On 4/11/18 4:19 pm, Eyal Lebedinsky wrote:
While configuring a new raid I ran iostat to see that it is idle. It was, and
there
was no io showing at all.
I then mounted it on a new mount point which I have no process using. I started
hearing
knocks from the PC case, and touching the disks revealed that they all had
activity 1-2
times a second, concurrently, leading to the louder than usual noise.
Here is what "iostat 60" is now showing on a totally idle system (this is a
very typical entry):
avg-cpu: %user %nice %system %iowait %steal %idle
0.21 0.00 0.19 1.36 0.00 98.24
Device tps kB_read/s kB_wrtn/s kB_read kB_wrtn
sda 0.73 0.93 4.07 56 244
sdb 5.87 8.53 20.78 512 1247
sdd 6.67 10.93 23.18 656 1391
sdf 8.50 40.27 52.52 2416 3151
sde 6.20 4.40 16.65 264 999
sdh 5.97 10.13 22.38 608 1343
sdg 7.77 38.00 50.25 2280 3015
sdc 5.87 8.53 20.78 512 1247
md127 1.80 0.00 40.27 0 2416
sda is the root fs (ext4).
md127 (ext4) is a RAID6 of 7 disks sd[b-h]1.
What is this io, and can it be stopped? I want to allow the disks to enter
low power mode (not spin down) when idle.
This is up-to-date recent install of f28 (this is a test system, so not
customized).
TIA
A summary of what I learnt so far:
I was pointed at the lazy init feature of ext4 as the culprit (read the thread).
I was not aware of this feature so there is a silver lining to this cloudy
issue.
After some searching I now see in mkfs.ext4 man page these two options
lazy_itable_init, lazy_journal_init
Furthermore, I read about it here
https://ext4.wiki.kernel.org/index.php/Ext4_Disk_Layout#Lazy_Block_Group_Initialization
One can see the activity using iotop:
$ iotop -oP
Total DISK READ : 0.00 B/s | Total DISK WRITE : 0.00 B/s
Actual DISK READ: 0.00 B/s | Actual DISK WRITE: 103.76 K/s
PID PRIO USER DISK READ DISK WRITE SWAPIN IO> COMMAND
1872 be/3 root 0.00 B/s 0.00 B/s 0.00 % 3.50 % [jbd2/md127-8]
1874 be/4 root 0.00 B/s 0.00 B/s 0.00 % 1.11 % [ext4lazyinit]
However it seems that there is no way to see how far the lazy init progressed
or how much
data needs to be written.
My other observation is that the RAID6 write amplification probably has a large
effect if the
init process is writing non sequential single blocks.
HTH
--
Eyal at Home (fed...@eyal.emu.id.au)
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