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

[Jan Kara <jack@suse.cz>]

On Fri 24-04-15 22:25:06, Andreas Dilger wrote:
> On Apr 24, 2015, at 3:51 PM, Eric Sandeen <sandeen@redhat.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...

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