[Lustre-discuss] Newbie question: File locking, synchronicity, order, and ownership

[Nochum Klein]

Hi Atul,

Thanks, I appreciate your help.  I apologize for not being clearer about my
intent.  The goal is to implement a fault-tolerant pair of application
component instances in a hot-warm configuration where the primary and
secondary instances exchange heartbeats to determine the health/availability
of their partner.  The partners would be on different nodes.  While the
primary instance is running it should have exclusive write access to the
application state.  If the configured number of heratbeats are missed, the
secondary component instance will try to retrieve the lock on
the application state (thereby becoming primary).  Given that networks are
often unreliable the design goal is that the clustered file system should
ensure that the secondary instance does not assume primary role while the
actual primary instance is still alive when a network disruption has
occurred.  So in a sense a controlled pingpong is actually the desired
effect (where the secondary and primary instances change roles whenever the
current primary instance fails).  Am I correct that the configuration
referenced below could support this behavior?

Thanks again for your help and your patience!

Nochum

On Sun, Jan 10, 2010 at 10:30 PM, Atul Vidwansa <Atul.Vidwansa at sun.com>wrote:

> Comments inline...
>
> Nochum Klein wrote:
>
>>
>> Hi Atul,
>>
>> Thanks a lot -- this is very helpful!
>>
>> So assuming the application is performing the following fcntl() call to
>> set a file segment lock:
>>
>>    struct flock fl;
>>    int err;
>>
>>    fl.l_type = F_WRLCK;
>>    fl.l_whence = 0;
>>    fl.l_start = 0;
>>    fl.l_len = 0;    /* len = 0 means until end of file */
>>
>>    err = fcntl(file, F_SETLK, &fl);
>>
>> I should be able to achieve the desired behavior
>>
>> What is your desired behavior ?
>
>
>> if I enable cluster-wide locking with the /-o flock/ mount option.  Is
>> this correct?
>>
>> Is your application writing to same file from multiple nodes? If yes, do
> writes from different nodes overlap? Above piece of code will work fine if
> each node is writing to its own file OR multiple nodes are writing to
> different sections of the file. Otherwise, it will result in lock pingpong.
>
> Cheers,
> _Atul
>
>>
>> Thanks again!
>>
>> Nochum
>>
>> ---------- Original message ----------
>>  From: Atul Vidwansa <Atul.Vidwa... at Sun.COM <mailto:Atul.Vidwa... at Sun.COM
>> >>
>> Date: Jan 8, 1:30 am
>> Subject: Newbie question: File locking, synchronicity, order, and
>> ownership
>> To: lustre-discuss-list
>>
>>
>> Some comments inline..
>>
>>
>>
>> Nochum Klein wrote:
>> > Hi Everyone,
>>
>> > Apologies for what is likely a simple question for anyone who has been
>> > working with Lustre for a while.  I am evaluating Lustre as part of a
>> > fault-tolerant failover solution for an application component.  Based
>> > on our design using heartbeats between the hot primary and warm
>> > secondary components, we have four basic requirements of the clustered
>> > file system:
>>
>> >    1. *Write Order *- The storage solution must write data blocks to
>> >       shared storage in the same order as they occur in the data
>> >       buffer.  Solutions that write data blocks in any other order
>> >       (for example, to enhance disk efficiency) do not satisfy this
>> >       requirement.
>> >    2.  *Synchronous Write Persistence* - Upon return from a
>> >       synchronous write call, the storage solution guarantees that all
>> >       the data have been written to durable, persistent storage.
>> >    3.  *Distributed File Locking* - Application components must be
>> >       able to request and obtain an exclusive lock on the shared
>> >       storage. The storage solution must not assign the locks to two
>> >       servers simultaneously.
>>
>> AFAIK Lustre does support distributed locking. From wiki.lustre.org <
>> http://wiki.lustre.org>:
>>
>>
>>    * /flock/lockf/
>>
>>    POSIX and BSD /flock/lockf/ system calls will be completely coherent
>>    across the cluster, using the Lustre lock manager, but are not
>>    enabled by default today. It is possible to enable client-local
>>    /flock/ locking with the /-o localflock/ mount option, or
>>    cluster-wide locking with the /-o flock/ mount option. If/when this
>>    becomes the default, it is also possible to disable /flock/ for a
>>    client with the /-o noflock/ mount option.
>>
>> >    1.  *Unique Write Ownership* - The application component that has
>> >       the file lock must be the only server process that can write to
>> >       the file. Once the system transfers the lock to another server,
>> >       pending writes queued by the previous owner must fail.
>>
>> It depends on what level of locking you do. Lustre supports byte-range
>> locking, so unless writes overlap, multiple writers can write to same
>> file.
>>
>> Cheers,
>> _Atul
>>
>>
>>
>>
>> >   1.
>>
>> > Can anyone confirm that these requirements would be met by Lustre 1.8?
>>
>> > Thanks a lot!
>>
>> > Nochum
>> > ------------------------------------------------------------------------
>>
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>>
>>
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