COMSTAR stands for Common Multiprotocol SCSI Target: it basically is a framework which can turn a Solaris host into a SCSI target. Before COMSTAR made its appearance, there was a very simple way to share a ZFS file system via iSCSI: just setting the
shareiscsi property on the file system was sufficient, such as you do to share it via NFS or CIFS with the
sharenfs and
sharesmb properties.
COMSTAR brings a more flexible and better solution: it's not as easy as using those ZFS properties, but it is not that hard, either. Should you need more complex setup and features, COMSTAR includes a wide set of advanced features such as:
- Scalability.
- Compatibility with generic host adapters.
- Multipathing.
- LUN masking and mapping functions.
The official COMSTAR documentation is very detailed and it's the only source of information about COMSTAR I use. If you want to read more about it, please check it out.
Enabling the COMSTAR service
COMSTAR runs as a SMF-managed service and enabling is no different than usual. First of all, check if the service is running:
# svcs \*stmf\*
STATE STIME FMRI
disabled 11:12:50 svc:/system/stmf:default
If the service is disable, enable it:
# svcadm enable svc:/system/stmf:default
After that, check that the service is up and running:
# svcs \*stmf\*
STATE STIME FMRI
online 11:12:50 svc:/system/stmf:default
# stmfadm list-state
Operational Status: online
Config Status : initialized
ALUA Status : disabled
ALUA Node : 0
Creating SCSI Logical Units
You're not required to master the SCSI protocols to setup COMSTAR but knowing the basics will help you understand the next steps you'll go through. Oversimplifying, a SCSI target is the endpoint which is waiting client (initiator) connections. For example, a data storage device is a target and your laptop may be an initiator. Each target can provide multiple logical units: each logical unit is the entity that performs "classical" storage operations, such as reading and writing from and to disk.
Each logical unit, then, is backed by some sort of storage device; Solaris and COMSTAR will let you create logical units backed by one of the following storage technologies:
- A file.
- A thin-provisioned file.
- A disk partition.
- A ZFS volume.
In this case, we'll choose the ZFS volume as our favorite backing storage technology.
Why ZFS volumes?
One of the wanders of ZFS is that it isn't just another filesystem: ZFS combines the volume manager and the file system providing you best of breed services from both world. With ZFS you can create a pool out of your drives and enjoy services such as mirroring and redundancy. In my case, I'll be using a RAID-Z pool made up of three eSATA drives for this test:
enrico@solaris:~$ zpool status tank-esata
pool: tank-esata
state: ONLINE
scrub: scrub completed after 1h15m with 0 errors on Sun Feb 14 06:15:16 2010
config:
NAME STATE READ WRITE CKSUM
tank-esata ONLINE 0 0 0
raidz1-0 ONLINE 0 0 0
c7t0d0 ONLINE 0 0 0
c8t0d0 ONLINE 0 0 0
c8t1d0 ONLINE 0 0 0
errors: No known data errors
Inside pools, you can create file systems or volumes, the latter being the equivalent of a raw drive connected to your machine. File systems and volumes use the storage of the pool without any need for further partitioning or slicing. You can create your file systems almost instantly. No more repartition hell or space estimation errors: file systems and volumes will use the space in the pool, according to the optional policies you might have established (such as quotas, space allocation, etc.)
ZFS, moreover, will let you snapshot (and clone) your file systems on the fly almost instantly: being a Copy-On-Write file system, ZFS will just write modification on the disk, without any overhead and when the blocks are no more referenced, they'll be automatically freed. ZFS snapshot are Solaris a much optimized version of Apple's time machine.
Creating a ZFS volume
Creating a volume, provided you've already have a ZFS pool, it's as easy as:
# zfs create -V 250G tank-esata/macbook0-tm
The previous command creates a 250GB volume called
macbook0-tm on pool
tank-esata. As expected you will find the raw device corresponding to this new volume:
# ls /dev/zvol/rdsk/tank-esata/
[...snip...] macbook0-tm [...snip...]
