Re: Atomic file data replace API

[Chris Mason <chris.mason@oracle.com>]

Excerpts from Olaf van der Spek's message of 2011-01-07 10:08:24 -0500:
> On Fri, Jan 7, 2011 at 4:05 PM, Chris Mason <chris.mason@oracle.com> =
wrote:
> >> > The problem is the write() // 0+ times. =C2=A0The kernel has no =
idea what
> >> > new result you want the file to contain because the application =
isn't
> >> > telling us.
> >>
> >> Isn't it safe for the kernel to wait until the first write or clos=
e
> >> before writing anything to disk?
> >
> > I'm afraid not. =C2=A0Picture an application that opens a thousand =
files and
> > writes 1MB to each of them, and then didn't close any. =C2=A0If we =
waited
> > until close, you'd have 1GB of memory pinned or staged somehow.
>=20
> That's not what I asked. ;)
> I asked to wait until the first write (or close). That way, you don't
> get unintentional empty files.
> One step further, you don't have to keep the data in memory, you're
> free to write them to disk. You just wouldn't update the meta-data
> (yet).

Sorry ;) Picture an application that truncates 1024 files without closi=
ng any
of them.  Basically any operation that includes the kernel waiting for
applications because they promise to do something soon is a denial of
service attack, or a really easy way to run out of memory on the box.

-chris
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