[libvirt] [PATCH 1/2] Introduce posix_fallocate() to allocate disk space

[Amit Shah]

On (Thu) Mar 19 2009 [10:39:05], Daniel P. Berrange wrote:
> On Thu, Mar 19, 2009 at 12:45:24PM +0530, Amit Shah wrote:
> > diff --git a/src/util.c b/src/util.c
> > index 66ad9a4..b69d33a 100644
> > --- a/src/util.c
> > +++ b/src/util.c
> > @@ -117,6 +117,26 @@ ssize_t safewrite(int fd, const void *buf, size_t count)
> >          return nwritten;
> >  }
> >  
> > +#if 1
> > +int safezero(int fd, int flags, off_t offset, off_t len)
> > +{
> > +    return posix_fallocate(fd, offset, len);
> > +}
> > +#else
> > +int safezero(int fd, int flags, off_t offset, off_t len)
> > +{
> > +    char *buf;
> > +    int r;
> > +
> > +    buf = calloc(len, sizeof(char));
> > +    if (buf == NULL)
> > +        return -ENOMEM;
> > +
> > +    r = safewrite(fd, buf, len);
> > +    return r;
> > +}
> > +#endif
> 
> For memory allocation you can use  
> 
>    if (VIR_ALLOC_N(buf, len) < 0)
>       return -1;
> 
> Also, you'll need a VIR_FREE(buf) call in there after the safewrite().
> 
> I'm a little worried about scalability of this impl though. The later
> patch will call this with 500 MB chunks if progress is turned on, or
> even allocate the whole file in one chunk - we don't want to be 
> allocating 20 GB of memory jus to write zero's to a file ! Is it
> perhaps easier just to  do something like
> 
>    char * buf = mmap(NULL, len, MAP_SHARED, MAP_ANONYMOUS, fd, offset)
>    memset(buf, 0, len);
>    munmap(buf, len);
> 
> Or, do your calloc() of a 1 MB chunk, and then call safewrite in a
> loop, just to avoid too large a memory allocation.

Yeah; I forgot the free().

My approach was to never allocate more than 500MiB. However we can call
safezero() itself with less than 500M; is 200M OK?

Amit