From: Theodore Tso Subject: Re: Is TRIM/DISCARD going to be a performance problem? Date: Mon, 11 May 2009 04:41:21 -0400 Message-ID: <20090511084121.GB29082@mit.edu> References: <20090511081216.GK4694@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Matthew Wilcox , Ric Wheeler , linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org To: Jens Axboe Return-path: Content-Disposition: inline In-Reply-To: <20090511081216.GK4694@kernel.dk> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Mon, May 11, 2009 at 10:12:16AM +0200, Jens Axboe wrote: > > I largely agree with this. I think that trims should be queued and > postponed until the drive is largely idle. I don't want to put this IO > tracking in the block layer though, it's going to slow down our iops > rates for writes. Providing the functionality in the block layer does > make sense though, since it sits between that and the fs anyway. So just > not part of the generic IO path, but a set of helpers on the side. Yes, I agree. However, in that case, we need two things from the block I/O path. (A) The discard management layer needs a way of knowing that the block device has become idle, and (B) ideally there should be a more efficient method for sending trim requests to the I/O submission path. If we batch the results, when we *do* send the discard requests, we may be sending several hundred discards, and it would be useful if we could pass into the I/O submission path a linked list of regions, so the queue can be drained *once*, and then a whole series of discards can be sent to the device all at once. Does that make sense to you? - Ted