On 30 Sep 2014, at 18:24, Emmanuel Dreyfus <m...@netbsd.org> wrote:
> On Tue, Sep 30, 2014 at 03:30:05PM +0000, Antti Kantee wrote:
>> Is it really better to sync fallocate, put stuff in the page cache and flush
>> the page cache some day instead of just having a write-through (or
>> write-first) page cache on the write() path? You also get rid of the
>> fallocate-not-implemented problem that way.
>
> GOP_ALLOC calls puffs_gop_alloc for chunks bigger than pages
> (I observed 1 MB for now). If we have fallocate implemented in the
> filesystem, this is really efficient, since fallocate saves us from
> sending any data to write. Hence IMO fallocate should be the preferred
> way if available.
>
> But if it is not there, indeed, doing a write on first attemps should
> do the trick.
>
>> Writing zeroes might be a bad emulation for distributed file systems, though
>> I guess you're the expert in that field and can evaluate the risks better
>> than me.
>
> I understand that areas fallocate'd should return zeroes, so it should be
> fine. The real problem is performances. I am not sure what approach is best.
>
> I first though about a puffs_gop_alloc like below, but that will not work,
> as VOP_PUTPAGES goes to genfs_putpages, which calls
> GOP_WRITE (genfs_gop_write), which calls VOP_STRATEGY without checking
> for failure.
genfs_gop_write calls genfs_do_io which does "error = biowait(mbp);"
near the end. This will catch errors from VOP_STRATEGY.
But why do you need GOP_ALLOC? Is there a consumer beside genfs_getpages
filling holes? Puffs doesn't return holes as its VOP_BMAP always
returns valid ( != -1 ) block addrs.
> Should I directly call VOP_STRATEGY?
>
> int
> puffs_gop_alloc(struct vnode *vp, off_t off, off_t len,
> int flags, kauth_cred_t cred)
> {
> int error;
>
> if (EXISTSOP(pmp, FALLOCATE))
> return _puffs_vnop_fallocate(vp, off, len);
> else
> return VOP_PUTPAGES(vp, off, off + len,
> PGO_CLEANIT|PGO_SYNCIO);
> }
>
> --
> Emmanuel Dreyfus
> m...@netbsd.org
--
J. Hannken-Illjes - hann...@eis.cs.tu-bs.de - TU Braunschweig (Germany)
