Monday, May 18, 2009

Make sure write cache is enabled on your RAID controller

Using a write cache protected by battery (BBWC) is well known and one of the best practices in RDBMS world. But I have frequently seen situations that people do not set write cache properly. Sometimes they just forget to enable write cache. Sometimes write cache is disabled even though they say they set properly.

Make sure that BBWC is enabled. If not enabled, you will be able to easily get better performance by just enabling it. The following is a DBT-2 example.

Write cache is disabled:

# iostat -xm 10
avg-cpu: %user %nice %system %iowait %steal %idle
21.16 0.00 6.14 29.77 0.00 42.93

Device: rrqm/s wrqm/s r/s w/s rMB/s
sdb 2.60 389.01 283.12 47.35 4.86
wMB/s avgrq-sz avgqu-sz await svctm %util
2.19 43.67 4.89 14.76 3.02 99.83

Write cache is enabled:

# iostat -xm 10
avg-cpu: %user %nice %system %iowait %steal %idle
40.03 0.00 16.51 16.52 0.00 26.94

Device: rrqm/s wrqm/s r/s w/s rMB/s
sdb 6.39 368.53 543.06 490.41 6.71
wMB/s avgrq-sz avgqu-sz await svctm %util
3.90 21.02 3.29 3.20 0.90 92.66

Both run same applications(DBT-2), but the server activity was significantly different each other. I got 85% better result when write cache is enabled.

Checking write cache and battery status

BBWC is mostly equipped with H/W raid controller so operational command depends on products. Here is an example of "arcconf" command result.

# /usr/StorMan/arcconf GETCONFIG 1 AL
Controller Battery Information
Status : Optimal
Over temperature : No
Capacity remaining : 99 percent
Time remaining (at current draw): 3 days, 1 hours, 11 minutes

Logical device information
Logical device number 0
Read-cache mode : Disabled
Write-cache mode : Enabled (write-back)
Write-cache setting: Enabled (write-back) when
protected by battery

Physical Device information
Device #0
Device is a Hard drive
Size : 140009 MB
Write Cache : Disabled (write-through)

Write cache should be enabled only when battery backup is working. 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 time

I recommend DBAs to monitor write cache status regularly (adding this to your monitoring scripts), including battery status checking. Long time ago I was asked for urgent help to fix a problem that application performance suddenly went down. I looked into problems then found that write cache unexpectedly turned off because a battery was expired. 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. allocating scheduled down time in order to replace the battery).

Quick health check with mysql commands

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 \

stored procedure:
create table t (c1 int) engine=innodb;
delimiter //
create procedure sp1(IN i INTEGER)
WHILE i > 0 DO
insert into t values (1);
SET i = i - 1;
delimiter ;

set global innodb_flush_log_at_trx_commit=1;
call sp1(100000);

You will be able to insert thousands of records per second if write cache is enabled. If disabled, only hundreds of inserts per second is possible, so you can easily check.


Sildenafil Citrate said...

My best friend told this that the best practices in RDBMS world was to use a write cache protected by battery. I tried it myself and I was impressed with the results, I recommend it to everybody now!

philip said...

hi yoshinori,

when linux server has hardward issue, for example:harddisk is broken, how to dispose the data which save in the BBWC?

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