Hi,
On 2024/06/04 12:48, Roger Heflin wrote:
Use the *_bytes values. If they are non-zero then they are used and
that allows setting even below 1% (quite large on anything with a lot
of ram).
I have been using this for quite a while:
vm.dirty_background_bytes = 3000000
vm.dirty_bytes = 5000000
crowsnest [13:32:48] ~ # sysctl vm.dirty_background_bytes=3000000
vm.dirty_background_bytes = 3000000
crowsnest [13:32:59] ~ # sysctl vm.dirty_bytes=500000000
vm.dirty_bytes = 500000000
And persisted via /etc/sysctl.conf
Thank you. Must be noted this host doesn't do much else other than disk
IO, so I'm hoping the 500MB value will be OK, this is just so IO won't
block CPU heavy-at-the-time tasks.
The purpose of 256GB RAM was so that we could have ~250GB worth of disk
cache (obviously we don't want all of that to be dirty, OS and "used"
used to be below 4GB, now generally around 8-12GB, currently it's in
"quiet" time, so a bit lower, just busy running some background
compression). As per iostat:
avg-cpu: %user %nice %system %iowait %steal %idle
7.73 18.43 18.96 37.86 0.00 17.01
Device tps MB_read/s MB_wrtn/s MB_dscd/s MB_read
MB_wrtn MB_dscd
md2 392.13 10.00 5.11 0.00 4244888
2167644 0
md3 2270.12 43.88 56.82 0.00 18626309
24120982 0
md4 1406.06 30.47 16.83 0.00
12934654 7143330 0
That's total 35805851 MB (34.1B) read and 33431956 MB (31.9TB) written
in just under 5 days.
What I am noticing immediately is that the "free" value as per "free -m"
is definitely much higher, which to me is indicative that we're not
caching as aggressively as can be done. Will monitor this for the time
being:
crowsnest [13:50:09] ~ # free -m
total used free shared buff/cache
available
Mem: 257661 6911 105313 7 145436 248246
Swap: 0 0 0
The Total DISK WRITE and Current DISK Write values in in iotop seems to
have a tighter correlation now (no longer seeing constant Total DISK
WRITE with spikes in current, seems to be more even now).
Kind regards,
Jaco
