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Message-ID: <20120509195056.GH5092@quack.suse.cz> Date: Wed, 9 May 2012 21:50:56 +0200 From: Jan Kara <jack@...e.cz> To: Daniel Pocock <daniel@...ock.com.au> Cc: linux-ext4@...r.kernel.org Subject: Re: ext4 barrier on SCSI vs SATA? On Mon 07-05-12 10:35:48, Daniel Pocock wrote: > > > 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? We always flush the complete cache. Actually, there's no interface for filesystem to tell lower layers that only some blocks should be flushed AFAIK. And even if we could, journaling is designed so that we need to flush caches for most of blocks because usually data blocks need to be on stable storage when transaction commits. Honza -- Jan Kara <jack@...e.cz> SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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