High performance SCST iSCSI Target on Linux software Raid
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Introduction
This HOW-TO is focusing on performance. This is why I made some decisions targeted on performance instead of security (like you are used in Alpine Linux). This means we are not using grsec and not using a firewall. I presume you will take security actions on another level.
To get started, you can download a boot cd here:
http://alpine.nethq.org/distfiles/alpine-scst-110106-x86_64.iso
Vanilla Linux kernel with SCST patches
The default Linux kernel will provide support for iSCSI. The problem with this implementation is it operates in user-space. SCST iSCSI will run in kernel-space and this is one of the reasons why it preforms much better. SCST performance depends on specific patches which need to be applied to the kernel. This is why we created a separate kernel just for SCST usage. SCST modules are already included by default so there is no need for a separate module package to be installed.
P.S. We only provide an x86_64 kernel for SCST because it will perform better on 64bit systems.
Linux software raid
In my personal setup i have 4 pieces of WD RE4 1TB drives which i want to use in the best performance raid level with redundancy. According to many mailing lists and opinion of the Linux raid author RAID10 with layout f2 (far) seems to preform best while still having redundancy. Please remember with RAID10 50% of your hard disk space will go to redundancy, but performance is almost the same as RAID0 (stripe).
For most up-to-date information regarding Linux software raid: https://raid.wiki.kernel.org/index.php/Overview
RAID10 has multiple layout types. f(far)2 in tests seem to preform the best. Please see above link for references.
mdadm -v --create /dev/md0 --level=raid10 --layout=f2 --raid-devices=4 /dev/sda /dev/sdb /dev/sdc /dev/sdd
I am not using partitions on my disks, although there are reasons to use partitions, see here:
https://raid.wiki.kernel.org/index.php/Partition_Types
You can now monitor your raid (re)building: cat /proc/mdstat
By default, the rebuild speed will be set and can be checked and changed here:
cat /proc/sys/dev/raid/speed_limit_max cat /proc/sys/dev/raid/speed_limit_min
make sure we have raid10 module loaded at boot
echo raid10 >> /etc/modules
When you are happy with your raid configuration, save its information to mdamd.conf file
mdadm --detail --scan >> /etc/mdadm.conf
It should display something like
ARRAY /dev/md0 metadata=1.2 name=scst:0 UUID=71fc93b8:3fef0057:9f5feec1:7d9e57e8
When you are ready with your raid setup and its functioning, you will need to make sure its starting at boot time
rc-update add mdadm-raid default
Monitor software raid
Linux software raid can be monitored with mdadm daemon option. Alpine Linux includes an initd script which can invoke the daemon
/etc/init.d/mdadm
It will be default monitor the array's defined in mdadm.conf. To receive email notifications about array issues, we need to provide our email address inside mdadm.conf:
MAILADDR me@inter.net
Because mdadm cannot send email itself, we need to setup an sendmail (replacement) program.
apk add ssmtp
Ssmtp can be configured by editing /etc/ssmtp/ssmtp.conf
You can monitor messages (syslog) for actions invoked by mmdadm.
If you have your own monitoring system active, you can also let mdadm issue a script and notify it.
Disk & Volume management with SCST
Volume management can be an interesting addition to your block device(s) but it does add is an extra layer between you block devices and SCST. If you are in need of any of the features provided by LVM then go ahead and use it, I didn't add it to my array cause i don't think i will need it. In my above RAID10 setup, md will provide me an 2TB block device md0. SCST provides 2 ways to access your disk subsystem, BLOCKIO and FILEIO.
BLOCKIO has access directly to the block device md0 (without extra layers) but FILEIO uses a filesystem on top of the block device and will use regular files. Altough the extra layer would seem to be bad but it actually isn't. The filesystem will provide a caching layer and in some situations you will see improved performance.
