I'm not a person necessarily in the "know", but I mess with these in udev for SSDs:
SUBSYSTEM=="block", ACTION=="add|change", KERNEL=="md*", ATTR{md/sync_speed_max}="2000000",ATTR{md/group_thread_cnt}="64", ATTR{md/stripe_cache_size}="8192"
Guidance on why I change the values this way - educated "guess", as I'm an experienced practitioner at best and not someone that messes with the code......I've seen resync rates as high as 900MB/s sustained on my SSD mdraids, even though the SSDs should be able to sustain higher.....
-----Original Message-----
From: Marcin Wanat <marcin.wanat@xxxxxxxxx>
Sent: Wednesday, August 25, 2021 6:06 AM
To: linux-raid@xxxxxxxxxxxxxxx
Subject: [Non-DoD Source] Re: Slow initial resync in RAID6 with 36 SAS drives
On Thu, Aug 19, 2021 at 11:28 AM Marcin Wanat <marcin.wanat@xxxxxxxxx> wrote:
>
> Sorry, this will be a long email with everything I find to be relevant.
> I have a mdraid6 array with 36 hdd SAS drives each able to do
> >200MB/s, but I am unable to get more than 38MB/s resync speed on a
> fast system (48cores/96GB ram) with no other load.
I have done a bit more research on 24 NVMe drives server and found that resync speed bottleneck affect RAID6 with >16 drives:
# mdadm --create --verbose /dev/md0 --level=6 --raid-devices=16
/dev/nvme1n1 /dev/nvme2n1 /dev/nvme3n1 /dev/nvme4n1 /dev/nvme5n1
/dev/nvme6n1 /dev/nvme7n1 /dev/nvme8n1 /dev/nvme9n1 /dev/nvme10n1
/dev/nvme11n1 /dev/nvme12n1 /dev/nvme13n1 /dev/nvme14n1 /dev/nvme15n1
/dev/nvme16n1
# iostat -dx 5
Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s
%rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
nvme0n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme1n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.88 0.00 0.99 470.84 2.25 2.51 86.04
nvme4n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.89 0.00 0.99 470.84 2.25 2.51 86.06
nvme5n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.89 0.00 0.99 470.84 2.25 2.51 86.14
nvme10n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.90 0.00 0.99 470.84 2.25 2.51 86.20
nvme9n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.91 0.00 1.00 470.84 2.25 2.53 86.76
nvme13n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.93 0.00 1.00 470.84 2.25 2.54 87.00
nvme12n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.94 0.00 1.01 470.84 2.25 2.54 87.08
nvme8n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.93 0.00 1.00 470.84 2.25 2.54 87.02
nvme14n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.96 0.00 1.01 470.84 2.25 2.56 87.64
nvme22n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme17n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme16n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 3.05 0.00 1.04 470.84 2.25 2.58 88.56
nvme19n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme2n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.94 0.00 1.01 470.84 2.25 2.54 87.20
nvme6n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.95 0.00 1.01 470.84 2.25 2.55 87.52
nvme7n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.94 0.00 1.01 470.84 2.25 2.54 87.22
nvme21n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme11n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.96 0.00 1.02 470.84 2.25 2.56 87.72
nvme15n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.99 0.00 1.02 470.84 2.25 2.53 86.84
nvme23n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme18n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme3n1 342.60 0.40 161311.20 0.90 39996.60 0.00
99.15 0.00 2.97 0.00 1.02 470.84 2.25 2.53 86.66
nvme20n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
as you can see, there are 342 iops with ~470 rareq-sz, but when i create RAID6 with 17 drives or more:
# mdadm --create --verbose /dev/md0 --level=6 --raid-devices=17
/dev/nvme1n1 /dev/nvme2n1 /dev/nvme3n1 /dev/nvme4n1 /dev/nvme5n1
/dev/nvme6n1 /dev/nvme7n1 /dev/nvme8n1 /dev/nvme9n1 /dev/nvme10n1
/dev/nvme11n1 /dev/nvme12n1 /dev/nvme13n1 /dev/nvme14n1 /dev/nvme15n1
/dev/nvme16n1 /dev/nvme17n1
# iostat -dx 5
Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s
%rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
nvme0n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme1n1 21484.20 0.40 85936.80 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.82 4.00 2.25 0.05 99.16
nvme4n1 21484.00 0.40 85936.00 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.74 4.00 2.25 0.05 99.16
nvme5n1 21484.00 0.40 85936.00 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.84 4.00 2.25 0.05 99.16
nvme10n1 21483.80 0.40 85935.20 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.65 4.00 2.25 0.04 83.64
nvme9n1 21483.80 0.40 85935.20 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.67 4.00 2.25 0.04 85.86
nvme13n1 21483.60 0.40 85934.40 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.63 4.00 2.25 0.04 83.66
nvme12n1 21483.60 0.40 85934.40 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.65 4.00 2.25 0.04 83.66
nvme8n1 21483.60 0.40 85934.40 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.81 4.00 2.25 0.05 99.22
nvme14n1 21481.80 0.40 85927.20 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.67 4.00 2.25 0.04 83.66
nvme22n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme17n1 21482.00 0.40 85928.00 0.90 0.00 0.00
0.00 0.00 0.02 0.00 0.49 4.00 2.25 0.03 67.12
nvme16n1 21481.60 0.40 85926.40 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.75 4.00 2.25 0.04 83.66
nvme19n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme2n1 21481.60 0.40 85926.40 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.95 4.00 2.25 0.05 99.26
nvme6n1 21481.60 0.40 85926.40 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.91 4.00 2.25 0.05 99.26
nvme7n1 21481.60 0.40 85926.40 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.87 4.00 2.25 0.05 99.24
nvme21n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme11n1 21481.20 0.40 85924.80 0.90 0.00 0.00
0.00 0.00 0.03 0.00 0.75 4.00 2.25 0.04 83.66
nvme15n1 21480.20 0.40 85920.80 0.90 0.00 0.00
0.00 0.00 0.04 0.00 0.80 4.00 2.25 0.04 83.66
nvme23n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme18n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
nvme3n1 21480.40 0.40 85921.60 0.90 0.00 0.00
0.00 0.00 0.05 0.00 1.02 4.00 2.25 0.05 99.26
nvme20n1 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
rareq-sz drops to 4, iops increase to 21483 and resync speed drops to 85MB/s.
Why is it like that? Could someone let me know which part of mdraid kernel code is responsible for this limitation ? Is changing this and recompiling the kernel on machine with 512GB+ ram safe ?
Regards,
Marcin Wanat
