From mboxrd@z Thu Jan 1 00:00:00 1970 From: NeilBrown Subject: Re: [PATCH 0/1] RFC: Unreliable discard performance can cripple RAID1 Date: Wed, 24 Jun 2015 17:55:56 +1000 Message-ID: <20150624175556.701b724c@noble> References: <1435105573-1373-1-git-send-email-Jes.Sorensen@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <1435105573-1373-1-git-send-email-Jes.Sorensen@redhat.com> Sender: linux-raid-owner@vger.kernel.org To: Jes.Sorensen@redhat.com Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids On Tue, 23 Jun 2015 20:26:12 -0400 Jes.Sorensen@redhat.com wrote: > From: Jes Sorensen > > Neil, > > I have been hitting issues with discard being ridiculously slow on > arrays with certain typs of SSDs that seem to serialize discard > processing. > > This is particularly bad as I have seen systems where the IMSM BIOS > defaults to 4KB chunk size, combined with these badly performing > drives, it could bump the mkfs on an array from seconds to over 40 > minutes. Most users will stick to the defaults and then hit the > problem during install without understanding why it goes wrong :( > > The problem is that there is no way to benchmark our way to this or > somehow test if a drive performs discard at reasonable speed. I > suggest we take an approach similar to that of RAID456 and default to > disabling discard, except for the case where the user knows the drives > are safe. > > Thoughts? > > Cheers, > Jes > > > Jes Sorensen (1): > raid0: Disable discard per default due to performance uncertainty > > drivers/md/raid0.c | 20 ++++++++++++++++++++ > 1 file changed, 20 insertions(+) > RAID1? RAID0?? I hate it when I do that! Doesn't the scheduler merge adjacent discard requests? Or is this some non-SATA/SCSI SSD that has a 'make_request_fn' driver? I think I came across one of those before (NVMe). In that case - the driver needs to be fixed. NeilBrown