Slow iSCSI performance on ZFS Volumes (zvol)

TL;DR: For reasons, don’t use ZVOLs for iSCSI volumes. Instead, just use a generic file.

I’ve been reorganizing my lab a bit to consolidate some storage and wanted to experiment with iSCSI. I thought “wow, what a great use-case for ZFS ZVOLS…”.

If you recall, ZFS has the ability to create block devices called ZVOLs. When you do this, you get a new device presented on the machine under /dev/zvol/<poolname>/ that you can use as you would any other disk. As part of my consolidation effort, I decided to use one and present it over iSCSI to my workstation. To my surprise, the performance was dismal, maxing out at around 30MB/s when writing to it over iSCSI.

Here are the steps I took to create the ZVOL and present over iSCSI. Note, I’m using FreeBSD as my storage server.

# zfs create -V 500gb zroot/luns/backup
# cat > /etc/ctl.conf <<EOF
portal-group pg0 {
	discovery-auth-group no-authentication
target {
	portal-group pg0
	lun 0 {
		path /dev/zvol/zroot/backup
		size 500G
# sysrc ctld_enable=YES
# service ctld start
# ctladm lunlist
(7:1:0/0): <FREEBSD CTLDISK 0001> Fixed Direct Access SPC-5 SCSI device

With this in place, we can move to the (Linux) client machine (initiator in iSCSI parlance) and initiate a connection to the iSCSI drive then format it.

# iscsiadm --mode discovery -t sendtargets --portal,-1

# iscsiadm --mode node --targetname --portal --login
Logging in to [iface: default, target:, portal:,3260]
Login to [iface: default, target:, portal:,3260] successful.

# dmesg |tail
[117514.525034] sd 9:0:0:0: Attached scsi generic sg5 type 0
[117514.525245] sd 9:0:0:0: Power-on or device reset occurred
[117514.527424] sd 9:0:0:0: [sdg] 1048576000 512-byte logical blocks: (537 GB/500 GiB)
[117514.527428] sd 9:0:0:0: [sdg] 131072-byte physical blocks
[117514.527706] sd 9:0:0:0: [sdg] Write Protect is off
[117514.527709] sd 9:0:0:0: [sdg] Mode Sense: 7f 00 10 08
[117514.528159] sd 9:0:0:0: [sdg] Write cache: enabled, read cache: enabled, supports DPO and FUA
[117514.528750] sd 9:0:0:0: [sdg] Optimal transfer size 8388608 bytes
[117514.675486] sd 9:0:0:0: [sdg] Attached SCSI disk

# mkfs.ntfs -Q /dev/sdg 

Let’s format it then mount it. Note this drive will eventually be mounted by a Windows machine thus we’re formatting it with NTFS.

# mkfs.ntfs -Q /dev/sdg
# mount -t ntfs3 /dev/sdg /mnt

At this point I proceeded to copy data onto the drive where it maxed out at 35MB/s. Abysmal. So, I decided to switch from ZVOL to a plain file on disk and use that instead.

# zfs destroy zroot/luns/backup
# zfs create -o mountpoint=/luns/backup zroot/luns/backup
# cd /luns/backup
# truncate -s 500G disk.img
# sed -i 's/\/dev\/zvol\/zroot/backup/\/luns\/backup\/disk.img/g' /etc/ctl.conf
# service ctld restart

After setting it up this way I was maxing out my 1Gb connnection with writes speeds of over 100MB/s, a 2x improvement in speed.

Lesson learned.

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