Как в PostgreSQL устроено взаимодействие с диском, какие проблемы производительности при этом бывают и как их решать выбором подходящего hardware, настройками операционной системы и настройками PostgreSQL

1. Êàê PostgreSQL ðàáîòàåò ñ äèñêîì
Èëüÿ Êîñìîäåìüÿíñêèé
ik@postgresql-consulting.com

2. Ïëàí
Çà÷åì áàçå äèñê?
×òî îñîáåííîãî â ðàáîòå ñ äèñêîì ó PostgreSQL?
Ïîòåíöèàëüíûå óçêèå ìåñòà
Ìîíèòîðèíã äèñêîâîé íàãðóçêè
Âûáîð àïïàðàòíîãî îáåñïå÷åíèÿ äëÿ ñåðâåðà ñ PostgreSQL
Íàñòðîéêà

3. Çà÷åì áàçå äèñê?
×èòàòü ñòðàíèöû ñ äèñêà
Ïèñàòü Write Ahead Log (WAL)
Ñèíõðîíèçèðîâàòü WAL ñ õðàíèëèùåì (CHECKPOINT)

4. Çà÷åì áàçå äèñê?
×èòàòü ñòðàíèöû ñ äèñêà
Ïèñàòü Write Ahead Log (WAL)
Ñèíõðîíèçèðîâàòü WAL ñ õðàíèëèùåì (CHECKPOINT)
Ñïåöèôèêà PostgreSQL
autovacuum
pg_clog
tmp, äèñêîâûå ñîðòèðîâêè, õýøèðîâàíèå

5. Çà÷åì áàçå äèñê?
×èòàòü ñòðàíèöû ñ äèñêà
Ïèñàòü Write Ahead Log (WAL)
Ñèíõðîíèçèðîâàòü WAL ñ õðàíèëèùåì (CHECKPOINT)
Ñïåöèôèêà PostgreSQL
autovacuum
pg_clog
tmp, äèñêîâûå ñîðòèðîâêè, õýøèðîâàíèå

6. Æèçíåííûé öèêë ñòðàíèöû â PostgreSQL
shared_buers
l
êýø ÎÑ
l
äèñêè

7. Checkpoint
Äëÿ ÷åãî íóæåí?
×èñòûåñòðàíèöû ïîäíèìàþòñÿâ shared_buers, åñëè õîòü îäèí tuple
èçìåíèëñÿ, ñòðàíèöà ïîìå÷àåòñÿ êàê ãðÿçíàÿ
COMMIT; íå âîçâðàùàåò óïðàâëåíèÿ, ïîêà ñòðàíèöà íå çàïèñàíà â WAL
Âðåìÿ îò âðåìåíè ïðîèñõîäèò CHECKPOINT: ãðÿçíûå ñòðàíèöû èç
shared_buers ïèøóòñÿ íà äèñê (fsync)
Ïðè áîëüøèõ çíà÷åíèÿõ shared_buers âîçíèêàþò ïðîáëåìû

8. Checkpoint
Äèàãíîñòèêà
Âñïëåñêè íà ìîíèòîðèíãå äèñêîâîé óòèëèçàöèè (iostat -d -x 1, ïîñëåäíÿÿ
êîëîíêà, %util)
pg_stat_bgwriter

9. pg_stat_bgwriter
pgbench=# select * from pg_stat_bgwriter ;
-[ RECORD 1 ]---------+------------------------------
checkpoints_timed | 29
checkpoints_req | 13
checkpoint_write_time | 206345
checkpoint_sync_time | 9989
buffers_checkpoint | 67720
buffers_clean | 1046
maxwritten_clean | 0
buffers_backend | 48142
buffers_backend_fsync | 0
buffers_alloc | 30137
stats_reset | 2014-10-24 17:59:15.812002-04
postgres=# select pg_stat_reset_shared(’bgwriter’);
-[ RECORD 1 ]--------+-
pg_stat_reset_shared |