Creating a logical unit
To create a logical unit for our ZFS volume, we can use the following command:
# sbdadm create-lu /dev/zvol/rdsk/tank-esata/macbook0-tm
Created the following LU:
GUID DATA SIZE SOURCE
-------------------------------- ------------------- ----------------
600144f00800271b51c04b7a6dc70001 268435456000 /dev/zvol/rdsk/tank-esata/macbook0-tm
Logical units are identified by a unique ID, which is the GUID shown in sbdadm output. To verify and get a list of the available logical units we can use the following command:
# sbdadm list-lu
Found 1 LU(s)
GUID DATA SIZE SOURCE
-------------------------------- ------------------- ----------------
600144f00800271b51c04b7a6dc70001 268435456000 /dev/zvol/rdsk/tank-esata/macbook0-tm
Indeed, it finds the only logical unit we created so far.
Mapping the logical unit
The logical unit we created in the previous section is not available to any initiator yet. To make your logical unit available, you must choose how to map them. Basically, you've got two choices:
- Mapping it for all initiators on every port.
- Mapping it selectively.
In this test, taking into account that it's a home setup on a private LAN, I'll go for simple mapping. Please, choose carefully your mapping strategy according to your needs. If you need more information on selective mapping, check the
official COMSTAR documentation.
To get the GUID of the logical unit you can use the
sbdadm or the
stmfadm commands:
# stmfadm list-lu -v
LU Name: 600144F00800271B51C04B7A6DC70001
Operational Status: Offline
Provider Name : sbd
Alias : /dev/zvol/rdsk/tank-esata/macbook0-tm
View Entry Count : 0
Data File : /dev/zvol/rdsk/tank-esata/macbook0-tm
Meta File : not set
Size : 268435456000
Block Size : 512
Management URL : not set
Vendor ID : SUN
Product ID : COMSTAR
Serial Num : not set
Write Protect : Disabled
Writeback Cache : Enabled
Access State : Active
To create the simple mapping for this logical unit, we run the following command:
# stmfadm add-view 600144f00800271b51c04b7a6dc70001
Configuring iSCSI target ports
As outlined in the introduction, with COMSTAR a new iSCSI transport implementation has been introduced that replaces the old implementation. Since the two implementation are incompatible and only one can run at a time, please check which one you're using. Nevertheless, consider switching to the new implementation as soon as you can.
The old implementation is registered as the SMF service svc:/system/iscsitgt:default and the new implementation is registered as svc:/network/iscsi/target.
enrico@solaris:~$ svcs \*scsi\*
STATE STIME FMRI
disabled Feb_03 svc:/system/iscsitgt:default
online Feb_03 svc:/network/iscsi/initiator:default
online Feb_16 svc:/network/iscsi/target:default
If you're running the new COMSTAR iSCSI transport implementation, you can now create a target with the following command:
# itadm create-target
Target iqn.1986-03.com.sun:02:7674e54f-6738-4c55-d57d-87a165eda163 successfully created
If you want to check and list the targets you can use the following command:
# itadm list-target
TARGET NAME STATE SESSIONS
iqn.1986-03.com.sun:02:7674e54f-6738-4c55-d57d-87a165eda163 online 0
Configuring the iSCSI target for discovery
The last thing left to do to have your iSCSI target configured for discovery. Discovery is the process which an initiator use to get a list of available targets. You can opt for one of the three iSCSI discovery methods:
- Static discovery: a static target address is configured.
- Dynamic discovery: targets are discovered by initiators using an intermediary iSNS servers.
- SendTargets discovery: configuring the SendTargets option on the initiator.
I will opt for static discovery because I've got a very small number of targets and I want to control which initiators connect to my target. To configure static discovery just run the following command:
# devfsadm -i iscsi
Next steps
Configuring a target is a matter of few commands. It took me much more time to write down this blog post than having my COMSTAR target running.
The next steps wil be having an initiator connect to your target. I detailed how to configure a Mac OS/X instance as an iSCSI initiator on another post.
