[LFS/VM TOPIC] Barriers and SYNCHRONZIE_CACHE

[Hannes Reinecke <hare@suse.de>]

Hi all,

[Topic]
Barriers and SYNCHRONIZE CACHE

[Abstract]
In recent years some design flaws in the barriers implementation
became apparent:
- No indication about _which_ blocks to flush, so all outstanding
  blocks are flushed
- No error handling for SYNCHRONIZE CACHE
- SYNCHRONIZE CACHE affects all I/Os, so if a LUN is used by
  several virtual guests _all_ guests are affected
- SYNCHRONIZE CACHE has a rather severe performance impact
  on RAID controllers (if executed directly)

To overcome these issues I would propose to modify the
barrier interface so that individual blocks can be
marked as 'flushing'/'immediate'. This would allow
the lower layers (eg. SCSI midlayer) to restrict
the barriers operation on the affected blocks only.
EG the SYNCHRONIZE_CACHE operation could be updated
with a block list. Or more advanced methods (like FUA
or tagging) could be used, avoiding the need to issue
a SYNCHRONIZE CACHE altogether.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: Markus Rex, HRB 16746 (AG Nürnberg)
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