fbrescue

internal Firebird consistency check (decompression overran buffer (179))

The full message usually reads internal Firebird consistency check (decompression overran buffer (179), file: sqz.cpp line: ...), in a query, in gbak or in gfix -v.

What it means

Firebird stores each record compressed with a simple run-length encoding. When it unpacks a record and the result does not fit the record’s layout, the stored bytes are damaged: by a failing disk, a bad copy, a crash in the middle of a write, or a page overwritten with something else. The engine stops the statement that touched the record, so a SELECT or a backup of that table fails at the same place every time. The other records of the table are usually fine.

First: make a copy, and work only on the copy

Stop the Firebird server, or at least make sure nothing is connected to the database. Then copy the file, together with any .delta file next to it (an nbackup lock) and the secondary files of a multi-file database. A copy taken while the server writes to the file is inconsistent, and every repair attempt changes the file it runs on. If the disk itself is failing, copy the file to another disk before anything else.

Then: try Firebird’s own tools

These come with every Firebird installation and often get a database back as a whole: indexes, triggers, users and all. On the copy:

gfix -v -full -n copy.fdb            # validate, report only
gfix -mend -full -ignore copy.fdb    # let gbak skip damaged records
gbak -b -v -ig -g copy.fdb copy.fbk  # back up: no checksums, no GC
gbak -c -v copy.fbk restored.fdb     # restore into a new database

Add -user SYSDBA -password ... as your server needs. If gbak -b completes, the restored database is the result: check the row counts of the important tables. gfix -mend throws away the records it cannot read, so compare with the original before you rely on the restored copy.

gfix -mend is the engine’s answer to exactly this error: it marks the damaged record so the next backup skips it.

If gbak still fails: get the data out with fbrescue

fbrescue reads the database file directly, without the Firebird engine, and exports every row it can read. It does not stop at the first damaged record or page, and it never writes to the file. Download the single binary for your system from Releases (Windows, Linux, macOS; nothing to install).

fbrescue scan copy.fdb                               # what is damaged, by table
fbrescue export copy.fdb --out rescued --format sql  # schema.sql + INSERTs per table

--format csv and --format jsonl are there too. Rows that could not be read are listed in rescued/_errors.jsonl with the page and the reason, and the exit status is 2 when anything was left out. To load the result into a new database:

isql -ch UTF8 -i rescued/schema.sql new.fdb
isql -ch UTF8 -i rescued/CUSTOMERS.sql new.fdb  # and so on for each table

If you would like help, open an issue and attach the output of fbrescue report copy.fdb: it describes the damage without any of your data, and with table and column names replaced by numbers.

For this error fbrescue reads every record of the table, lists the ones that do not unpack in _errors.jsonl with their page and line, and exports the rest. A record that continues on another page (records larger than a page) is lost only if one of its pieces is damaged.