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Index corruption. Validation put "missing entries" message into firebird.log [CORE3515] #3873

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firebird-issue-importer opened this issue Jun 5, 2011 · 10 comments

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@firebird-issue-importer firebird-issue-importer commented Jun 5, 2011

Submitted by: @hvlad

Relate to CORE3921

Imagine a table T with two indices PK (unique, it is important) and IDX2 (in my case it is very bad index with 2 different values only but this is not so important).

So, we have :

CREATE TABLE T (
ID INT NOT NULL,
VAL INT
);

ALTER TABLE T ADD CONSTRAINT PK PRIMARY KEY ON (ID);
CREATE INDEX IDX2 ON T (VAL);

INSERT INTO T VALUES (1, 0);
COMMIT;

The sequence of actions is following:

1. tx1: insert into T values (1, 0)
2. tx1: VIO_store
returns OK, new record have recno = 1
3. tx1: IDX_store
4. tx1: insert_key
index == PK, key == 1, recno == 1
returns duplicate error
i.e. we have unique violation in index PK
note, there was no attempt to insert key into index IDX2

5. tx1: VIO_backout
6. tx1: delete_record ... OK

7. tx2: insert into T values (2, 0)
8. tx2: VIO_store ...
returns OK, new record have recno = 1, yes same recno !!!
9. tx2: IDX_store
10. tx2: insert_key
index == PK, key == 2, recno == 1
returns OK
11. tx2: insert_key ... returns OK
index == IDX2, key == 0, recno == 1
returns OK
12. tx2: commit

13. tx1: IDX_garbage_collect
14. tx1: BTR_remove
index == PK, key == 1, recno == 1
15. tx1: BTR_remove
index == IDX2, key == 0, recno == 1
here we removed not ours index entry !!!

So, we have at disk \(ignoring pre\-existing record with recno = 0\): 

after (4) : recno = 1, PK : entry {key = 1, rec = 1}, IDX2 : no entries
after (6) : no records, PK : entry {1, 1}, IDX2 : no entries
after (12) : recno = 1, PK : entries {1, 1} and {2, 1}, IDX2 : entry {0, 1}
after (14) : recno = 1, PK : entry {2, 1}, IDX2 : entry {0, 1}
after (15) : recno = 1, PK : entry {1, 1}, IDX2 : no entries

and finally we have missed entry at index IDX2 for record 1.

The issue happens when all conditions are met at the same time :

a) first insert violates indexed constraint (unique or foreign key)
b) this indexed constraint is not a physically last index, so some indices have no entries for the failed record
c) VIO_backout deleted record and at the same time new record is inserted at the same slot on data page and have assigned the same record number
d) at least one index from the second group in (b) have the same key value in the new record as in failed record
e) second insert completes before backout started to remove index entries of failed record

I think the problem could happen no only with two inserts, but with two updates too (first update failed, backout started and second update came at inappropriate moment). Also it seems possible to have "blob not found" errors by the same reason.

To fix the issue i offer to delay physical record removal (in case of backout) until all going index entries will be removed. This will prevent concurrent insert from creating new record with the same record number until backout maintains indices.

To do it, i offer to split backout on the two phases. At the first phase don't remove record from disk but mark it with current transaction number and flag rpb_gc_active. Then cleanup indices and after remove backed out record version completely.

It seems safe as :

a) marking record with current transaction number prevents concurrent backouts
b) marking record with rpb_gc_active flag allow readers to skip this record version and read previous one (which will be primary record version after backout completes)
c) if our process will die during backout, next process will see our transaction as dead and will start all over again

The patch passed my tests and also runs in production over month by the Pavel Zotov who reported the bug and made great help to investigate it.

