I have setup ZFS RAID0 for PostgreSQL database. The Storage and Instances are in AWS EC2 and EBS volumes.
NAME SIZE ALLOC FREE EXPANDSZ FRAG CAP DEDUP HEALTH ALTROOT
pgpool 479G 289G 190G - 70% 60% 1.00x ONLINE -
xvdf 59.9G 36.6G 23.3G - 71% 61%
xvdg 59.9G 34.7G 25.2G - 70% 57%
xvdh 59.9G 35.7G 24.2G - 71% 59%
xvdi 59.9G 35.7G 24.2G - 71% 59%
xvdj 59.9G 36.3G 23.6G - 71% 60%
xvdk 59.9G 36.5G 23.4G - 71% 60%
xvdl 59.9G 36.6G 23.3G - 71% 61%
xvdm 59.9G 36.6G 23.2G - 71% 61%
Previously the FRAG is at 80% on most of the devices and we have suffered a heavy write IOPS. As the pool capacity is previously at 75% utilization (400GB), provisioned additional 10GB to each device (400GB + 80GB). Now the FRAG is reduced to 70%. One important metric is that write IOPS is much lesser for the same workload now.
As per the Cloudwatch metrics after increase in EBS size, write IOPS drastically reduced to 1200 – 1400 IOPS from 4000 IOPS for Master PG and reduced to 600 IOPS from 3000 IOPS for Slave PG. I have suspected that this is due to how FRAG amplifies writes as explained in this answer.
We have set recordsize=128K as the compressratio is much better than recordsize=8K. I think due to higher recordsize, FRAG is increased quickly and results in write amplification and heavy write IOPS. Will decreasing the record size would prevent write amplification or any other problem which I am missing?
Database Info
PostgreSQL 9.6
Checkpoint Timeout: 10min
Total Databases: ~1100
Tables / DB: ~1300
Files / DB: ~6000 to 8000 (1300 Tables + Indexes + Toast)
Total Files for All DBs: ~7.7m
In average for every 5 mins, ~60000 to 70000 files (Tables + Indexes + Toast) would be affected (as per postgres checkpoint completion log). So any tuning need to be done regarding the huge number of files? Or atleast what I'm hoping is why the drastic difference in IOPS when additional space is added to the pool?
Also I've seen the same kind of IOPS difference a month back when additional space is added to the pool but getting back to the high IOPS rate after two days. So I guess the same would repeat after two to three days and that's why I'm suspecting Free Space Fragmentation FRAG would be the culprit..!!
ZPOOL Props
ubuntu@ip-10-0-1-59:~$ sudo zpool get all
NAME PROPERTY VALUE SOURCE
pgpool size 479G -
pgpool capacity 60% -
pgpool altroot - default
pgpool health ONLINE -
pgpool guid 1565875598252756833 -
pgpool version - default
pgpool bootfs - default
pgpool delegation on default
pgpool autoreplace off default
pgpool cachefile - default
pgpool failmode wait default
pgpool listsnapshots off default
pgpool autoexpand on local
pgpool dedupditto 0 default
pgpool dedupratio 1.00x -
pgpool free 190G -
pgpool allocated 289G -
pgpool readonly off -
pgpool ashift 0 default
pgpool comment - default
pgpool expandsize - -
pgpool freeing 0 -
pgpool fragmentation 71% -
pgpool leaked 0 -
pgpool multihost off default
pgpool feature@async_destroy enabled local
pgpool feature@empty_bpobj enabled local
pgpool feature@lz4_compress active local
pgpool feature@multi_vdev_crash_dump enabled local
pgpool feature@spacemap_histogram active local
pgpool feature@enabled_txg active local
pgpool feature@hole_birth active local
pgpool feature@extensible_dataset active local
pgpool feature@embedded_data active local
pgpool feature@bookmarks enabled local
pgpool feature@filesystem_limits enabled local
pgpool feature@large_blocks enabled local
pgpool feature@large_dnode enabled local
pgpool feature@sha512 enabled local
pgpool feature@skein enabled local
pgpool feature@edonr enabled local
pgpool feature@userobj_accounting active local
ZFS Props
ubuntu@ip-10-0-1-59:~$ sudo zfs get all
NAME PROPERTY VALUE SOURCE
pgpool type filesystem -
pgpool creation Mon Oct 8 18:45 2018 -
pgpool used 289G -
pgpool available 175G -
pgpool referenced 288G -
pgpool compressratio 5.06x -
pgpool mounted yes -
pgpool quota none default
pgpool reservation none default
pgpool recordsize 128K default
pgpool mountpoint /mnt/PGPOOL local
pgpool sharenfs off default
pgpool checksum on default
pgpool compression lz4 local
pgpool atime off local
pgpool devices on default
pgpool exec on default
pgpool setuid on default
pgpool readonly off default
pgpool zoned off default
