Re: ENSOPC on a 10% used disk

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 18, 2018 at 06:44:54PM +0300, Avi Kivity wrote:
> 
> On 18/10/2018 14.00, Avi Kivity wrote:
> >
> >
> >This can happen, and indeed I see our default hint is 1MB, so our
> >small files use a 1MB hint. Looks like we should remove that 1MB
> >hint since it's reducing allocation flexibility for XFS without a
> >good return.
> 
> 
> I convinced myself that this is the root cause, it fits perfectly
> with your explanation. I still think that XFS should allocate
> *something* rather than ENOSPC, but I can also understand someone
> wanting a guarantee.

Yup, it's a classic catch 22.

> >On the other hand, I worry that because we bypass the page cache,
> >XFS doesn't get to see the entire file at one time and so it will
> >get fragmented.
> 
> 
> That's what happens. I write 1000 4k writes to 400 files, in
> parallel, AIO+DIO. I got 400 perfectly-fragmented files, each had
> 1000 extents.

Yup, you wrote them all in the one directory, didn't you? :)

> So I'll remove the default hint for small files, and replace it with
> larger buffer sizes so we batch more and don't get 8k-sized extents
> (which is our default buffer size).

Or you could just mount with the "noalign" mount option to turn off
stripe alignment. After all, you don't need stripe alignment for a
single spindle....

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com