs. The LDM thread running the backup receives the whole write rate budget for the backup, plus its (reduced) share of the write rate budget for local checkpoints.

The default value for this parameter is 50 (interpreted as 50%).

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