amcheck: Verify the logical consistency of PostgreSQL B-Tree indexes

Current version: 0.1

Author: Peter Geoghegan <[email protected]>

License: PostgreSQL license

Minimum supported version: PostgreSQL 9.4

Overview

The amcheck module provides functions that allow you to verify the logical consistency of the structure of PostgreSQL indexes. If the structure appears to be valid, no error is raised. Currently, only B-Tree indexes are supported, although since in practice the majority of PostgreSQL indexes are B-Tree indexes, amcheck is likely to be effective as a general corruption smoke-test in production PostgreSQL installations.

See "Using amcheck effectively" below for information about the kinds of real-world problems amcheck is intended to detect.

Invariants

amcheck provides functions that specifically verify various invariants in the structure of the representation of particular indexes. The correctness of the access method functions behind index scans and other important operations is predicated on these invariants always holding. For example, certain functions verify, among other things, that all B-Tree pages have items in "logical", sorted order (e.g., for B-Tree indexes on text, index tuples should be in collated lexical order). If that particular invariant somehow fails to hold, we can expect binary searches on the affected page to incorrectly guide index scans, resulting in wrong answers to SQL queries. Problems like this can be very subtle, and might otherwise remain undetected.

Verification is performed using the same procedures as those used by index scans themselves, which may be user-defined operator class code. For example, B-Tree index verification relies on comparisons made with one or more B-Tree support function 1 routines, much like B-Tree index scans rely on the routines to guide the scan to a point in the underlying table; see http://www.postgresql.org/docs/current/static/xindex.html for details of operator class support functions.

Project background

amcheck is proposed as a contrib extension for PostgreSQL 10, scheduled for release in 2017. This externally maintained version of the extension exists to target earlier versions of PostgreSQL (PostgreSQL 9.4+).

Bugs

Report bugs using the Github issue tracker. Feature requests will not accepted, since amcheck is targeted for future inclusion into PostgreSQL as a contrib module (however, amcheck may in the future support building packages on additional platforms).

Installation

Building using PGXS (generic)

The module can be built using the standard PGXS infrastructure. For this to work, you will need to have the pg_config program available in your $PATH.

If you are using a packaged PostgreSQL build and have pg_config available (and in your OS user's $PATH), the procedure is as follows:

tar xvzf amcheck-0.1.tar.gz
cd amcheck-0.1
make
make install

Note that just because pg_config is located in one user's $PATH does not necessarily make it so for the root user.

Building Debian/Ubuntu packages

The Makefile also provides a target for building Debian packages. The target has a dependency on debhelper, devscripts, postgresql-server-dev-all, and the PostgreSQL source package itself (e.g. postgresql-server-dev-9.4).

The packages can be created and installed from the amcheck directory as follows:

sudo aptitude install debhelper devscripts postgresql-server-dev-all
make deb
sudo dpkg -i ./build/postgresql-9.4-amcheck_*.deb

Setting up PostgreSQL

Once amcheck is built and installed, it should be created as a PostgreSQL extension in every database that requires it:

mydb=# CREATE EXTENSION amcheck;

amcheck functions may be used only by superusers.

Interface

The amcheck extension has a simple interface. amcheck consists of just a few functions that can be used for verification of a named B-Tree index. Note that currently, no function inspects the structure of the underlying heap representation (table).

regclass function arguments are used by amcheck to identify particular index relations. This allows amcheck to accept arguments using various SQL calling conventions:

  -- Use string literal regclass input:
  SELECT bt_index_check('pg_database_oid_index');
  -- Use oid regclass input (both perform equivalent verification):
  SELECT bt_index_check(2672);
  SELECT bt_index_check(oid) FROM pg_class
  WHERE relname = 'pg_database_oid_index';

See the PostgreSQL documentation on Object identifier types for more information.

bt_index_check(index regclass) returns void

bt_index_check tests that its target, a B-Tree index, respects a variety of invariants. Example usage:

  SELECT bt_index_check(c.oid), c.relname, c.relpages
  FROM pg_index i
  JOIN pg_opclass op ON i.indclass[0] = op.oid
  JOIN pg_am am ON op.opcmethod = am.oid
  JOIN pg_class c ON i.indexrelid = c.oid
  JOIN pg_namespace n ON c.relnamespace = n.oid
  WHERE am.amname = 'btree' AND n.nspname = 'pg_catalog'
  -- Don't check pg_class (bt_index_parent_check() requires this):
  AND c.relname NOT LIKE 'pg_class%'
  -- Don't check temp tables, which may be from another session:
  AND c.relpersistence != 't'
  -- Function may throw an error when this is omitted:
  AND i.indisready AND i.indisvalid
  ORDER BY c.relpages DESC LIMIT 10;

   bt_index_check |             relname             | relpages 
  ----------------+---------------------------------+----------
                  | pg_depend_reference_index       |       43
                  | pg_depend_depender_index        |       40
                  | pg_proc_proname_args_nsp_index  |       31
                  | pg_description_o_c_o_index      |       21
                  | pg_attribute_relid_attnam_index |       14
                  | pg_proc_oid_index               |       10
                  | pg_attribute_relid_attnum_index |        9
                  | pg_amproc_fam_proc_index        |        5
                  | pg_amop_opr_fam_index           |        5
                  | pg_amop_fam_strat_index         |        5