Some speed tests:
http://scst.sourceforge.net/vl_res.txt
When using FILEIO we need to create a filesystem. To create an XFS filesystem we need xfsprogs installed
apk add xfsprogs
We also need XFS support in our kernel
modprobe xfs
And we need it next time we boot
echo xfs >> /etc/modules
XFS will automatically choose the correct settings for the target block device
mkfs.xfs /dev/md0
Our filesystem should be ready to be mounted
mkdir -p /mnt/array1 && mount /dev/md0 /mnt/array1
Because of FILEIO we need files on our filesystem which will act as iSCSI disks. We will create them with dd
dd if=/dev/zero of=/mnt/array1/disk01 bs=512k count=100000
You can also user sparse files which are instantly created (no need to wait for every bit to be written to the filesystem).
More info about sparse files here
http://en.wikipedia.org/wiki/Sparse_file
SCST and iSCSI management
SCST is managed by the sysfs filesystem. You can create your own scripts to control it, or use the included scstadmin like I will show here.
For a detailed overview of the sysfs filesystem please check here:
http://lwn.net/Articles/378658/
When starting with SCST management we need to have the SCST framework (kernel module) and the iSCSI kernel module loaded
modprobe scst scst_vdisk iscsi_scst
After these are loaded we can start the iSCSI deamon
/etc/init.d/iscsi-scst start
This command will return some information based on the current configuration located in: /etc/scst.conf
The basic config when only having iSCSI loaded is:
TARGET_DRIVER iscsi {
enabled 0
}
We start add adding a target to the correct target driver
scstadmin -add_target iqn.2010-12.org.alpinelinux:tgt -driver iscsi
The config at this point should be:
TARGET_DRIVER iscsi {
enabled 0
TARGET iqn.2010-12.org.alpinelinux:tgt {
enabled 0
}
}
Now that we have a target in our configuration we need to add a device:
scstadmin -open_dev disk01 -handler vdisk_fileio -attributes filename=/mnt/array1/disk01,nv_cache=1
The config at this point should be:
HANDLER vdisk_fileio {
DEVICE disk01 {
t10_dev_id "disk01 b8ceed65"
usn b8ceed65
filename /mnt/array1/disk01
nv_cache 1
}
}
TARGET_DRIVER iscsi {
enabled 0
TARGET iqn.2010-12.org.alpinelinux:tgt {
enabled 0
}
}
To add the device to the target we need to specify which LUN it will be. (we always need to start with 0)
scstadmin -add_lun 0 -driver iscsi -target iqn.2010-12.org.alpinelinux:tgt -device disk01
The config at this point should be:
HANDLER vdisk_fileio {
DEVICE disk01 {
t10_dev_id "disk01 b8ceed65"
usn b8ceed65
filename /mnt/array1/disk01
nv_cache 1
}
}
TARGET_DRIVER iscsi {
enabled 0
TARGET iqn.2010-12.org.alpinelinux:tgt {
enabled 0
LUN 0 disk01
}
}
This is the default minimum configuration for a working iSCSI setup. We now need to activate it:
scstadmin -enable_target iqn.2010-12.org.alpinelinux:tgt -driver iscsi
The config at this point should be:
HANDLER vdisk_fileio {
DEVICE disk01 {
t10_dev_id "disk01 b8ceed65"
usn b8ceed65
filename /mnt/array1/disk01
nv_cache 1
}
}
TARGET_DRIVER iscsi {
enabled 0
TARGET iqn.2010-12.org.alpinelinux:tgt {
rel_tgt_id 1
enabled 1
LUN 0 disk01
}
}
NOTE: I'm currently not sure why the target driver is not activated by the above command. Looks like the only option in scstadmin which could do it. You can enable it by running:
echo 1 >/sys/kernel/scst_tgt/targets/iscsi/enabled
To disable the target
scstadmin -disable_target iqn.2010-12.org.alpinelinux:tgt -driver iscsi
To remove the lun
scstadmin -rem_lun 0 -driver iscsi -target iqn.2010-12.org.alpinelinux:tgt -device disk01
To close a device
scstadmin -close_dev disk01 -handler vdisk_fileio
To remove a target
scstadmin -rem_target iqn.2010-12.org.alpinelinux:tgt -driver iscsi