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

[Eric Sandeen <sandeen@redhat.com>]

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@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...

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

-Eric