Re: New option for pg_basebackup, to specify a different directory for pg_xlog

On Tue, Nov 19, 2013 at 2:41 PM, Fujii Masao <masao(dot)fujii(at)gmail(dot)com> wrote:

> On Tue, Nov 19, 2013 at 9:14 PM, Haribabu kommi
> <haribabu(dot)kommi(at)huawei(dot)com> wrote:
> > On 18 November 2013 23:30 Fujii Masao wrote:
> >> On Tue, Nov 19, 2013 at 12:01 AM, Haribabu kommi
> >> <haribabu(dot)kommi(at)huawei(dot)com> wrote:
> >> > On 18 November 2013 18:45 Fujii Masao wrote:
> >> >> On Mon, Nov 18, 2013 at 6:31 PM, Haribabu kommi
> >> >> <haribabu(dot)kommi(at)huawei(dot)com> wrote:
> >> >> >
> >> >> > On 18 November 2013 11:19 Haribabu kommi wrote:
> >> >> >> On 17 November 2013 00:55 Fujii Masao wrote:
> >> >> >> > On Sat, Nov 16, 2013 at 2:27 PM, Haribabu kommi
> >> >> >> > <haribabu(dot)kommi(at)huawei(dot)com> wrote:
> >> >> >> > > on 15 November 2013 17:26 Magnus Hagander wrote:
> >> >> >> > >
> >> >> >> > >>On Fri, Nov 15, 2013 at 12:10 PM, Haribabu kommi
> >> >> >> > >><haribabu(dot)kommi(at)huawei(dot)com> wrote:
> >> >> >> > >
> >> >> >> > >>On 14 November 2013 23:59 Fujii Masao wrote:
> >> >> >> > >>> On Thu, Nov 14, 2013 at 9:08 PM, Haribabu kommi
> >> >> >> > >>> <haribabu(dot)kommi(at)huawei(dot)com> wrote:
> >> >> >> > >>> > Please find attached the patch, for adding a new option
> >> >> >> > >>> > for pg_basebackup, to specify a different directory for
> >> pg_xlog.
> >> >> >> > >>>>
> >> >> >> > >>> Sounds good! Here are the review comments:
> >> >> >> > >>>> Don't we need to prevent users from specifying the same
> >> >> >> directory
> >> >> >> > >>>> in both --pgdata and --xlogdir?
> >> >> >> > >
> >> >> >> > >>>I feel no need to prevent, even if user specifies both
> >> >> >> > >>>--pgdata
> >> >> >> and
> >> >> >> > >>>--xlogdir as same directory all the transaction log files
> >> >> >> > >>>will be created in the base directory instead of pg_xlog
> >> directory.
> >> >> >> > >
> >> >> >> > >
> >> >> >> > >
> >> >> >> > >>Given how easy it would be to prevent that, I think we should.
> >> >> It
> >> >> >> > >>would be an easy misunderstanding to specify that when you
> >> >> >> actually
> >> >> >> > >>want it in <wherever>/pg_xlog. Specifying that would be
> >> >> >> > >>redundant in the first place, but people ca do that, but it
> >> >> >> > >
> >> >> >> > >>would also be very easy to do it by mistake, and you'd end up
> >> >> >> > >>with something that's really bad, including a recursive
> >> symlink.
> >> >> >> > >
> >> >> >> > >
> >> >> >> > >
> >> >> >> > > Presently with initdb also user can specify both data and
> >> xlog
> >> >> >> > > directories as same.
> >> >> >> > >
> >> >> >> > > To prevent the data directory and xlog directory as same,
> >> >> >> > > there is
> >> >> >> a
> >> >> >> > > way in windows (_fullpath api) to get absolute path from a
> >> >> >> > > relative path, but I didn't get any such possibilities in
> >> linux.
> >> >> >> > >
> >> >> >> > > I didn't find any other way to check it, if anyone have any
> >> >> >> > > idea regarding this please let me know.
> >> >> >> >
> >> >> >> > What about make_absolute_path() in miscinit.c?
> >> >> >>
> >> >> >> The make_absoulte_patch() function gets the current working
> >> >> directory
> >> >> >> and adds The relative path to CWD, this is not giving proper
> >> >> absolute
> >> >> >> path.
> >> >> >>
> >> >> >> I have added a new function verify_data_and_xlog_dir_same() which
> >> >> >> will change the Current working directory to data directory and
> >> >> >> gets the CWD and the same way for transaction log directory.
