On Apr 24, 2015, at 3:51 PM, Eric Sandeen <sandeen@xxxxxxxxxx> 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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