This example shows a session that performs verification of every catalog index in the database "test" (except those associated with the pg_class catalog). Details of just the 10 largest indexes verified are displayed. Since no error is raised, all indexes tested appear to be logically consistent. Naturally, this query could easily be changed to call bt_index_check for every index in the database where verification is supported. An AccessShareLock is acquired on the target index by bt_index_check. This lock mode is the same lock mode acquired on relations by simple SELECT statements.

bt_index_check does not verify invariants that span child/parent relationships, nor does it verify that the target index is consistent with its heap relation. When a routine, lightweight test for corruption is required in a live production environment, using bt_index_check often provides the best trade-off between thoroughness of verification and limiting the impact on application performance and availability.

bt_index_parent_check(index regclass) returns void

bt_index_parent_check tests that its target, a B-Tree index, respects a variety of invariants. The checks performed by bt_index_parent_check are a superset of the checks performed by bt_index_check. bt_index_parent_check can be thought of as a more thorough variant of bt_index_check: unlike bt_index_check, bt_index_parent_check also checks invariants that span parent/child relationships. However, it does not verify that the target index is consistent with its heap relation. bt_index_parent_check follows the general convention of raising an error if it finds a logical inconsistency or other problem.

An ExclusiveLock is required on the target index by bt_index_parent_check (a ShareLock is also acquired on the heap relation). These locks prevent concurrent data modification from INSERT, UPDATE, and DELETE commands. The locks also prevent the underlying relation from being concurrently processed by VACUUM (and other utility commands). Note that the function holds locks for as short a duration as possible, so there is no advantage to verifying each index individually in a series of transactions, unless long running queries happen to be of particular concern.

bt_index_parent_check's additional verification is more likely to detect various pathological cases. These cases may involve an incorrectly implemented B-Tree operator class used by the index that is checked, or, hypothetically, undiscovered bugs in the underlying B-Tree index access method code. Note that bt_index_parent_check cannot be called when Hot Standby is enabled (i.e., on read-only physical replicas), unlike bt_index_check.

Using amcheck effectively

Causes of corruption

amcheck can be effective at detecting various types of failure modes that data page checksums will always fail to catch. These include:

• Structural inconsistencies caused by incorrect operator class implementations.

This includes issues caused by the comparison rules of operating system collations changing. Comparisons of datums of a collatable type like text must be immutable (just as all comparisons used for B-Tree index scans must be immutable), which implies that operating system collation rules must never change.

Though rare, updates to operating system collation rules can cause these issues. More commonly, an inconsistency in the collation order between a master server and a standby server is implicated, possibly because the major operating system version in use is inconsistent. Such inconsistencies will generally only arise on standby servers, and so can generally only be detected on standby servers.

If a problem like this arises, it may not affect each individual index that is ordered using an affected collation, simply because indexed values might happen to have the same absolute ordering regardless of the behavioral inconsistency.

• Corruption caused by hypothetical undiscovered bugs in the underlying PostgreSQL access method code or sort code.

Automatic verification of the structural integrity of indexes plays a role in the general testing of new or proposed PostgreSQL features that could plausibly allow a logical inconsistency to be introduced. One obvious testing strategy is to call amcheck functions continuously when running the standard regression tests.

• Filesystem or storage subsystem faults where checksums happen to simply not be enabled.

Note that amcheck examines a page as represented in some shared memory buffer at the time of verification if there is only a shared buffer hit when accessing the block. Consequently, amcheck does not necessarily examine data read from the filesystem at the time of verification. Note that when checksums are enabled, amcheck may raise an error due to a checksum failure when a corrupt block is read into a buffer.

• Corruption caused by faulty RAM, and the broader memory subsystem and operating system.

PostgreSQL does not protect against correctable memory errors and it is assumed you will operate using RAM that uses industry standard Error Correcting Codes (ECC) or better protection. However, ECC memory is typically only immune to single-bit errors, and should not be assumed to provide absolute protection against failures that result in memory corruption.

Overhead

The overhead of calling bt_index_check for every index on a live production system is roughly comparable to the overhead of vacuuming; like VACUUM, verification uses a "buffer access strategy", which limits its impact on which pages are cached within shared_buffers. A major design goal of amcheck is to support routine verification of all indexes on busy production systems.

No amcheck routine will ever modify data, and so no pages will ever be "dirtied", which is not the case with VACUUM. On the other hand, amcheck may be required to verify a large number of indexes all at once, which is typically not a behavior that autovacuum exhibits. amcheck exhaustively accesses every page in each index verified. This behavior is useful in part because verification may detect a checksum failure, which may have previously gone undetected only because no process needed data from the corrupt page in question, including even an autovacuum worker process.

Note also that bt_index_check and bt_index_parent_check access the contents of indexes in "logical" order, which, in the worst case, implies that all I/O operations are performed at random positions on the filesystem. In contrast, VACUUM always removes dead index tuples from B-Tree indexes while accessing the contents of B-Tree indexes in sequential order.

Acting on information about corruption

No error concerning corruption raised by amcheck should ever be a false positive. In practice, amcheck is more likely to find software bugs than problems with hardware. amcheck raises errors in the event of conditions that, by definition, should never happen. It seems unlikely that there could ever be a useful general remediation to problems it detects.

In general, an explanation for the root cause of an invariant violation should be sought. The pageinspect tool can play a useful role in diagnosing corruption that amcheck highlights. A REINDEX may or may not be effective in repairing corruption, depending on the exact details of how the corruption originated.

In general, amcheck can only prove the presence of corruption; it cannot prove its absence.