Commits: af4fab8 5ac9733 945a1bd

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@firebird-issue-importer firebird-issue-importer commented Jun 5, 2011

Modified by: @hvlad

assignee: Vlad Khorsun [ hvlad ]

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@firebird-issue-importer firebird-issue-importer commented Jun 5, 2011

Modified by: @hvlad

status: Open [ 1 ] => Resolved [ 5 ]

resolution: Fixed [ 1 ]

Fix Version: 2.5.1 [ 10333 ]

Fix Version: 3.0 Alpha 1 [ 10331 ]

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@firebird-issue-importer firebird-issue-importer commented Jun 10, 2011

Commented by: Sean Leyne (seanleyne)

Edit for formatting/readability

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@firebird-issue-importer firebird-issue-importer commented Jun 10, 2011

Modified by: Sean Leyne (seanleyne)

description:
Imagine a table T with two indices PK (unique, it is important) and IDX2 (in my case
it is very bad index with 2 different values only but this is not so important).
So, we have :

CREATE TABLE T (
ID INT NOT NULL,
VAL INT
);

ALTER TABLE T ADD CONSTRAINT PK PRIMARY KEY ON (ID);
CREATE INDEX IDX2 ON T (VAL);

INSERT INTO T VALUES (1, 0);
COMMIT;

The sequence of actions is following:

1. tx1: insert into T values (1, 0)
2. tx1: VIO_store
returns OK, new record have recno = 1
3. tx1: IDX_store
4. tx1: insert_key
index == PK, key == 1, recno == 1
returns duplicate error
i.e. we have unique violation in index PK
note, there was no attempt to insert key into index IDX2

5. tx1: VIO_backout
6. tx1: delete_record ... OK

7. tx2: insert into T values (2, 0)
8. tx2: VIO_store ...
returns OK, new record have recno = 1, yes same recno !!!
9. tx2: IDX_store
10. tx2: insert_key
index == PK, key == 2, recno == 1
returns OK
11. tx2: insert_key ... returns OK
index == IDX2, key == 0, recno == 1
returns OK
12. tx2: commit

13. tx1: IDX_garbage_collect
14. tx1: BTR_remove
index == PK, key == 1, recno == 1
15. tx1: BTR_remove
index == IDX2, key == 0, recno == 1
here we removed not ours index entry !!!

So, we have at disk \(ignoring pre\-existing record with recno = 0\): 

after (4) : recno = 1, PK : entry {key = 1, rec = 1}, IDX2 : no entries
after (6) : no records, PK : entry {1, 1}, IDX2 : no entries
after (12) : recno = 1, PK : entries {1, 1} and {2, 1}, IDX2 : entry {0, 1}
after (14) : recno = 1, PK : entry {2, 1}, IDX2 : entry {0, 1}
after (15) : recno = 1, PK : entry {1, 1}, IDX2 : no entries

and finally we have missed entry at index IDX2 for record 1.

The issue happens when all conditions are met at the same time :

a) first insert violates indexed constraint (unique or foreign key)
b) this indexed constraint is not a physically last index, so some indices have
no entries for the failed record
c) VIO_backout deleted record and at the same time new record is inserted at
the same slot on data page and have assigned the same record number
d) at least one index from the second group in (b) have the same key value in
the new record as in failed record
e) second insert completes before backout started to remove index entries of
failed record

I think the problem could happen no only with two inserts, but with two 

updates too (first update failed, backout started and second update came at
inappropriate moment). Also it seems possible to have "blob not found" errors
by the same reason.

To fix the issue i offer to delay physical record removal \(in case of backout\) until 

all going index entries will be removed. This will prevent concurrent insert from
creating new record with the same record number until backout maintains indices.

To do it, i offer to split backout on the two phases\. At the first phase don't remove 

record from disk but mark it with current transaction number and flag rpb_gc_active.
Then cleanup indices and after remove backed out record version completely.

It seems safe as :
a) marking record with current transaction number prevents concurrent backouts
b) marking record with rpb_gc_active flag allow readers to skip this record
version and read previous one (which will be primary record version after
backout completes)
c) if our process will die during backout, next process will see our transaction
as dead and will start all over again

The patch passed my tests and also runs in production over month by the Pavel 

Zotov who reported the bug and made great help to investigate it.

=>

Imagine a table T with two indices PK (unique, it is important) and IDX2 (in my case it is very bad index with 2 different values only but this is not so important).

