Re: NetBSD Raid5, slow write speeds, using big disks?!

On Sat, 20 Jun 2026, J. Hannken-Illjes wrote:
> Others will know better, but I always thought a Raid5 with four disks spends most time with read-modify-write cycles as usually no requests will fit the stripe size.

Yes, with RAIDframe RAID-5 you want (2^n)+1 disks because of parity being 
separate, so your MAXPHYS data needs to split neatly across the remaining 
data disks. You are trying to split your data across 3 disks...

Also make sure you align everything for 4K sectors.

But, yes, I vote for using ZFS too.

>> On 20. Jun 2026, at 18:55, smurfd <[email protected]> wrote:
>>
>> Hey,
>>
>> This will be a long one, sorry for this.
>> I have created a RAID5 NAS using NetBSD 10.1, and im experiencing slow writing speeds. Or is my expectations all wrong? :)
>> I earlier had FreeBSD and ZFS zraid on the same Ugreen DXP4800 plus (https://nas-eu.ugreen.com/products/ugreen-nasync-dxp4800-plus-nas-storage)
>> The NAS drives are 4x7TB, less than a year old Segate Ironwolf NAS 3.5".
>> NetBSD is installed first on the SSD, using a base install. Then the Raid is created manually...
>>
>> Have asked here:
>> https://www.unitedbsd.com/d/1646-raid5-slow-write-on-big-disks/8
>> Just now redid the raid, for the Xth time, to compare the speeds i got using FreeBSD (because i didnt remember/had that written down)
>> I know, comparing zraid vs raid5 is like comparing apples to oranges, right?...
>>
>> What write speeds should i expect?! is probably the most important question...
>> Or where did i go wrong, because user error is VERY likely.
>>
>> First, i tried to follow the Summary section in the https://man.netbsd.org/raidctl.8, realizing that Disklabel supports no larger than 2TB disks.
>> So found this: https://wiki.netbsd.org/users/mlelstv/using-large-disks/ and tried to follow the Raidframe section.
>> Then i got the correct size i expected...
>>
>> The speed though, is slow! (again compared to zfs zraid)
>> When i had FreeBSD installed, i did the following. Installed the system to the SSD.
>> Then, did this, where **** is repeaded 4 times for the disks...
>> # gpart destroy -F ada0 ****
>> # gpart create -s gpt ada0 ****
>> # gpart add -t freebsd-zfs ada0 ****
>> # zpool create -f island raidz /dev/ada0p1 /dev/ada1p1 /dev/ada2p1 /dev/ada3p1 (this was instant)
>> island/scumm           21T    140K     21T     0% /zstorage/scumm
>>
>> This is the speed...
>> # dd if=/dev/zero bs=1024k count=1000 of=/zstorage/scumm/test.txt
>> 1048576000 bytes transferred in 0.259733 secs (4037123661 bytes/sec)
>> # dd if=/dev/zero bs=4096k count=1000 of=/zstorage/scumm/test.txt
>> 4194304000 bytes transferred in 1.534966 secs (2732505282 bytes/sec)
>> # dd if=/dev/zero bs=1024 count=1000 of=/zstorage/scumm/test.txt
>> 1024000 bytes transferred in 0.004667 secs (219403356 bytes/sec)
>>
>> Then on NetBSD, installed on the SSD i did this:
>>
>> # gpt destroy wd0 ****
>> # gpt create -Af wd0 ****
>> # gpt add -t raid -l raid5@wd0 -b $(( 2048 )) -s 15628051053 wd0 ****
>>
>> # raidctl -C /etc/raid5.conf raid5
>> # raidctl -I 13371337 raid5
>> # raidctl -iv raid5 (This took around 72+ hours)
>> # gpt create -Af raid5
>> # gpt add -a 1024 -t ffs -l island raid5
>> /dev/rraid5: Partition 1 added: 49f48d5a-b10e-11dc-b99b-0019d1879648 34 46884152860
>> # dkctl raid5 addwedge island 34 46884152860 ffs
>> # newfs -O2 -b 65536 -s -1g /dev/dk7 (trying to enable https://man.netbsd.org/wapbl.4)
>> # tunefs -l 1g /dev/dk7
>> # mount -o log /dev/dk7 /mnt
>> # umount /mnt
>> # mount /dev/dk7 /mnt/island/
>> /dev/dk7        22T   8.0K    21T   0% /mnt/island
>>
>> from /etc/fstab
>> /dev/dk7        /mnt/island     ffs     rw,noatime,log   1 1
>>
>> This one, was done before initializing the raid, on one disk
>> nas1# dd if=/dev/zero bs=1024k count=1000 of=/mnt/test.txt
>> 1048576000 bytes transferred in 2.201 secs (476408905 bytes/sec)
>>
>> Writing to the raid:
>> nas1# dd if=/dev/zero bs=1024k count=1000 of=/mnt/island/test.txt
>> 1048576000 bytes transferred in 68.984 secs (15200278 bytes/sec)
>> nas1# dd if=/dev/zero bs=4096k count=1000 of=/mnt/island/test.txt
>> 4194304000 bytes transferred in 478.951 secs (8757271 bytes/sec)
>> nas1# dd if=/dev/zero bs=1024 count=1000 of=/mnt/island/test.txt
>> 1024000 bytes transferred in 0.054 secs (18962962 bytes/sec)
>>
>> If i tried bs=512k and count=1000, i get around 1.6Billion bytes per sek write, woohoo...
>> Then try the same with count=2000 and get 1.3 million bytes per sek write
>> hmmm ....
>>
>> This is /etc/raid5.conf
>> START array
>> # numRow numCol numSpare
>> 1 4 0
>>
>> START disks
>> /dev/dk3
>> /dev/dk4
>> /dev/dk5
>> /dev/dk6
>>
>> START layout
>> # sectPerSU SUsPerParityUnit SUsPerReconUnit RAID_level_5
>> 32 1 1 5
>>
>> START queue
>> fifo 100
>>
>>
>> Summary:
>> It seems like this is 200 times slower than FreeBSD zraid, is that reasonable? or where did i go wrong?!
>> Again, only curious. Was thinking about installing OpenBSD, they have a more similar raid system compared to netbsd(more or less exactly the same:))
>>
>> Could it be worth adding a Big disk section to the raidctl manpage: https://man.netbsd.org/raidctl.8
>>
>> Thanks in advance!
>> Br Nicklas
>>
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