Re: LMDB crash consistency, again
Hallvard Breien Furuseth wrote:
Catching up with old mail...
On 20/10/14 12:44, Howard Chu wrote:
> This paper
> https://www.usenix.org/conference/osdi14/technical-sessions/presentation/...
>
> describes a potential crash vulnerability in LMDB due to its use of
> fdatasync instead of fsync when syncing writes to the data file. The
> vulnerability exists because fdatasync omits syncs of the file metadata;
> if the data file needed to grow as a result of any writes then this
> requires a metadata update.
Looks like an OS bug. fdatasync() should not break data integrity, it
may only skip metadata which are unneeded for retrieving the data. So
size changes are synced. So say the Posix spec and the Linux manpage.
Ah good point. If you check out their slides, #103 of 106 asks the
question; the only failure they found in LMDB occurred on ext3 (and not
on XFS) so we may just chalk this up to a flaw in ext3 instead.
Given that ext3 has already been superseded by ext4, this result of
theirs may not be all that useful in the real world. We already have
disrecommended ext3 for performance reasons, perhaps we should just note
this and move on.
--
-- Howard Chu
CTO, Symas Corp. http://www.symas.com
Director, Highland Sun http://highlandsun.com/hyc/
Chief Architect, OpenLDAP http://www.openldap.org/project/