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Re: [linux-lvm] fsync() and LVM



On Mon, Mar 16, 2009 at 2:50 PM, Les Mikesell <lesmikesell gmail com> wrote:
> Stuart D. Gathman wrote:
>>
>>> No, it's more than that. PostgreSQL gurus say LVM doesn't honor fsync(),
>>
>> That is clearly wrong - since fsync() isn't LVM's responsibility.
>> I think they mean that fsync() can't garrantee that any writes are
>> actually on the platter.
>>
>>> 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,
>>
>> That is clearly wrong.  If writes don't work, nothing works.
>>
>>> 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.  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.

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.

Greg
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