Re: Performance testing of various barrier reduction patches [was: Re: [RFC v4] ext4: Coordinate fsync requests]

[Christoph Hellwig <hch@lst.de>]

On Mon, Oct 18, 2010 at 03:49:36PM -0700, Darrick J. Wong wrote:
> Through this table, I'm looking for a performance characteristic that typifies
> storage with a battery-backed write cache (BBWC).  As we can see from
> lldd_flush_rtt_avg, the BBWC storage features a very low flush time, about 1ms
> or less.  Everything else, including SSDs, are over that amount.  The other odd
> result I see is that it takes a significant amount of time to get a flush
> command from the top of the block layer to the LLDD, though I suspect some of
> that might be waiting for the device to process earlier writes.  Christoph has
> a patch that looks like it streamlines that, but it triggered various BUG_ONs
> when I booted with it, so I took the patch out.

We currently synchronize flush requests.  There's no real reason to do
it except that we'll either need to make drivers accept flush requests
with a bio attached to them or find a workaround in the block layer to
submit it without bio without synchronizing them.

I thin kthat should be the first angle of attack before adding
complexity to filesystems.