| SCST, DRBD and Dual Primary Mode |
| ******************************** |
| |
| Introduction |
| ------------ |
| One possible approach to build a highly available storage solution is to |
| combine SCST with DRBD. While designing such a solution one has to choose an |
| operation mode for DRBD. While single primary mode is the easiest to set up, |
| with dual primary mode it becomes possible to balance the load for a single |
| LUN over the clustered servers. However, SCST and DRBD settings have to be |
| chosen carefully in order to avoid data corruption with dual primary |
| mode. One must make sure that SCST only confirms a write as finished to the |
| multipath initiator after the data involved in write operation has been |
| passed on to DRBD on both nodes. |
| |
| SCST Configuration |
| ------------------ |
| In order to avoid data corruption it is crucial to use the following SCST |
| settings for each DRBD block device exported via SCST: |
| * One can use either the vdisk_blockio device handler or vdisk_fileio. |
| * Caching MUST be disabled: set the nv_cache parameter to 0 in scst.conf. |
| * Write-through MUST be enabled: set the write_through parameter to 1 in |
| scst.conf. |
| * If SCSI reservations are used, their sync between nodes must be enabled, |
| see README.dlm. |
| |
| DRBD Configuration |
| ------------------ |
| As documented in the DRBD manual, dual primary mode only works with the DRBD |
| replication protocol C and not with protocol A nor with protocol B. |
| |
| Initiator Configuration |
| ----------------------- |
| When using VMware, do NOT use multipath I/O in round-robin mode. This mode |
| namely depends on persistent reservations to guarantee data |
| consistency. When running an SCST cluster, the SCST persistent reservation |
| state is not shared between the two nodes. |
| |
| References |
| ---------- |
| * DRBD authors, [Dual Primary Mode], DRBD Manual, 2011. |
| * VMware, [iSCSI SAN Configuration Guide 4.1], VMware website, September 2010. |
| * Phil White, [VMware ESX, MPIO, SCST and DRBD in Dual Primary Mode], SCST |
| Developers Mailing List, June 2010. |
| * Stefan Berger, [SCST and DRBD in Dual Primary Mode], DRBD users mailing |
| list, June 2010. |
| * Bart Van Assche [SCST and DRBD in Dual Primary Mode], SCST Developers |
| Mailing List, May 2011. |