For transactional log files, sync happens per commit. And your printed scale doesn't have to match your drawing's scale.Writing small volume of data (Bytes-MBs) with sync (fsync()/fdatasync()/O_SYNC/O_DSYNC) is very common for RDBMS and is needed to guarantee durability. You can print to scale just as easily. Choose among common standard architectural scales, a metric scale, and more. You can easily change the scale at any time. Draw and Print to Scale Easily SmartDraw has the most complete feature set for creating scaled CAD drawings.
This really matters when you writes data with fsync() very frequently. Increasing file size requires a lot of overheads such as allocating space within the filesystem, updating & flushing metadata. Overwriting does not change file size, while appending does. In this case, overwriting is much faster than appending for most filesystems/storages. Typically RDBMS does syncing data very frequently.
Four times difference is amazing but this is real and I got similar results on other filesystems except zfs (I tested xfs, reiserfs and zfs). The difference between no.1 and no.2 is just appending or overwriting. This is actually does the same thing with no.2)Apparently no.2(and no.4) are much faster than no.1 and no.3. Zero-filling 1GB, sleeping 20 seconds, then writing 8KB 128*1024 times with fdatasync()(Zero-filling finished within 20 seconds. Zero-filling 1GB, then writing 8KB 128*1024 times with fdatasync() immediately(Zero-filling causes massive disk writes so killing application performance)4. Creating a 1GB data file, then writing 8KB 128*1024 times with fdatasync()(Emulating current InnoDB log file behavior)3.
If preallocation happens *dynamically during heavy loads* (no.3), application performance is seriously degraded.No.3 is close to current InnoDB system tablespace (ibdata) mechanism. Overwriting requires preallocation : allocating space before writing. I am currently directly working on this and implementing " preallocating binlog" functionality.The difference between no.3 and no.4 is also interesting.
Unfortunately currently most of enterprise Linux distributions/filesystems/glibc don't behave as expected, but internally doing zero-filling instead(including RHEL5.3).Preallocating large enough space before going into production or low-load hours (midnight etc) is a current workaround.Talking about InnoDB deployment, innodb_file_per_table is popular and easy-to-manage, but it's currently not overwriting architecture(Updated in May 28: Preallocating GBs of data beforehand is not possible unless you explicitly load & delete data into/from tables. Posix_fallocate() preallocates space without any overhead. But as you see above, dynamically preallocating with zero is not good for performance.Using posix_fallocate() instead of zero-filling will fix this issue.
But I have frequently seen situations that people do not set write cache properly. Ibdata1,2,3., over 100GB in total) can often get better performance.Using a write cache protected by battery (BBWC) is well known and one of the best practices in RDBMS world. See bug#31592 for detail.) Not using innodb_file_per_table, but preallocating large InnoDB tablespaces before going into production (i.e. Ibd file is 4MB at one time, regardless of innodb_autoextend_increment setting.
In other words:- Write cache on logical device (H/W raid controller) is enabled when protected by battery- Write cache on physical device is disabled- The battery has enough capacity and long enough remaining timeI recommend DBAs to monitor write cache status regularly (adding this to your monitoring scripts), including battery status checking. Here is an example of "arcconf" command result.Time remaining (at current draw): 3 days, 1 hours, 11 minutesWrite-cache setting: Enabled (write-back) whenWrite cache should be enabled only when battery backup is working. I got 85% better result when write cache is enabled.BBWC is mostly equipped with H/W raid controller so operational command depends on products. The following is a DBT-2 example.Avg-cpu: %user %nice %system %iowait %steal %idleWMB/s avgrq-sz avgqu-sz await svctm %utilBoth run same applications(DBT-2), but the server activity was significantly different each other. If not enabled, you will be able to easily get better performance by just enabling it. Sometimes write cache is disabled even though they say they set properly.Make sure that BBWC is enabled.
Now I'm pretty confident that storing tables on SSD, redo/Binlog/SYSTEM-tablespace on HDD will be one of the best practices for the time being.This post is a detailed benchmarking report.(This post is very long and focusing on InnoDB only. If disabled, only hundreds of inserts per second is possible, so you can easily check.I recently did a disk bound DBT-2 benchmarking on SSD/HDD (MySQL 5.4.0, InnoDB). Allocating scheduled down time in order to replace the battery).If you are not familiar with H/W raid controller specific command but want to check write cache status quickly, using mysqlslap or stored procedure is easy.$ mysql -e "set global innodb_flush_log_at_trx_commit=1"$ mysqlslap -concurrency=1 -iterations=1 -engine=innodb \-auto-generate-sql -auto-generate-sql-load-type=write \Set global innodb_flush_log_at_trx_commit=1 You will be able to insert thousands of records per second if write cache is enabled. If you successfully detected that battery capacity was decreased before write cache was disabled, you would be able to take an action before server performance suddenly goes down (i.e. I looked into problems then found that write cache unexpectedly turned off because a battery was expired.
In the near future many people will use SSD instead of HDD.From DBA's standpoint, you have a couple of choices for storage allocation.- Storing all files on SSD, not using HDD at all- Storing all files on HDD, not using SSD at all- Using SSD and HDD altogether (some files on SSD, others on HDD).Which is the best approach? My favorite approach is storing tables on SSD, storing Redo Log files, Binary Log files, and SYSTEM-tablespace(ibdata) on HDD. Currently storage capacity is much smaller and unit price is much higher than HDD, but the situation is very rapidly changing. )SSD is often called as a disruptive storage technology.
Average disk rotation overhead is 1/2 round. Redo logs are sequentially written, so disk seek doesn’t happen when the disk is dedicated for redo log files, but disk rotation still happens. You can change this behavior by changing innodb_flush_log_at_trx_commit parameter to 0 or 2, but default setting (1) is recommended if you need durability.Without write cache, flushing to disks require disk rotation and disk seek. Normally InnoDB flushes data to disks (redo log files) per each commit to guarantee durability. BBWC is normally equipped with hardware raid controller.
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