linux-ext4 - Re: [PATCH v2] e2fsprogs: Limit number of reserved gdt blocks on small fs

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Date:	Tue, 28 Apr 2015 14:21:02 +0200
From:	Jan Kara <jack@...e.cz>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	Jan Kara <jack@...e.cz>, Andreas Dilger <adilger@...ger.ca>,
	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.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 <sandeen@...hat.com> 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...

								Honza
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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