> >> >> >> Compare the both data and xlog directories and throw an error.
> >> >> >> Please check it
> >> >> once.
> >> >> >>
> >> >> >> Is there any other way to identify that both data and xlog
> >> >> >> directories are pointing to the same Instead of comparing both
> >> >> absolute paths?
> >> >> >>
> >> >> >> Updated patch attached in the mail.
> >> >> >
> >> >> > Failure when the xlogdir doesn't exist is fixed in the updated
> >> >> > patch
> >> >> attached in the mail.
> >> >>
> >> >> Thanks for updating the patch!
> >> >>
> >> >> With the patch, when I specify the same directory in both -D and --
> >> >> xlogdir, I got the following error.
> >> >>
> >> >> $ cd /tmp
> >> >> $ pg_basebackup -D hoge --xlogdir=/tmp/hoge -X fetch
> >> >> pg_basebackup: could not change directory to "hoge": No such file or
> >> >> directory
> >> >
> >> > Thanks. After finding the xlog directory absolute path returning back
> >> > to current working Directory is missed, because of this reason it is
> >> > failing while finding the absolute Path for data directory. Apart
> >> from this change the code is reorganized a bit.
> >> >
> >> > Updated patch is attached in the mail. Please review it once.
> >>
> >> Thanks for newer version of the patch!
> >>
> >> I found that the empty base directory is created and remains even when
> >> the same directory is specified in both -D and --xlogdir and the error
> >> occurs.
> >> I think that it's
> >> better to throw an error in that case before creating any new directory.
> >> Thought?
> >
> > By creating the base directory only the patch finds whether provided
> base and
> > Xlog directories are same or not? To solve this problem following
> options are possible
> >
> > 1. No problem as it is just an empty base directory, so it can be reused
> in the next time
> > Leave it as it is.
> > 2. Once the error is identified, the base directory can be deleted.
> > 3. write a new function to remove the parent references from the
> provided path to identify
> > the absolute path used for detecting base and Xlog directories are
> same or not?
> >
> > Please provide your suggestions.
> >
> >> + xlogdir = get_absolute_path(xlog_dir);
> >>
> >> xlog_dir must be an absolute path. ISTM we don't do the above. No?
> >
> > It is required. As user can provide the path as
> /home/installation/bin/../bin/data.
> > The provided path is considered as absolute path only but while
> comparing the same
> > With data directory path it will not match.
>
> Okay, maybe I understand you. In order to know the real absolute path,
> basically
> we need to create the directory specified in --xlogdir, change the
> working directory
> to it and calculate the parent path. You're worried about the cases as
> follows, for example.
> Right?
>
> * path containing ".." like /aaa/bbb/../ccc is specified in --xlogdir
> * symbolic link is specified in --xlogdir
>
> On the second thought, I'm thinking that it might be overkill to add
> such not simple code for that small benefit.
>

What I actually was *looking* for when I said I thought we should have that
check, was just to find *manual* errors. As in, I wanted to cover the case
where the user said "-D /my/backup --xlogdir /my/backup", thinking that it
had to be specified. If they end up that way through a series of symlinks
or something like that, I think we can just punt that as user error and
they'll find out on their own later, we don't need to catch it and throw an
error.

--
Magnus Hagander
Me: http://www.hagander.net/
Work: http://www.redpill-linpro.com/