On Sun, Jul 24, 2016 at 12:28:26PM +0200, Benoît Canet wrote:
> On Sun, Jul 24, 2016 at 12:20 PM, Nadav Har'El <n...@scylladb.com> wrote:
>
> >
> > On Sun, Jul 24, 2016 at 1:06 PM, Gleb Natapov <g...@scylladb.com> wrote:
> >
> >>
> >> > So it appears that unless you're using mmap(), OSv does *not* have any
> >> > writeback buffer, not even for ZFS filesystems: If you do a write()
> >> system
> >> > call to write 10 bytes, you will create a 10-byte disk operation; If
> >> > understand you correctly, there is no attempt to somehow coalesce many
> >> > small writes to one operation, nor is there any attempt to reorder the
> >> I/O
> >> > operations to better fit some assumptions on disk performance.
> >> >
> >> No, unless you're using mmap() _our_ write page cache will not be used,
> >>
> >
> > Yes, that we already understood after your previous clarifications on the
> > bug tracker. None of Benoit's tests are using mmap(), so our pagecache.cc
> > is irrelevant to this discussion.
> >
> >
> >> instead write will go directly into zfs layer which should not generate
> >> disk write for each write operation unless it is extremely stupid (which
> >> I doubt) or mounted/configured incorrectly or we use some kind of wrong
> >> write.
> >>
> >
> > Ok, so I think that what Benoit was seeing is that ZFS *is* being
> > extremely stupid and generating many small writes.
> >
> > But, Benoit, is it possible that ZFS *is* trying to coalesce writes, just
> > somehow misconfigured to aim at the wrong size? What I mean is, if you do
> > write() operations of 10 bytes, will you see 10-byte operations, or are
> > they still coalesced to 512-byte sizes?
> > If there is coalescing, just to 512 bytes (not 4096 as you would have
> > hoped), then clearly ZFS can coalesce the writes but is somehow
> > misconfigured to make them so small.
> >
>
> It round writes to 512 bytes.
>
>
It cannot write less than block size. Even if it wants to it will do
RMW, so no matter size of your write on wire you will see at least 512
byte writes.
--
Gleb.
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