From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH 2/2] xfs: add 'discard_sync' mount flag To: "Luis R. Rodriguez" , Jens Axboe Cc: Brian Foster , linux-xfs@vger.kernel.org, linux-block@vger.kernel.org, hch@lst.de References: <1525102372-8430-1-git-send-email-axboe@kernel.dk> <1525102372-8430-3-git-send-email-axboe@kernel.dk> <20180430171945.GB22176@bfoster.bfoster> <24df628d-c861-6f39-96a8-d759902d1fe3@kernel.dk> <20180430182525.GE27875@wotan.suse.de> From: Eric Sandeen Message-ID: <4cb68c8e-bf6c-b717-73f0-20e850582004@sandeen.net> Date: Mon, 30 Apr 2018 14:19:27 -0500 MIME-Version: 1.0 In-Reply-To: <20180430182525.GE27875@wotan.suse.de> Content-Type: text/plain; charset=utf-8 List-ID: On 4/30/18 1:25 PM, Luis R. Rodriguez wrote: > On Mon, Apr 30, 2018 at 12:07:31PM -0600, Jens Axboe wrote: >> On 4/30/18 11:19 AM, Brian Foster wrote: >>> On Mon, Apr 30, 2018 at 09:32:52AM -0600, Jens Axboe wrote: >>>> XFS recently added support for async discards. While this can be >>>> a win for some workloads and devices, there are also cases where >>>> async bursty discard will severly harm the latencies of reads >>>> and writes. >>>> >>>> Add a 'discard_sync' mount flag to revert to using sync discard, >>>> issuing them one at the time and waiting for each one. This fixes >>>> a big performance regression we had moving to kernels that include >>>> the XFS async discard support. >>>> >>>> Signed-off-by: Jens Axboe >>>> --- >>> >>> Hm, I figured the async discard stuff would have been a pretty clear win >>> all around, but then again I'm not terribly familiar with what happens >>> with discards beneath the fs. I do know that the previous behavior would >>> cause fs level latencies due to holding up log I/O completion while >>> discards completed one at a time. My understanding is that this lead to >>> online discard being pretty much universally "not recommended" in favor >>> of fstrim. >> >> It's not a secret that most devices suck at discard. > > How can we know if a device sucks at discard? I was going to ask the same thing. ;) "Meh, punt to the admin!" I'm having deja vu but can't remember why. Seems like this has come up before and we thought it should be a block device tunable, not pushed down from the filesystem. Is that possible? -Eric