[Gluster-devel] Gluster driver for Archipelago - Development process

Anand Avati avati at gluster.org
Wed Dec 4 01:15:13 UTC 2013

On Tue, Dec 3, 2013 at 7:34 AM, Vijay Bellur <vbellur at redhat.com> wrote:

> Adding gluster-devel as there is a good amount of detail on the ongoing
> integration with Archipelago.
> 1. There are no async operations for open/create/close/stat/unlink,
>>>> which are necessary for various operations of Archipelago.
>>> Is there more description on how various operations of Archipelago
>>> rely on async operations for open/create etc.? I must admit that I
>>> haven't gone through your code but will definitely do so to get a
>>> better understanding.
>> Sure, I 'll explain our rationale but first, let me provide some insight
>> on the fundamental logic of Archipelago to understand the context on
>> which we operate:
>> An Archipelago volume is a COW volume, consisting of many contiguous
>> pieces (volume chunks), typically 4MB in size. It is COW since it may
>> share read-only chunks with other volumes (e.g. if the volumes are
>> created from the same OS image) and creates new chunks to write to
>> them. In order to refer to a chunk, we assign a name to it (e.g.
>> volume1_0002) which can be considered as the object name (Rados) or file
>> name (Gluster, NFS).
>> The above logic is handled by separate Archipelago entities (mappers,
>> volume composers). This means that the storage driver’s only task is to
>> read/write chunks from and to the storage. Also, given that there is one
>> such driver per host - where 50 VMs can be running - means that it must
>> handle a lot of chunks.
>> Now, back to our storage driver and the need for full asynchronism. When
>> it receives a read/write request for a chunk, it will generally need to
>> open the file, create it if it doesn’t exist, perform the I/O and
>> finally close the file. Having non-blocking read/write but blocking
>> open/create/close essentially makes this request a blocking request.
>> This means that if the driver supports e.g. 64 in-flight requests, it
>> needs to have 64 threads to be able to manage all of them.
> open/create/close are not completely synchronous in gluster with
> open-behind and write-behind translators loaded in the client side graph.
> open-behind and write-behind translators by default are part of the client
> side graph in GlusterFS. With open-behind translator, open() system call is
> short circuited by GlusterFS and the actual open happens in the background.
> For create, an open with O_CREAT | O_EXCL flags would be handled by
> open-behind. Similarly an actual close is done in the background by the
> write-behind translator. As such, Archipelago should not experience
> significant latency with open & close operations.

A create (O_CREAT with or without O_EXCL) is currently not handled by
open-behind and will always be synchronous with a network round trip. An
open() on an existing file is cut-short by open-behind. But even that might
not be sufficient because the path resolver (lookup() etc.) always works
synchronously with network round-drops for path based operations. We will
need to design new APIs for true async path based operations (with path
resolver also executed asynchronously).

> Let’s assume that open/create/close are indeed non-blocking or virtually
>> nonexistent [1]. Most importantly, this would greatly reduce the
>> read/write latency, especially for 4k requests. Another benefit is the
>> ability to use a much smaller number of threads. However, besides
>> read/write, there are also other operations that the driver must support
>> such as stat()ing or deleting a file. If these operations are
>> blocking, this means that a spurious delete and stat can stall our
>> driver. Once more, it needs to have a lot of threads to be operational.
Currently stat() and unlink() are synchronous too.

Note that internally all the operations in gluster are asynchronous in
their true nature. gfapi provides "convenience wrappers" for these calls in
a synchronous way. It is trivial to expose the asynchronous calls through
gfapi, but we haven't done so for the path based operations because there
hasn't been a need thus far. And without even a single consumer, we did not
want to reason about the semantics of async path calls.

Do you have an example driver/header which shows the ideal behavior for the
async path based calls? Will you provide a context pointer and expect to
receive it in the callback? Or do you expect the API to return a stub for
the async call dispatch and poll on it?

 2. There is no way to create notifications on a file (as Rados can
>>>> with its objects).
>>> How are these notifications consumed?
>> They are consumed by the lock/unlock operations that are also handled by
>> our driver. For instance, the Rados driver can wait asynchronously for
>> someone to unlock an object by registering a watch to the object and a
>> callback function. Conversely, the unlock operation makes sure to send a
>> notification to all watchers of the object. Thus, the lock/unlock
>> operation can happen asynchronously [2].
>> I have read that Gluster supports Posix locks, but this is not the
>> locking scheme we have in mind. We need a persistent type of lock that
>> would stay on a file even if the process closed the file descriptor or
>> worse, crashed.
> How would you recover if a process that held the lock goes away forever?
> We can provide an option to make this kind of behavior possible with
> posix-locks translator.
>  Our current solution is to  create a “lock file” e.g.
>> “volume1_0002_lock” with the owner name written in it. Thus, the
>> lock/unlock operations generally happen as follows:
>> a) Lock: Try to exclusively create a lock file. If successful, write the
>> owner id to it. If not, sleep for 1 second and retry.
>> b) Unlock: Read a lock file and its owner. If we are the owner, delete
>> it. Else, fail.
>> As you can see, this is not an elegant way and is subject to race
>> conditions. If Gluster can provide a better solution, we would be more
>> than happy to know about it.
Currently our locking API exposed through gfapi is POSIX-like (synchronous,
non-persistent). We have other internal locking mechanisms (entry-lk for
locking an abstract name/string in a directory and inode-lk - nested range
locks in a file) which are currently not exposed through gfapi. But these
are not persistent either. Providing async API version of these calls is
not hard (if we have a good understanding about things like whether the
caller provides a context pointer or expects a call specific stub from the
API etc). However I'm not sure (yet) how to provide persistent locks (and
what the right behavior should be in the event of crash of the caller). You
may be able to simulate (somewhat) persistent locks in the driver using a
combination of sync/async locking APIs + xattrs.

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