Re: CREATE DATABASE with filesystem cloning

Em qua., 8 de mai. de 2024 às 04:37, Nazir Bilal Yavuz <byavuz81(at)gmail(dot)com>
escreveu:

> Hi Ranier,
>
> Thanks for looking into this!
>
> I am not sure why but your reply does not show up in the thread, so I
> copied your reply and answered it in the thread for visibility.
>
> On Tue, 7 May 2024 at 16:28, Ranier Vilela <ranier(dot)vf(at)gmail(dot)com> wrote:
> >
> > I know it's coming from copy-and-paste, but
> > I believe the flags could be:
> > - dstfd = OpenTransientFile(tofile, O_RDWR | O_CREAT | O_EXCL |
> PG_BINARY);
> > + dstfd = OpenTransientFile(tofile, O_CREAT | O_WRONLY | O_TRUNC |
> O_EXCL | PG_BINARY);
> >
> > The flags:
> > O_WRONLY | O_TRUNC
> >
> > Allow the OS to make some optimizations, if you deem it possible.
> >
> > The flag O_RDWR indicates that the file can be read, which is not true
> in this case.
> > The destination file will just be written.
>
> You may be right about the O_WRONLY flag but I am not sure about the
> O_TRUNC flag.
>
> O_TRUNC from the linux man page [1]: If the file already exists and is
> a regular file and the access mode allows writing (i.e., is O_RDWR or
> O_WRONLY) it will be truncated to length 0. If the file is a FIFO or
> terminal device file, the O_TRUNC flag is ignored. Otherwise, the
> effect of O_TRUNC is unspecified.
>
O_TRUNC is usually used in conjunction with O_WRONLY.
See the example at:
open.html
<https://pubs.opengroup.org/onlinepubs/009696699/functions/open.html>

O_TRUNC signals the OS to forget the current contents of the file, if it
happens to exist.
In other words, there will be no seeks, only and exclusively writes.

> But it should be already checked if the destination directory already
> exists in dbcommands.c file in createdb() function [2], so this should
> not happen.
>
I'm not sure what you're referring to here.

best regards,
Ranier Vilela