Re: [Question] About XFS random buffer write performance

[Matthew Wilcox <willy@infradead.org>]

On Thu, Jul 30, 2020 at 09:05:03AM +1000, Dave Chinner wrote:
> On Wed, Jul 29, 2020 at 07:50:35PM +0100, Matthew Wilcox wrote:
> > I had a bit of a misunderstanding.  Let's discard that proposal
> > and discuss what we want to optimise for, ignoring THPs.  We don't
> > need to track any per-block state, of course.  We could implement
> > __iomap_write_begin() by reading in the entire page (skipping the last
> > few blocks if they lie outside i_size, of course) and then marking the
> > entire page Uptodate.
> 
> __iomap_write_begin() already does read-around for sub-page writes.
> And, if necessary, it does zeroing of unwritten extents, newly
> allocated ranges and ranges beyond EOF and marks them uptodate
> appropriately.

But it doesn't read in the entire page, just the blocks in the page which
will be touched by the write.

> > Buffer heads track several bits of information about each block:
> >  - Uptodate (contents of cache at least as recent as storage)
> >  - Dirty (contents of cache more recent than storage)
> >  - ... er, I think all the rest are irrelevant for iomap
> 
> 
> Yes, it is. And we optimised out the dirty tracking by just using
> the single dirty bit in the page.
> 
> > I think I just talked myself into what you were arguing for -- that we
> > change the ->uptodate bit array into a ->dirty bit array.
> > 
> > That implies that we lose the current optimisation that we can write at
> > a blocksize alignment into the page cache and not read from storage.
> 
> iomap does not do that. It always reads the entire page in, even for
> block aligned sub-page writes. IIRC, we even allocate on page
> granularity for sub-page block size filesystems so taht we fill
> holes and can do full page writes in writeback because this tends to
> significantly reduce worst case file fragmentation for random sparse
> writes...

That isn't what __iomap_write_begin() does today.

Consider a 1kB block size filesystem and a 4kB page size host.  Trace through
writing 1kB at a 2kB offset into the file.
We call iomap_write_begin() with pos of 2048, len 1024.
Allocate a new page
Call __iomap_write_begin(2048, 1024)
block_start = 2048
block_end = 3072
iomap_adjust_read_range() sets poff and plen to 2048 & 1024
from == 2048, to == 3072, so we continue
block_start + plen == block_end so the loop terminates.
We didn't read anything.

> Modern really SSDs don't care about runs of zeros being written.
> They compress and/or deduplicate such things on the fly as part of
> their internal write-amplification reduction strategies. Pretty much
> all SSDs on the market these days - consumer or enterprise - do this
> sort of thing in their FTLs and so writing more than the exact
> changed data really doesn't make a difference.

You're clearly talking to different SSD people than I am.