[linux-lvm] fsync() and LVM

Les Mikesell lesmikesell at gmail.com
Mon Mar 16 20:28:51 UTC 2009

Greg Freemyer wrote:
>>>> 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.
>>> fsync() is a file system call that writes dirty buffers, and then waits
>>> for the physical writes to complete.  It is only the waiting part that
>>> is broken.
>> It's a yes or no question...  Fsync() either guarantees that the write is
>> committed to physical media so the application can continue knowing that
>> it's own transactional expectations are met (i.e. you can crash and recover
>> that piece of data), or it is broken.  If it doesn't wait for completion, it
>> can't possibly report the correct status.
> This discussion seems a bit bizarre to me.

You can't avoid a discussion of expected but missing functionality.

> Many apps require data get
> to stable memory in a well defined way.  Barriers is certainly one way
> to do that, but I don't think barriers are supported by LVM, mdraid,
> or drbd.
> Those are some very significant subsystems.  I have to believe
> filesystems have another way to implement fsync if barriers are not
> supported in the stack of block susbsystems.

If you can't get the completion status from the underlying layer, how 
can a filesystem possibly implement it?

> Maybe this discussion needs to move to a filesystem list, since it is
> the filesystem that is responsible for making fsync() work even in the
> absence of barriers.

I though linux ended up doing a sync of the entire outstanding buffered 
data for a partition with horrible performance, at least on ext3.

   Les Mikesell
    lesmikesell at gmail.com

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