linux-ext4 - ext4 barrier on SCSI vs SATA?

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Date:	Mon, 07 May 2012 10:35:48 +0000
From:	Daniel Pocock <daniel@...ock.com.au>
To:	linux-ext4@...r.kernel.org
Subject: ext4 barrier on SCSI vs SATA?

I understand that for barriers to work, the fs needs to be able to tell
the drive when to move data from hardware cache to the platter.

I notice various pages mention the SYNCHRONIZE CACHE command (SCSI) and
the FLUSH_CACHE_EXT command (ATA) as if they are equivalent.

Looking more closely, I found the SYNCHRONIZE CACHE supports a block
range, whereas it appears that FLUSH_CACHE_EXT always flushes the entire
cache (maybe 32MB or 64MB on a SATA drive)

Does ext4 always flush all of the cache contents?  Or if the system is
SCSI, does it only selectively flush the blocks that must be flushed to
maintain coherency?

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