From: Neil Brown Subject: Re: [PATCH v6 0/4] ext4: Coordinate data-only flush requests sent by fsync Date: Tue, 30 Nov 2010 11:39:06 +1100 Message-ID: <20101130113906.176ffcad@notabene.brown> References: <20101129220536.12401.16581.stgit@elm3b57.beaverton.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Jens Axboe , "Theodore Ts'o" , Andreas Dilger , Alasdair G Kergon , Jan Kara , Mike Snitzer , linux-kernel , linux-raid@vger.kernel.org, Keith Mannthey , dm-devel@redhat.com, Mingming Cao , Tejun Heo , linux-ext4@vger.kernel.org, Ric Wheeler , Christoph Hellwig , Josef Bacik To: "Darrick J. Wong" Return-path: In-Reply-To: <20101129220536.12401.16581.stgit@elm3b57.beaverton.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, 29 Nov 2010 14:05:36 -0800 "Darrick J. Wong" wrote: > On certain types of hardware, issuing a write cache flush takes a considerable > amount of time. Typically, these are simple storage systems with write cache > enabled and no battery to save that cache after a power failure. When we > encounter a system with many I/O threads that write data and then call fsync > after more transactions accumulate, ext4_sync_file performs a data-only flush, > the performance of which is suboptimal because each of those threads issues its > own flush command to the drive instead of trying to coordinate the flush, > thereby wasting execution time. > > Instead of each fsync call initiating its own flush, there's now a flag to > indicate if (0) no flushes are ongoing, (1) we're delaying a short time to > collect other fsync threads, or (2) we're actually in-progress on a flush. > > So, if someone calls ext4_sync_file and no flushes are in progress, the flag > shifts from 0->1 and the thread delays for a short time to see if there are any > other threads that are close behind in ext4_sync_file. After that wait, the > state transitions to 2 and the flush is issued. Once that's done, the state > goes back to 0 and a completion is signalled. I haven't seen any of the preceding discussion do I might be missing something important, but this seems needlessly complex and intrusive. In particular, I don't like adding code to md to propagate these timings up to the fs, and I don't the arbitrary '2ms' number. Would it not be sufficient to simply gather flushes while a flush is pending. i.e - if no flush is pending, set the 'flush pending' flag, submit a flush, then clear the flag. - if a flush is pending, then wait for it to complete, and then submit a single flush on behalf of all pending flushes. That way when flush is fast, you do a flush every time, and when it is slow you gather multiple flushes together. I think it would issues a few more flushes than your scheme, but it would be a much neater solution. Have you tried that and found it to be insufficient? Thanks, NeilBrown