From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jes Sorensen Subject: Re: [PATCH 0/1] RFC: Unreliable discard performance can cripple RAID1 Date: Wed, 24 Jun 2015 07:04:43 -0400 Message-ID: References: <1435105573-1373-1-git-send-email-Jes.Sorensen@redhat.com> <20150624100014.7d1e9842@natsu> Mime-Version: 1.0 Content-Type: text/plain Return-path: In-Reply-To: <20150624100014.7d1e9842@natsu> (Roman Mamedov's message of "Wed, 24 Jun 2015 10:00:14 +0500") Sender: linux-raid-owner@vger.kernel.org To: Roman Mamedov Cc: neilb@suse.de, linux-raid@vger.kernel.org List-Id: linux-raid.ids Roman Mamedov writes: > 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? > > It's very unfortunate if you would cripple all the good SSD models because of > a few bad ones. No one will remember to explicitly put the override to enable > TRIM, or perhaps even know that it gets disabled in md in the first place. The > only thing they will later notice is lowered performance and lifespan of their > SSDs. We already disable discard per default on raid456 in a similar manner because some of them unreliably reports discard_zeroes_data when they in reality don't. If there was a way to reliably detect these things it would be fine, unfortunately there isn't. Jes