我们在PostgreSQL配置方面遇到了一些问题.在一些基准测试之后,我发现非常简单的查询需要相对较长的时间,在进一步检查后,看起来实际的COMMIT命令确实很慢.
我使用下表进行了一个非常简单的测试：
CREATE TABLE test (
id serial primary key,foo varchar(16),);
打开所有语句后,我运行了以下查询10000次：
BEGIN;
INSERT INTO test (a) VALUES ('bar');
COMMIT;
BEGIN和INSERT正在采用< 1ms完成,但COMMIT平均需要22ms才能完成. 在我自己的PC上运行相同的基准测试,速度要慢得多,产生的BEGIN和INSERT语句的平均值相同,但平均COMMIT约为0.4ms(快20倍). 经过一些阅读后,我尝试使用pg_test_fsync工具来解决问题.在服务器上我得到这些结果：
$./pg_test_fsync -o 1024
1024 operations per test
O_DIRECT supported on this platform for open_datasync and open_sync.
Compare file sync methods using one 8kB write:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 14.875 ops/sec
fdatasync 11.920 ops/sec
fsync 30.524 ops/sec
fsync_writethrough n/a
open_sync 30.425 ops/sec
Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 19.956 ops/sec
fdatasync 23.299 ops/sec
fsync 21.955 ops/sec
fsync_writethrough n/a
open_sync 3.619 ops/sec
Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB
in different write open_sync sizes.)
16kB open_sync write 5.923 ops/sec
8kB open_sync writes 3.120 ops/sec
4kB open_sync writes 10.246 ops/sec
2kB open_sync writes 1.787 ops/sec
1kB open_sync writes 0.830 ops/sec
Test if fsync on non-write file descriptor is honored:
(If the times are similar,fsync() can sync data written
on a different descriptor.)
write,fsync,close 34.371 ops/sec
write,close,fsync 36.527 ops/sec
Non-Sync'ed 8kB writes:
write 248302.619 ops/sec
在我自己的电脑上,我得到：
$./pg_test_fsync -o 1024
1024 operations per test
O_DIRECT supported on this platform for open_datasync and open_sync.
Compare file sync methods using one 8kB write:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 69.862 ops/sec
fdatasync 68.871 ops/sec
fsync 34.593 ops/sec
fsync_writethrough n/a
open_sync 26.595 ops/sec
Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 26.872 ops/sec
fdatasync 59.056 ops/sec
fsync 34.031 ops/sec
fsync_writethrough n/a
open_sync 17.284 ops/sec
Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB
in different write open_sync sizes.)
16kB open_sync write 7.412 ops/sec
8kB open_sync writes 3.942 ops/sec
4kB open_sync writes 8.700 ops/sec
2kB open_sync writes 4.161 ops/sec
1kB open_sync writes 1.492 ops/sec
Test if fsync on non-write file descriptor is honored:
(If the times are similar,close 35.086 ops/sec
write,fsync 34.043 ops/sec
Non-Sync'ed 8kB writes:
write 240544.985 ops/sec
服务器的配置：
CPU: Intel(R) Core(TM) i7-3770 CPU @ 3.40GHz RAM: 32GB Disk: 2x 2TB SATA disk in Software RAID 1
用于比较的机器是带有16GB RAM和普通SATA磁盘的i5(没有raid).
更多信息：
>操作系统：Ubuntu服务器12.10
>内核：Linux … 3.5.0-22-generic#34-Ubuntu SMP Tue Jan 8 21:47:00 UTC 2013 x86_64 x86_64 x86_64 GNU / Linux
>软件RAID 1
>文件系统是ext4
>未指定其他安装选项.
> Postgres版本9.1
> Linux mdadm raid
dump2efs的输出：
dumpe2fs 1.42.5 (29-Jul-2012) Filesystem volume name: <none> Last mounted on: / Filesystem UUID: 16e30b20-0531-4bcc-877a-818e1f5d5fb2 Filesystem magic number: 0xEF53 Filesystem revision #: 1 (dynamic) Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize Filesystem flags: signed_directory_hash Default mount options: (none) Filesystem state: clean Errors behavior: Continue Filesystem OS type: Linux Inode count: 182329344 Block count: 729289039 Reserved block count: 36464451 Free blocks: 609235080 Free inodes: 182228152 First block: 0 Block size: 4096 Fragment size: 4096 Reserved GDT blocks: 850 Blocks per group: 32768 Fragments per group: 32768 Inodes per group: 8192 Inode blocks per group: 256 RAID stride: 1 Flex block group size: 16 Filesystem created: Sat Jan 19 12:42:19 2013 Last mount time: Wed Jan 23 16:23:11 2013 Last write time: Sat Jan 19 12:46:13 2013 Mount count: 8 Maximum mount count: 30 Last checked: Sat Jan 19 12:42:19 2013 Check interval: 15552000 (6 months) Next check after: Thu Jul 18 13:42:19 2013 Lifetime writes: 257 GB Reserved blocks uid: 0 (user root) Reserved blocks gid: 0 (group root) First inode: 11 Inode size: 128 Journal inode: 8 First orphan inode: 17304375 Default directory hash: half_md4 Directory Hash Seed: a71fa518-7696-4a28-bd89-b21c10d4265b Journal backup: inode blocks Journal features: journal_incompat_revoke Journal size: 128M Journal length: 32768 Journal sequence: 0x000df5a4 Journal start: 31733
Mdadm – 详细输出：
/dev/md2:
Version : 1.2
Creation Time : Sat Jan 19 12:42:05 2013
Raid Level : raid1
Array Size : 2917156159 (2782.02 GiB 2987.17 GB)
Used Dev Size : 2917156159 (2782.02 GiB 2987.17 GB)
Raid Devices : 2
Total Devices : 2
Persistence : Superblock is persistent
Update Time : Fri Mar 22 11:16:45 2013
State : clean
Active Devices : 2
Working Devices : 2
Failed Devices : 0
Spare Devices : 0
Name : rescue:2
UUID : d87b98e7:d584a4ed:5dac7907:ae5639b0
Events : 38
Number Major Minor RaidDevice State
0 8 3 0 active sync /dev/sda3
1 8 19 1 active sync /dev/sdb3
更新2013-03-25：
我对两个磁盘都进行了长时间的智能测试,结果没有问题.两个磁盘均来自Seagate,型号：ST3000DM001-9YN166.
