Re: WIP: Restricting pg_rewind to data/wal dirs

From: Chris Travers <chris(dot)travers(at)adjust(dot)com>
To: PostgreSQL mailing lists <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: WIP: Restricting pg_rewind to data/wal dirs
Date: 2017-11-01 08:58:05
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Attached is the patch as submitted for commitfest.

Please note, I am not adverse to adding an additional --Include-path
directive if that avoids backwards-compatibility problems. However the
patch is complex enough I would really prefer review on the rest of it to
start first. This doesn't strike me as perfectly trivial and I think it is
easier to review pieces separately. I have already started on the
--Include-path directive and could probably get it attached to a later
version of the patch very soon.

I would also like to address a couple of important points here:

1. I think default restrictions plus additional paths is the best, safest
way forward. Excluding shell-globs doesn't solve the "I need this
particular config file" very well particularly if we want to support this
outside of an internal environment. Shell globs also tend to be overly
inclusive and so if you exclude based on them, you run into a chance that
your rewind is corrupt for being overly exclusive.

2. I would propose any need for an additional paths be specified using an
--Include-path directive. This could be specified multiple times and could
point to a file or directory which would be added to the paths rewound. I
have a patch for this mostly done but I am concerned that these sorts of
changes result in a combination of changes that are easier to review
separately than together. So if needed, I can add it or in a separate
patch following.

3. I think it would be a mistake to tie backup solutions in non-replicated
environments to replication use cases, and vice versa. Things like
replication slots (used for streaming backups) have different
considerations in different environments. Rather than build the same
infrastructure first, I think it is better to support different use cases
well and then build common infrastructure to support the different cases.
I am not against building common infrastructure for pg_rewind and
pg_basebackup. I am very much against having the core guarantees being the
exact same.

Best Wishes,
Chris Travers

On Sat, Oct 28, 2017 at 1:22 PM, Chris Travers <chris(dot)travers(at)adjust(dot)com>

> Hi;
> There are still some cleanup bits needed here but I wanted to get feedback
> on my general direction.
> I hope to submit for commit fest soon if the general feedback is good.
> Tests are passing (with adjustments intended for change of behaviour in one
> test script). I want to note that Crimson Thompson (also with Adjust) has
> provided some valuable discussion on this code.
> The Problem:
> pg_rewind makes no real guarantees regarding the final state of non-data
> files or directories. It.checks to see if the timeline has incremented
> (and therefore guarantees that if successful the data directories are on
> the same timeline) but for non-data files, these are clobbered if we rewind
> and left intact if not. These other files include,
> replication slots, and can include log files.
> Copying logs over to the new slave is something one probably never wants
> to do (same with replication slots), and the behaviours here can mask
> troubleshooting regarding what a particular master failed, cause wal
> segments to build up, automatic settings changes, and other undesirable
> behaviours. Together these make pg_rewind very difficult to use properly
> and push tasks to replication management tooling that the management tools
> are not in a good position to handle correctly.
> Designing the Fix:
> Two proposed fixes have been considered and one selected: Either we
> whitelist directories and only rewind those. The other was to allow shell
> globs to be provided that could be used to exclude files. The whitelisting
> solution was chosen for a number of reasons.
> When pg_rewind "succeeds" but not quite correctly the result is usually a
> corrupted installation which requires a base backup to replace it anyway.
> In a recovery situation, sometimes pressures occur which render human
> judgment less effective, and therefore glob exclusion strikes me as
> something which would probably do more harm than good, but maybe I don't
> understand the use case (comments as to why some people look at the other
> solution as preferable would be welcome).
> In going with the whitelisting solution, we chose to include all
> directories with WAL-related information. This allows more predicable
> interaction with other parts of the replication chain. Consequently not
> only do we copy pg_wal and pg_xact but also commit timestamps and so forth.
> The Solution:
> The solution is a whitelist of directories specified which are the only
> ones which are synchronised. The relevant part of this patch is:
> +/* List of directories to synchronize:
> + * base data dirs (and ablespaces)
> + * wal/transaction data
> + * and that is it.
> + *
> + * This array is null-terminated to make
> + * it easy to expand
> + */
> +
> +const char *rewind_dirs[] = {
> + "base",
> + "global",
> + "pg_commit_ts",
> + "pg_logical",
> + "pg_multixact",
> + "pg_serial",
> + "pg_subtrans",
> + "pg_tblspc",
> + "pg_twophase",
> + "pg_wal",
> + "pg_xact",
> + NULL
> +};
> From there we iterate over this array for directory-based approaches in
> copy_fetch.c and with one query per directory in libpq_fetch.c. This also
> means shifting from the basic interface from PQexec to PQexecParams. It
> would be possible to move to binary formats too, but this was not done
> currently in order to simplify code review (that could be a separate
> independent patch at a later time).
> Testing Done:
> The extra files tests correctly test this change in behaviour. The tests
> have been modified, and diagnostics in cases of failures expanded, in this
> case. The other tests provide good coverage of whether pg_rewind does what
> it is supposed to do.
> Cleanup still required:
> I accidentally left Carp::Always in the PM in this perl module. It will
> be fixed.
> I renamed one of the functions used to have a more descriptive name but
> currently did not remove the old function yet.
> Feedback is very welcome. pg_rewind is a very nice piece of software. I
> am hoping that these sorts of changes will help ensure that it is easier to
> use and provides more predictable results.
> --
> Best Regards,
> Chris Travers
> Database Administrator
> Tel: +49 162 9037 210 <+49%20162%209037210> | Skype: einhverfr |
> Saarbrücker Straße 37a, 10405 Berlin

Best Regards,
Chris Travers
Database Administrator

Tel: +49 162 9037 210 | Skype: einhverfr |
Saarbrücker Straße 37a, 10405 Berlin

Attachment Content-Type Size
pg_rewind_restrict_dirs.v2.patch application/octet-stream 11.0 KB

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