Re: Triggering non-integrity writeback from userspace

[Dave Chinner <david@fromorbit.com>]

On Thu, Oct 29, 2015 at 12:23:12AM +0100, Andres Freund wrote:
> Hi,
> 
> On 2015-10-29 07:48:34 +1100, Dave Chinner wrote:
> > > The idea of using SYNC_FILE_RANGE_WRITE beforehand is that
> > > the fsync() will only have to do very little work. The language in
> > > sync_file_range(2) doesn't inspire enough confidence for using it as an
> > > actual integrity operation :/
> > 
> > So really you're trying to minimise the blocking/latency of fsync()?
> 
> The blocking/latency of the fsync doesn't actually matter at all *for
> this callsite*. It's called from a dedicated background process - if
> it's slowed down by a couple seconds it doesn't matter much.
> The problem is that if you have a couple gigabytes of dirty data being
> fsync()ed at once, latency for concurrent reads and writes often goes
> absolutely apeshit. And those concurrent reads and writes might
> actually be latency sensitive.

Right, but my point is with an async fsync/fdatasync you don't need
this background process - you can just trickle out async fdatasync
calls instead of trckling out calls to sync_file_range().

> By calling sync_file_range() over small ranges of pages shortly after
> they've been written we make it unlikely (but still possible) that much
> data has to be flushed at fsync() time.

Right, but you still need the fsync call, whereas with a async fsync
call you don't - when you gather the completion, no further action
needs to be taken on that dirty range.

> At the moment using fdatasync() instead of fsync() is a considerable
> performance advantage... If I understand the above proposal correctly,
> it'd allow specifying ranges, is that right?

Well, the patch I sent doesn't do ranges, but it could easily be
passed in as the iocb has offset/len parameters that are used by
IOCB_CMD_PREAD/PWRITE. io_prep_fsync/io_fsync both memset the iocb
to zero, so if we pass in a non-zero length, we could treat it as a
ranged f(d)sync quite easily.

> There'll be some concern about portability around this - issuing
> sync_file_range() every now and then isn't particularly invasive. Using
> aio might end up being that, not sure.

It's still a non-portable/linux only solution, because it is using
the linux native aio interface, not the glibc one...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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