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