ext3/ext4 fsync hack (was: openldap.git branch mdb.master updated. 0018eeb2c3b2239c30def9d47c9d194a4ebf35fe)
4:04 a.m.
On 18/12/14 05:40, openldap-commit2devel(a)OpenLDAP.org wrote:
commit 0018eeb2c3b2239c30def9d47c9d194a4ebf35fe
Author: Howard Chu <hyc(a)openldap.org>
Date: Thu Dec 18 04:38:53 2014 +0000
Hack for potential ext3/ext4 corruption issue
Use regular fsync() if we think this commit grew the DB file.
This does not catch all cases:
If the new pages below mt_next_pgno were freed instead of
written, me_size becomes too big. Later when the file does
grow, me_size may be >= actual filesize so it fdatasync()s.
Similar to b09e46904c1c059bd5086243e3915b6be510e57d
"ITS#7886 fix mdb_copy write size".
We can fix me_size, grow the file anyway (ftruncate), or
give the pages back to mt_next_pgno in mdb_freelist_save().
Another issue: After an MDB_NOSYNC commit, mdb_env_sync()
only fdatasync()s. It does not know when the file grew.
The planned "group commits" may get the same problem if
the user checkpoints with mdb_env_sync().
6:18 a.m.
New subject: ext3/ext4 fsync hack
Hallvard Breien Furuseth wrote:
On 18/12/14 05:40, openldap-commit2devel(a)OpenLDAP.org wrote:
> commit 0018eeb2c3b2239c30def9d47c9d194a4ebf35fe
> Author: Howard Chu <hyc(a)openldap.org>
> Date: Thu Dec 18 04:38:53 2014 +0000
>
> Hack for potential ext3/ext4 corruption issue
>
> Use regular fsync() if we think this commit grew the DB file.
This does not catch all cases:
If the new pages below mt_next_pgno were freed instead of
written, me_size becomes too big.
Huh? mt_next_pgno definitively tells how many pages have ever been used
in the DB file.
Later when the file does
grow, me_size may be >= actual filesize so it fdatasync()s.
Similar to b09e46904c1c059bd5086243e3915b6be510e57d
"ITS#7886 fix mdb_copy write size".
We can fix me_size, grow the file anyway (ftruncate), or
give the pages back to mt_next_pgno in mdb_freelist_save().
Another issue: After an MDB_NOSYNC commit, mdb_env_sync()
only fdatasync()s. It does not know when the file grew.
I suppose we can change the FORCE flag to also cause fsync() to be used.
The planned "group commits" may get the same problem if
the user checkpoints with mdb_env_sync().
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/
6:27 a.m.
New subject: ext3/ext4 fsync hack
On 01/06/2015 03:18 PM, Howard Chu wrote:
Hallvard Breien Furuseth wrote:
> (....)
> If the new pages below mt_next_pgno were freed instead of
> written, me_size becomes too big.
Huh? mt_next_pgno definitively tells how many pages have ever been used in
the DB file.
No, see ITS#7886:
"Allocate an ovpage from mt_next_pgno, mdb_ovpage_free() it
and commit: The datafile may end before MDB_meta.mm_last_pg
since the ovpage was never written. mdb_env_copyfd() & co
break when they read the file to mm_last_pg."
> Later when the file does
> grow, me_size may be >= actual filesize so it fdatasync()s.
> Similar to b09e46904c1c059bd5086243e3915b6be510e57d
> "ITS#7886 fix mdb_copy write size".
> We can fix me_size, grow the file anyway (ftruncate), or
> give the pages back to mt_next_pgno in mdb_freelist_save().
--
Hallvard
6:40 a.m.
New subject: ext3/ext4 fsync hack
Sorry, forgot this one.
On 01/06/2015 03:18 PM, Howard Chu wrote:
Hallvard Breien Furuseth wrote:
> Another issue: After an MDB_NOSYNC commit, mdb_env_sync()
> only fdatasync()s. It does not know when the file grew.
I suppose we can change the FORCE flag to also cause fsync() to be used.
Insufficient if the user commits with MDB_NOSYNC (maybe when creating
the DB), then turns off MDB_NOSYNC and does mdb_env_sync(env, 0).
Or another process without MDB_NOSYNC doing mdb_env_sync(env, 0).
The lockfile could track what has been synced how, though.
Except it won't know at init, so if someone does a lot of
<open env, commit, close env> they'll end up fsync'ing each time.
--
Hallvard
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