So, we have :

CREATE TABLE T (
ID INT NOT NULL,
VAL INT
);

ALTER TABLE T ADD CONSTRAINT PK PRIMARY KEY ON (ID);
CREATE INDEX IDX2 ON T (VAL);

INSERT INTO T VALUES (1, 0);
COMMIT;

The sequence of actions is following:

1. tx1: insert into T values (1, 0)
2. tx1: VIO_store
returns OK, new record have recno = 1
3. tx1: IDX_store
4. tx1: insert_key
index == PK, key == 1, recno == 1
returns duplicate error
i.e. we have unique violation in index PK
note, there was no attempt to insert key into index IDX2

5. tx1: VIO_backout
6. tx1: delete_record ... OK

7. tx2: insert into T values (2, 0)
8. tx2: VIO_store ...
returns OK, new record have recno = 1, yes same recno !!!
9. tx2: IDX_store
10. tx2: insert_key
index == PK, key == 2, recno == 1
returns OK
11. tx2: insert_key ... returns OK
index == IDX2, key == 0, recno == 1
returns OK
12. tx2: commit

13. tx1: IDX_garbage_collect
14. tx1: BTR_remove
index == PK, key == 1, recno == 1
15. tx1: BTR_remove
index == IDX2, key == 0, recno == 1
here we removed not ours index entry !!!

So, we have at disk \(ignoring pre\-existing record with recno = 0\): 

after (4) : recno = 1, PK : entry {key = 1, rec = 1}, IDX2 : no entries
after (6) : no records, PK : entry {1, 1}, IDX2 : no entries
after (12) : recno = 1, PK : entries {1, 1} and {2, 1}, IDX2 : entry {0, 1}
after (14) : recno = 1, PK : entry {2, 1}, IDX2 : entry {0, 1}
after (15) : recno = 1, PK : entry {1, 1}, IDX2 : no entries

and finally we have missed entry at index IDX2 for record 1.

The issue happens when all conditions are met at the same time :

a) first insert violates indexed constraint (unique or foreign key)
b) this indexed constraint is not a physically last index, so some indices have no entries for the failed record
c) VIO_backout deleted record and at the same time new record is inserted at the same slot on data page and have assigned the same record number
d) at least one index from the second group in (b) have the same key value in the new record as in failed record
e) second insert completes before backout started to remove index entries of failed record

I think the problem could happen no only with two inserts, but with two updates too (first update failed, backout started and second update came at inappropriate moment). Also it seems possible to have "blob not found" errors by the same reason.

To fix the issue i offer to delay physical record removal (in case of backout) until all going index entries will be removed. This will prevent concurrent insert from creating new record with the same record number until backout maintains indices.

To do it, i offer to split backout on the two phases. At the first phase don't remove record from disk but mark it with current transaction number and flag rpb_gc_active. Then cleanup indices and after remove backed out record version completely.

It seems safe as :

a) marking record with current transaction number prevents concurrent backouts
b) marking record with rpb_gc_active flag allow readers to skip this record version and read previous one (which will be primary record version after backout completes)
c) if our process will die during backout, next process will see our transaction as dead and will start all over again

The patch passed my tests and also runs in production over month by the Pavel Zotov who reported the bug and made great help to investigate it.

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@firebird-issue-importer firebird-issue-importer commented Jun 15, 2011

Commented by: @hvlad

Backported into v2.1.5

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@firebird-issue-importer firebird-issue-importer commented Jun 15, 2011

Modified by: @hvlad

Fix Version: 2.1.5 [ 10420 ]

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@firebird-issue-importer firebird-issue-importer commented Apr 23, 2013

Modified by: @pcisar

status: Resolved [ 5 ] => Closed [ 6 ]

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@firebird-issue-importer firebird-issue-importer commented Jul 9, 2013

Modified by: @dyemanov

Link: This issue relate to CORE3921 [ CORE3921 ]

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@firebird-issue-importer firebird-issue-importer commented Jan 18, 2016

Modified by: @pavel-zotov

QA Status: No test

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@firebird-issue-importer firebird-issue-importer commented Mar 12, 2016

Modified by: @pavel-zotov

status: Closed [ 6 ] => Closed [ 6 ]

QA Status: No test => Cannot be tested

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