pgpool snapdir hidden default
pgpool aclinherit restricted default
pgpool createtxg 1 -
pgpool canmount on default
pgpool xattr sa local
pgpool copies 1 default
pgpool version 5 -
pgpool utf8only off -
pgpool normalization none -
pgpool casesensitivity sensitive -
pgpool vscan off default
pgpool nbmand off default
pgpool sharesmb off default
pgpool refquota none default
pgpool refreservation none default
pgpool guid 571000568545391306 -
pgpool primarycache all default
pgpool secondarycache all default
pgpool usedbysnapshots 0B -
pgpool usedbydataset 288G -
pgpool usedbychildren 364M -
pgpool usedbyrefreservation 0B -
pgpool logbias throughput local
pgpool dedup off default
pgpool mlslabel none default
pgpool sync standard default
pgpool dnodesize legacy default
pgpool refcompressratio 5.07x -
pgpool written 288G -
pgpool logicalused 1.42T -
pgpool logicalreferenced 1.42T -
pgpool volmode default default
pgpool filesystem_limit none default
pgpool snapshot_limit none default
pgpool filesystem_count none default
pgpool snapshot_count none default
pgpool snapdev hidden default
pgpool acltype off default
pgpool context none default
pgpool fscontext none default
pgpool defcontext none default
pgpool rootcontext none default
pgpool relatime off default
pgpool redundant_metadata most local
pgpool overlay off default
ARC
ubuntu@ip-10-0-1-59:~$ cat /proc/spl/kstat/zfs/arcstats
13 1 0x01 96 4608 8003814189 8133508755293587
name type data
hits 4 48883641586
misses 4 1321425301
demand_data_hits 4 15606800719
demand_data_misses 4 786648720
demand_metadata_hits 4 32729793033
demand_metadata_misses 4 278006868
prefetch_data_hits 4 315020107
prefetch_data_misses 4 207366277
prefetch_metadata_hits 4 232027727
prefetch_metadata_misses 4 49403436
mru_hits 4 10685966527
mru_ghost_hits 4 85159850
mfu_hits 4 37807765384
mfu_ghost_hits 4 40046050
deleted 4 960331018
mutex_miss 4 15489677
access_skip 4 17626
evict_skip 4 21812713742
evict_not_enough 4 267500438
evict_l2_cached 4 0
evict_l2_eligible 4 108797848903680
evict_l2_ineligible 4 2000957528064
evict_l2_skip 4 0
hash_elements 4 313401
hash_elements_max 4 769926
hash_collisions 4 587807552
hash_chains 4 11108
hash_chain_max 4 6
p 4 3794526616
c 4 7022898344
c_min 4 1004520576
c_max 4 16072329216
size 4 6944858120
compressed_size 4 3732648960
uncompressed_size 4 20043857408
overhead_size 4 1682603008
hdr_size 4 110128312
data_size 4 3359916544
metadata_size 4 2055335424
dbuf_size 4 246193488
dnode_size 4 943126912
bonus_size 4 230157440
anon_size 4 33360384
anon_evictable_data 4 0
anon_evictable_metadata 4 0
mru_size 4 3718656000
mru_evictable_data 4 1940126208
mru_evictable_metadata 4 224441856
mru_ghost_size 4 1684268032
mru_ghost_evictable_data 4 0
mru_ghost_evictable_metadata 4 1684268032
mfu_size 4 1663235584
mfu_evictable_data 4 1288254464
mfu_evictable_metadata 4 83499008
mfu_ghost_size 4 1506922496
mfu_ghost_evictable_data 4 1391591424
mfu_ghost_evictable_metadata 4 115331072
l2_hits 4 0
l2_misses 4 0
l2_feeds 4 0
l2_rw_clash 4 0
l2_read_bytes 4 0
l2_write_bytes 4 0
l2_writes_sent 4 0
l2_writes_done 4 0
l2_writes_error 4 0
l2_writes_lock_retry 4 0
l2_evict_lock_retry 4 0
l2_evict_reading 4 0
l2_evict_l1cached 4 0
l2_free_on_write 4 0
l2_abort_lowmem 4 0
l2_cksum_bad 4 0
l2_io_error 4 0
l2_size 4 0
l2_asize 4 0
l2_hdr_size 4 0
memory_throttle_count 4 0
memory_direct_count 4 3209752
memory_indirect_count 4 15181019
memory_all_bytes 4 32144658432
memory_free_bytes 4 6916120576
memory_available_bytes 3 6413860864
arc_no_grow 4 0
arc_tempreserve 4 0
arc_loaned_bytes 4 0
arc_prune 4 252427
arc_meta_used 4 3584941576
arc_meta_limit 4 12054246912
arc_dnode_limit 4 1205424691
arc_meta_max 4 12420274128
arc_meta_min 4 16777216
sync_wait_for_async 4 4231322
demand_hit_predictive_prefetch 4 195199007
arc_need_free 4 0
arc_sys_free 4 502260288
zdb -C
ubuntu@ip-10-0-1-59:~$ sudo zdb -C pgpool
MOS Configuration:
version: 5000
name: 'pgpool'
state: 0
txg: 4739409
pool_guid: 1565875598252756833
errata: 0
hostname: 'ip-10-0-1-59'
com.delphix:has_per_vdev_zaps
vdev_children: 8
vdev_tree:
type: 'root'
id: 0
guid: 1565875598252756833
create_txg: 4
children[0]:
type: 'disk'
id: 0
guid: 1507251152420866879
path: '/dev/xvdf1'
whole_disk: 1
metaslab_array: 64
