[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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