How to specify a thin provisioned LUN in SCST

The other day I wrote about how to install SCST 3.4.0 on Ubuntu 18.04. If you are not familiar with SCST, it is basically a SAN target software that you can run on Linux so you can build your own low cost SAN storage. I've been using it for years, and just recently I've started to learn a few new things about it.

For instance, I used to think that for thin provisioning, all you had to do was to create a thin provisioned disk file to present as a LUN. To do that you just run the following:

sudo dd if=/dev/zero of=lun1 bs=1 count=0 seek=1T

The above creates a thinly provisioned 1TB LUN file called lun1. Simple right?

Well, this is great and all, but if you want to use features like TRIM or UNMAP to reclaim disk space, you also need to tell SCST that this LUN file is a thin provisioned LUN. To do that, you need to add the thin_provisioned parameter to the device section of your /etc/scst.conf file. See below for an example:

 HANDLER vdisk_fileio {  
     DEVICE lun1 {  
         filename /data/lun1  
         nv_cache 1  
         thin_provisioned 1  
     }  
 }  
 TARGET_DRIVER iscsi {  
     enabled 1  
     TARGET iqn.2018-05.bauer-power.net:iscsi.lun1 {  
         enabled 1  
         rel_tgt_id 1  
         GROUP VMWARE {  
             LUN 0 lun1  
             INITIATOR iqn.2018-05.com.vmware1:8bfdfcd0  
         }  
     }         
 }  

After making this change, you can either restart the scst daemon, or reboot your SAN. If you can't reboot the SAN you will have to actually remove the LUN on the fly to make this change. To do that you have to do the following:

• sudo scstadmin -rem_lun 0 -driver iscsi -target iqn.2018-05.bauer-power.net:iscsi.lun1 -group VMWARE
• sudo scstadmin -close_dev lun1 -handler vdisk_fileio
• sudo scstadmin -open_dev lun1 -handler vdisk_fileio -attributes filename=/data/lun1 thin_provisioned=1 
• sudo scstadmin -add_lun 0 -driver iscsi -target iqn.2018-05.bauer-power.net:iscsi.lun1 -group VMWARE -device lun1

Obviously, you need to change the lun names, file names and target names to match your environment. Special thanks to Gilbert Standen from the Scst-devel mailing list for the above steps on making this change on the fly! Check out his blog here: (brandydandyoracle)

There are a lot of parameters you can add to your config file as well. Here's a list from SCST's Source Forge page:

  - filename - contains path and file name of the backend file.  
  - blocksize - contains block size used by this virtual device.  
  - write_through - contains status of write back caching of this virtual  
   device.  
  - read_only - contains read only status of this virtual device.  
  - o_direct - contains O_DIRECT status of this virtual device.  
  - nv_cache - contains NV_CACHE status of this virtual device.  
  - thin_provisioned - contains thin provisioning status of this virtual  
   device.  
  - removable - contains removable status of this virtual device.  
  - rotational - contains rotational status of this virtual device.  
  - size_mb - contains size of this virtual device in MB.  
  - t10_dev_id - contains and allows to set T10 vendor specific  
   identifier for Device Identification VPD page (0x83) of INQUIRY data.  
   By default VDISK handler always generates t10_dev_id for every new  
   created device at creation time based on the device name and  
   scst_vdisk_ID scst_vdisk.ko module parameter (see below).  
  - usn - contains the virtual device's serial number of INQUIRY data. It  
   is created at the device creation time based on the device name and  
   scst_vdisk_ID scst_vdisk.ko module parameter (see below).  
  - type - contains SCSI type of this virtual device.  
  - resync_size - write only attribute, which makes vdisk_fileio to  
   rescan size of the backend file. It is useful if you changed it, for  
   instance, if you resized it.  

Pretty cool right? Let us know what you think in the comments!