Slow disk performance inside VM

Cloudjiffy

New Member
Aug 12, 2019
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Hello,

We have installed Proxmox6 on Server with ZFS RAID-1 and Created Virtuozzo 7 VM in the same.

Server specs -
2 x 256GB SSD ( Samsung PM 883 ) for OS ZFS RAID1
2 x 4TB SSD ( Samsung PM883 ) for VM storage ZFS RAID1
There is no RAID controller and disks are directly connected to the Motherboard.

Now we are running the following commands on both server, i.e on main node and in VM to check performance
" dd bs=1M count=2560 if=/dev/zero of=test conv=fdatasync "
" pveperf "

The Output of command from Main node:-
Code:
$dd bs=1M count=2560 if=/dev/zero of=test conv=fdatasync
2560+0 records in
2560+0 records out
2684354560 bytes (2.7 GB, 2.5 GiB) copied, 1.14534 s, 2.3 GB/s


Code:
$pveperf
CPU BOGOMIPS:      240045.12
REGEX/SECOND:      3204623
HD SIZE:           215.06 GB (rpool/ROOT/pve-1)
FSYNCS/SECOND:     3666.42
DNS EXT:           18.77 ms
DNS INT:           383.84 ms


The Output from VM-
Code:
$dd bs=1M count=2560 if=/dev/zero of=test conv=fdatasync
2560+0 records in
2560+0 records out
2684354560 bytes (2.7 GB) copied, 10.0709 s, 267 MB/s

There is nothing running within the VM while testing.

What could be the reason for slow IO performance ?
 
tldr: dd is not a benchmark, use fio or something similar

in detail: writing zeroes to zfs (with compression on; which is the default) results in not much write activity on the disk -> use random data -> use a real benchmark tool
 

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