Re: [Lsf-pc] [LSF/MM TOPIC] really large storage sectors - going beyond 4096 bytes

[Chris Mason <clm@fb.com>]

On Thu, 2014-01-23 at 13:27 -0800, Joel Becker wrote:
> On Wed, Jan 22, 2014 at 10:47:01AM -0800, James Bottomley wrote:
> > On Wed, 2014-01-22 at 18:37 +0000, Chris Mason wrote:
> > > On Wed, 2014-01-22 at 10:13 -0800, James Bottomley wrote:
> > > > On Wed, 2014-01-22 at 18:02 +0000, Chris Mason wrote:
> > [agreement cut because it's boring for the reader]
> > > > Realistically, if you look at what the I/O schedulers output on a
> > > > standard (spinning rust) workload, it's mostly large transfers.
> > > > Obviously these are misalgned at the ends, but we can fix some of that
> > > > in the scheduler.  Particularly if the FS helps us with layout.  My
> > > > instinct tells me that we can fix 99% of this with layout on the FS + io
> > > > schedulers ... the remaining 1% goes to the drive as needing to do RMW
> > > > in the device, but the net impact to our throughput shouldn't be that
> > > > great.
> > > 
> > > There are a few workloads where the VM and the FS would team up to make
> > > this fairly miserable
> > > 
> > > Small files.  Delayed allocation fixes a lot of this, but the VM doesn't
> > > realize that fileA, fileB, fileC, and fileD all need to be written at
> > > the same time to avoid RMW.  Btrfs and MD have setup plugging callbacks
> > > to accumulate full stripes as much as possible, but it still hurts.
> > > 
> > > Metadata.  These writes are very latency sensitive and we'll gain a lot
> > > if the FS is explicitly trying to build full sector IOs.
> > 
> > OK, so these two cases I buy ... the question is can we do something
> > about them today without increasing the block size?
> > 
> > The metadata problem, in particular, might be block independent: we
> > still have a lot of small chunks to write out at fractured locations.
> > With a large block size, the FS knows it's been bad and can expect the
> > rolled up newspaper, but it's not clear what it could do about it.
> > 
> > The small files issue looks like something we should be tackling today
> > since writing out adjacent files would actually help us get bigger
> > transfers.
> 
> ocfs2 can actually take significant advantage here, because we store
> small file data in-inode.  This would grow our in-inode size from ~3K to
> ~15K or ~63K.  We'd actually have to do more work to start putting more
> than one inode in a block (thought that would be a promising avenue too
> once the coordination is solved generically.

Btrfs already defaults to 16K metadata and can go as high as 64k.  The
part we don't do is multi-page sectors for data blocks.

I'd tend to leverage the read/modify/write engine from the raid code for
that.

-chris

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