10. ×òî äåëàòü?
Hardware: RAID
Äåøåâûé RAID êîíòðîëëåð - çëî, óæ ëó÷øå software RAID
RAID êîíòðîëëåð äîëæåí áûòü ñ áàòàðåéêîé
Ïðîèçâîäèòåëè: megaraid èëè perc - OK, à íàïðèìåð HP èëè ARECA - íå
âñåãäà
Áàòàðåéêà - ïðèñóòñòâóåò è èñïðàâíà
cache mode ! write back
io mode ! direct
Disk Write Cache Mode ! disabled

11. ×òî äåëàòü?
Hardware: Äèñêè
2,5SAS (âïîëíå áûâàåò 15K) - seek â 2-3 ðàçà áûñòðåå ÷åì 3,5
Íå âñå SSD îäèíàêîâî ïîëåçíû: enterprise level Intel p3700 vs desktop-level
Samsung
Èñïîëüçîâàòü òîëüêî SSD äëÿ PostgreSQL îçíà÷àåò áûòü ãîòîâûìè ê ðÿäó
ïðîáëåì
RAID 1+0
Eñëè íåò õîðîøèõ äèñêîâ èëè êîíòðîëëåðà - synchronous_commit ! o

12. ×òî äåëàòü?
Ôàéëîâàÿ ñèñòåìà
xfs èëè ext4 - ÎÊ, zfs è ïðî÷èå lvm - íå ïðî ïðîèçâîäèòåëüíîñòü
barrier=0, noatime

13. ×òî äåëàòü?
Îïåðàöèîííàÿ ñèñòåìà
Ïî óìîë÷àíèþ vm:dirty_ratio = 20 vm:dirty_background_ratio = 10 - ïëîõî
Áîëåå ðàçóìíûå çíà÷åíèÿ vm:dirty_background_bytes = 67108864
vm:dirty_bytes = 536870912 (512Mb êýø íà RAID)
Åñëè íåò êýøà íà RAID - ðàçäåëèòü íà 4

14. ×òî äåëàòü?
postgresql.conf
wal_buers (768kB ! 16Mb)
checkpoint_segments (3 - ÷åêïîéíò êàæäûå 48Mb ! 256 - 4Gb)
checkpoint_timeout = 60 (÷òî ïåðâîå íàñòóïèò)
checkpoint_completion_target = 0:9 (äëÿ ðàñïðåäåëåíèÿ ÷åêïîéíòîâ)

15. Êàê ñåáÿ ïðîâåðèòü?
pgdev@pg-dev-deb:~$ tt_pg/bin/pg_test_fsync
5 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 11396.056 ops/sec 88 usecs/op
fdatasync 11054.894 ops/sec 90 usecs/op
fsync 10692.608 ops/sec 94 usecs/op
fsync_writethrough n/a
open_sync 67.045 ops/sec 14915 usecs/op
Compare file sync methods using two 8kB writes:
(in wal_sync_method preference order, except fdatasync

16. Íåáîëüøàÿ õèòðîñòü: ïóñòü bgwriter ðàáîòàåò
postgres=# select name, setting, context, max_val, min_val from pg_settings where name ~ ’bgwr’;
name | setting | context | max_val | min_val
-------------------------+---------+---------+---------+---------
bgwriter_delay | 200 | sighup | 10000 | 10
bgwriter_lru_maxpages | 100 | sighup | 1000 | 0
bgwriter_lru_multiplier | 2 | sighup | 10 | 0
(3 rows)

17. Ïðî ÷òî íå çàáûòü: autovacuum
postgres=# select name, setting, context from pg_settings where category ~ ’Autovacuum’;
name | setting | context
-------------------------------------+-----------+------------
autovacuum | on | sighup
autovacuum_analyze_scale_factor | 0.02 | sighup
autovacuum_analyze_threshold | 50 | sighup
autovacuum_freeze_max_age | 200000000 | postmaster
autovacuum_max_workers | 3 | postmaster
autovacuum_multixact_freeze_max_age | 400000000 | postmaster
autovacuum_naptime | 60 | sighup
autovacuum_vacuum_cost_delay | 20 | sighup
autovacuum_vacuum_cost_limit | -1 | sighup
autovacuum_vacuum_scale_factor | 0.001 | sighup
autovacuum_vacuum_threshold | 50 | sighup
(11 rows)

18. Âîïðîñû?
ik@postgresql-consulting.com