更新2013-03-27：
我在完全空闲的机器上运行了最新版本(9.2.3)的pg_test_fsync：
$./pg_test_fsync -s 3
3 seconds per test
O_DIRECT supported on this platform for open_datasync and open_sync.
Compare file sync methods using one 8kB write:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 39.650 ops/sec
fdatasync 34.283 ops/sec
fsync 19.309 ops/sec
fsync_writethrough n/a
open_sync 55.271 ops/sec
它比之前略好,但仍然令人遗憾.两个磁盘上的分区对齐：
$sudo parted /dev/sdb unit s print Model: ATA ST3000DM001-9YN1 (scsi) Disk /dev/sdb: 5860533168s Sector size (logical/physical): 512B/4096B Partition Table: gpt Number Start End Size File system Name Flags 4 2048s 4095s 2048s bios_grub 1 4096s 25169919s 25165824s raid 2 25169920s 26218495s 1048576s raid 3 26218496s 5860533134s 5834314639s raid
挂载-v输出：
$mount -v | grep ^/dev/ /dev/md2 on / type ext4 (rw,noatime) /dev/md1 on /boot type ext3 (rw)
md2设备正用于测试.要破坏交换分区并在各个磁盘上运行pg_test_fsync.
如果我在两个磁盘上单独运行pg_test_fsync,我得到大致相同的性能,分区是用noatime挂载的：
$pg_test_fsync/pg_test_fsync -s 3
3 seconds per test
O_DIRECT supported on this platform for open_datasync and open_sync.
Compare file sync methods using one 8kB write:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 75.111 ops/sec
fdatasync 71.925 ops/sec
fsync 37.352 ops/sec
fsync_writethrough n/a
open_sync 33.746 ops/sec
Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order,except fdatasync
is Linux's default)
open_datasync 38.204 ops/sec
fdatasync 49.907 ops/sec
fsync 32.126 ops/sec
fsync_writethrough n/a
open_sync 13.642 ops/sec
Compare open_sync with different write sizes:
(This is designed to compare the cost of writing 16kB
in different write open_sync sizes.)
1 * 16kB open_sync write 25.325 ops/sec
2 * 8kB open_sync writes 12.539 ops/sec
4 * 4kB open_sync writes 6.207 ops/sec
8 * 2kB open_sync writes 3.098 ops/sec
16 * 1kB open_sync writes 1.208 ops/sec
Test if fsync on non-write file descriptor is honored:
(If the times are similar,close 27.275 ops/sec
write,fsync 20.561 ops/sec
Non-Sync'ed 8kB writes:
write 562902.020 ops/sec
在阵列和单个磁盘上运行测试几次后,数字似乎变化很大.最坏的情况是性能大约是我在这里发布的50％(第一次测试大约30次操作/秒).这是正常的吗？机器一直处于空闲状态.
此外,根据dmesg输出,控制器处于AHCI模式.
解决方法
服务器具有令人难以置信的,令人难以置信的,非常慢的fsync性能.您的软件RAID 1设置存在严重问题.可怕的fsync性能几乎肯定是导致性能问题的原因.
桌面只有非常慢的fsync.
通过设置synchronous_commit = off并设置commit_delay,您可以以崩溃后丢失一些数据为代价来解决性能问题.但是,你确实需要在服务器上整理磁盘性能,这是令人不快的.
为了比较,这是我在笔记本电脑上得到的东西(i7,8GB RAM,中档128G SSD,9.2中的pg_test_fsync)：
Compare file sync methods using one 8kB write:
open_datasync 4445.744 ops/sec
fdatasync 4225.793 ops/sec
fsync 2742.679 ops/sec
fsync_writethrough n/a
open_sync 2907.265 ops/sec
不可否认,这款SSD可能不是硬电源丢失安全的,但是当我们谈论服务器成本时,它并不像一个体面的电源故障安全SSD成本很高.
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