[linux-lvm] fsync() and LVM

Marco Colombo linux-lvm at esiway.net
Mon Mar 16 14:36:18 UTC 2009

Charles Marcus wrote:
> On 3/14/2009 8:51 PM, Marco Colombo wrote:
>> Stuart D. Gathman wrote:
>>> On Sat, 14 Mar 2009, Dietmar Maurer wrote: 
>>> It just means that write barriers won't get passed to the device.
>>> This is only a problem if the devices have write caches.  Note 
>>> that with multiple devices, even a FIFO write cache could cause 
>>> reordering between devices (one device could finish faster than another).
>> No, it's more than that. PostgreSQL gurus say LVM doesn't honor fsync(),
>> that data doesn't even get to the controller, and it doesn't matter
>> if the disks have write caches enabled or not. Or if they have battery backed
>> caches. Please read the thread I linked. If what they say it's true,
>> you can't use LVM for anything that needs fsync(), including mail queues
>> (sendmail), mail storage (imapd), as such. So I'd really like to know.
> Seeing as my /var (with both postfix & courier-imap using it for mail
> storage) has been on lvm for almost 4 years, that would be news to me...
> ;)

Believe me or not, they both depend on fsync(). Anyway, even if you lost
a message, how do you expect to know? If you have any user base large
enough, you're used to 'missing' messages (99% of the user-deleted or
user-never-sent kind). A truly lost one may have gone missed in the noise.

A lying fsync() doesn't blow all your mail repository up, just you may
loose one/two messages on a crash. Or a transaction, speaking of databases.
If that's the case, I would like to know, that's all.


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