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

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Date:	Fri, 24 Apr 2015 22:25:06 -0600
From:	Andreas Dilger <adilger@...ger.ca>
To:	Eric Sandeen <sandeen@...hat.com>
Cc:	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 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.

> The reproducer, for those playing along at home, is something like:
> 
> mkfs.ext4 /dev/sda 20M
> mount /dev/sda /mnt/test
> resize2fs /dev/sda 200M
> 
> -Eric
> 
> 
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Cheers, Andreas





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