From: =?ISO-8859-15?Q?Luk=E1=A8_Czerner?= Subject: Re: [PATCH v2] e2fsprogs: Limit number of reserved gdt blocks on small fs Date: Tue, 28 Apr 2015 14:24:46 +0200 (CEST) Message-ID: References: <1427280382-31120-1-git-send-email-lczerner@redhat.com> <553ABAF0.2020702@redhat.com> <20150427161451.GA22448@quack.suse.cz> <553E6277.3040800@redhat.com> <20150428122102.GA9955@quack.suse.cz> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Eric Sandeen , Andreas Dilger , linux-ext4@vger.kernel.org To: Jan Kara Return-path: Received: from mx1.redhat.com ([209.132.183.28]:56923 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S965535AbbD1MYy (ORCPT ); Tue, 28 Apr 2015 08:24:54 -0400 In-Reply-To: <20150428122102.GA9955@quack.suse.cz> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Tue, 28 Apr 2015, Jan Kara wrote: > Date: Tue, 28 Apr 2015 14:21:02 +0200 > From: Jan Kara > To: Eric Sandeen > Cc: Jan Kara , Andreas Dilger , > Lukas Czerner , linux-ext4@vger.kernel.org > Subject: Re: [PATCH v2] e2fsprogs: Limit number of reserved gdt blocks on > small fs > > On Mon 27-04-15 11:23:19, Eric Sandeen wrote: > > On 4/27/15 11:14 AM, Jan Kara wrote: > > > On Fri 24-04-15 22:25:06, Andreas Dilger wrote: > > >> On Apr 24, 2015, at 3:51 PM, Eric Sandeen wrote: > > >>> On 3/25/15 5:46 AM, Lukas Czerner wrote: > > >>>> Currently we're unable to online resize very small (smaller than 32 MB) > > >>>> file systems with 1k block size because there is not enough space in the > > >>>> journal to put all the reserved gdt blocks. > > >>> > > >>> So, I'll get to the patch review if I need to, but this all seemed a little > > >>> odd; this is a regression, so do we really need to restrict things at mkfs > > >>> time? > > >>> > > >>> On the userspace side, things were ok until: > > >>> > > >>> 9f6ba88 resize2fs: add support for new in-kernel online resize ioctl > > >>> > > >>> and even with that, on the kernelspace side, things were ok until: > > >>> > > >>> 8f7d89f jbd2: transaction reservation support > > >>> > > >>> I guess I'm trying to understand why that jbd2 commit regressed this. > > >>> I've not been paying enough attention to ext4 lately. ;) > > >>> > > >>> I mean, the threshold got chopped in half: > > >>> > > >>> - if (nblocks > journal->j_max_transaction_buffers) { > > >>> + /* > > >>> + * 1/2 of transaction can be reserved so we can practically handle > > >>> + * only 1/2 of maximum transaction size per operation > > >>> + */ > > >>> + if (WARN_ON(blocks > journal->j_max_transaction_buffers / 2)) { > > >>> printk(KERN_ERR "JBD2: %s wants too many credits (%d > %d)\n", > > >>> - current->comm, nblocks, > > >>> - journal->j_max_transaction_buffers); > > >>> + current->comm, blocks, > > >>> + journal->j_max_transaction_buffers / 2); > > >>> return -ENOSPC; > > >>> } > > >>> > > >>> so it's clear why the behavior changed, I guess, but it feels like I > > >>> must be missing something here. > > >> > > >> Is there some way to reserve these journal blocks only in the case of > > >> delalloc usage? This has caused a performance regression with Lustre > > >> servers on 3.10 kernels because the journal commits twice as often. > > >> We've worked around this for now by doubling the journal size, but it > > >> seems a bit of a hack since we can never use the whole journal anymore. > > > Hum, so the above hunk only limits maximum number of credits used by a > > > single handle. Multiple handles can still consume upto maximum transaction > > > size buffers (at least that's the intention :). So I don't see how that can > > > cause the problem you describe. What can happen though is that there are > > > quite a few outstanding reserved handles and so we have to reserve space > > > for them in the running transaction. Do you use dioread_nolock option? That > > > enables the use of reserved handles in ext4 for conversion of unwritten > > > extents... > > > > You're probably asking Andreas, but just in case, for my testcase, it's > > all defaults & standard options. > > > > i.e. just this fails, after the above commit, whereas it worked before. > > > > mkfs.ext4 /dev/sda 20M > > mount /dev/sda /mnt/test > > resize2fs /dev/sda 200M > Yeah, I understand your failure - transaction reservation has reduced > max transaction size to a half. After that your fs resize exceeds max > transaction size and we are in trouble. I'd prefer solution for that to be > in resize code though because it's really a corner case and I wouldn't like > to slow down the common transaction start path for it... Hi Jan, if you have not already, please see the patch which started the discussion. Also I think that aside from the userspace fix, I need to add some safety to the kernel as well so that people who do run into this know what's going on. Thanks! -Lukas > > Honza >