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1425
create_txg: 4
com.delphix:vdev_zap_leaf: 129
com.delphix:vdev_zap_top: 130
children[1]:
type: 'disk'
id: 1
guid: 12461793154748882472
path: '/dev/xvdg1'
whole_disk: 1
metaslab_array: 68
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1424
create_txg: 9
com.delphix:vdev_zap_leaf: 131
com.delphix:vdev_zap_top: 132
children[2]:
type: 'disk'
id: 2
guid: 15169312858460766498
path: '/dev/xvdh1'
whole_disk: 1
metaslab_array: 69
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1427
create_txg: 12
com.delphix:vdev_zap_leaf: 133
com.delphix:vdev_zap_top: 134
children[3]:
type: 'disk'
id: 3
guid: 8533762672700299025
path: '/dev/xvdi1'
whole_disk: 1
metaslab_array: 74
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1422
create_txg: 15
com.delphix:vdev_zap_leaf: 71
com.delphix:vdev_zap_top: 72
children[4]:
type: 'disk'
id: 4
guid: 6612366135198079494
path: '/dev/xvdj1'
whole_disk: 1
metaslab_array: 139
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1428
create_txg: 18
com.delphix:vdev_zap_leaf: 136
com.delphix:vdev_zap_top: 137
children[5]:
type: 'disk'
id: 5
guid: 15947515837120747219
path: '/dev/xvdk1'
whole_disk: 1
metaslab_array: 142
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1421
create_txg: 21
com.delphix:vdev_zap_leaf: 140
com.delphix:vdev_zap_top: 141
children[6]:
type: 'disk'
id: 6
guid: 15626684534009192025
path: '/dev/xvdl1'
whole_disk: 1
metaslab_array: 146
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1426
create_txg: 24
com.delphix:vdev_zap_leaf: 143
com.delphix:vdev_zap_top: 144
children[7]:
type: 'disk'
id: 7
guid: 19815078749163274
path: '/dev/xvdm1'
whole_disk: 1
metaslab_array: 149
metaslab_shift: 27
ashift: 9
asize: 64418480128
is_log: 0
DTL: 1423
create_txg: 27
com.delphix:vdev_zap_leaf: 147
com.delphix:vdev_zap_top: 148
features_for_read:
com.delphix:hole_birth
com.delphix:embedded_data
zfs.conf
ubuntu@ip-10-0-1-59:~$ sudo cat /etc/modprobe.d/zfs.conf
options zfs zfs_arc_max=77147180237
zdb metaslabs & spacemap
From the metaslabs and spacemap info, it look like the metaslabs and spacemaps at end of each vdev have lots of free space (due to latest size expansion) compared with the upper regions. So this correlates with the problem explained here as how the degrade in performance when the metaslabs and spacemap is fragmented.
I think it's better to add mirrors and remove the old vdevs so to promote the new devices as primary stripe. This would help the metaslabs and spacemap as if it's a fresh pool without much fragmentation (??? not sure !!!). Anyway, this would be kept as a last resort, until then I'll look into verifying the problem nature and any tunings needed regarding metaslabs / spacemap from zfs side.
Also to reduce fragmentation, planned to add zfs log device to a seperate device and moving postgres pg_xlog directory to seperate device with ext4 partition.
Best Answer
Without deep debug, it is difficult to give you a definitive answer. Anyway, some things to note are:
ZFS allocate blocks via spacemaps. When a spacemap is >= 96% full (80% for older build), ZFS will switch from first-fit to best-fit allocator. Note that this is a per-spacemap decision: you can have an 80% full pool with some spacemaps well over that value, maybe already at over 96%. When writing to such spacemaps, ZFS will use the slower best-fit allocator
a fragmented spacemap will use much more memory than a non-fragmented one. This added memory pressure can lead to spacemap trashing. You can avoid that by setting
metaslab_debug_load=1; if it does not work, try re-importing your pool and/or settingmetaslab_debug_unload=1. Note that persistently locking all spacemaps in memory will inevitably consume more RAMyou could be burned by gang blocks but, again, it is difficult to tell if it is the case without further debug. Surely a 128K recordsize, with such a good
compressratio, is doing you no favor with regard to fragmentation. You can read some more information here and here.Side note: I see your pool has
ashift=9. I think that pure 512B devices are quite rare nowadays, especially in cloud environment. In a bid to increase performance, you can/would re-create your pool withashift=12.