On Tue, May 8, 2012 at 2:34 AM, Xavier Hernandez <xhernandez@xxxxxxxxxx> wrote:
The goals you state above are all valid. What would really help (adoption) is if you can implement this as a modification of AFR by utilizing all the work already done, and you get brownie points if it is backward compatible with existing AFR. If you already have any code in a publishable state, please share it with us (github link?).
Avati
Hello developers,
I would like to expose some ideas we are working on to create a new kind of translator that should be able to unify and simplify to some extent the healing procedures of complex translators.
Currently, the only translator with complex healing capabilities that we are aware of is AFR. We are developing another translator that will also need healing capabilities, so we thought that it would be interesting to create a new translator able to handle the common part of the healing process and hence to simplify and avoid duplicated code in other translators.
The basic idea of the new translator is to handle healing tasks nearer the storage translator on the server nodes instead to control everything from a translator on the client nodes. Of course the heal translator is not able to handle healing entirely by itself, it needs a client translator which will coordinate all tasks. The heal translator is intended to be used by translators that work with multiple subvolumes.
I will try to explain how it works without entering into too much details.
There is an important requisite for all client translators that use healing: they must have exactly the same list of subvolumes and in the same order. Currently, I think this is not a problem.
The heal translator treats each file as an independent entity, and each one can be in 3 modes:
1. Normal mode
This is the normal mode for a copy or fragment of a file when it is synchronized and consistent with the same file on other nodes (for example with other replicas. It is the client translator who decides if it is synchronized or not).2. Healing mode
This is the mode used when a client detects an inconsistency in the copy or fragment of the file stored on this node and initiates the healing procedures.3. Provider mode (I don't like very much this name, though)
This is the mode used by client translators when an inconsistency is detected in this file, but the copy or fragment stored in this node is considered good and it will be used as a source to repair the contents of this file on other nodes.Initially, when a file is created, it is set in normal mode. Client translators that make changes must guarantee that they send the modification requests in the same order to all the servers. This should be done using inodelk/entrylk.
When a change is sent to a server, the client must include a bitmap mask of the clients to which the request is being sent. Normally this is a bitmap containing all the clients, however, when a server fails for some reason some bits will be cleared. The heal translator uses this bitmap to early detect failures on other nodes from the point of view of each client. When this condition is detected, the request is aborted with an error and the client is notified with the remaining list of valid nodes. If the client considers the request can be successfully server with the remaining list of nodes, it can resend the request with the updated bitmap.
The heal translator also updates two file attributes for each change request to mantain the "version" of the data and metadata contents of the file. A similar task is currently made by AFR using xattrop. This would not be needed anymore, speeding write requests.
The version of data and metadata is returned to the client for each read request, allowing it to detect inconsistent data.
When a client detects an inconsistency, it initiates healing. First of all, it must lock the entry and inode (when necessary). Then, from the data collected from each node, it must decide which nodes have good data and which ones have bad data and hence need to be healed. There are two possible cases:
1. File is not a regular file
In this case the reconstruction is very fast and requires few requests, so it is done while the file is locked. In this case, the heal translator does nothing relevant.2. File is a regular file
For regular files, the first step is to synchronize the metadata to the bad nodes, including the version information. Once this is done, the file is set in healing mode on bad nodes, and provider mode on good nodes. Then the entry and inode are unlocked.When a file is in provider mode, it works as in normal mode, but refuses to start another healing. Only one client can be healing a file.
When a file is in healing mode, each normal write request from any client are handled as if the file were in normal mode, updating the version information and detecting possible inconsistencies with the bitmap. Additionally, the healing translator marks the written region of the file as "good".
Each write request from the healing client intended to repair the file must be marked with a special flag. In this case, the area that wants to be written is filtered by the list of "good" ranges (if there are any intersection with a good range, it is removed from the request). The resulting set of ranges are propagated to the lower translator and added to the list of "good" ranges but the version information is not updated.
Read requests are only served if the range requested is entirely contained into the "good" regions list.
There are some additional details, but I think this is enough to have a general idea of its purpose and how it works.
The main advantages of this translator are:
1. Avoid duplicated code in client translators
2. Simplify and unify healing methods in client translators
3. xattrop is not needed anymore in client translators to keep track of changes
4. Full file contents are repaired without locking the file
5. Better detection and prevention of some split brain situations as soon as possible
I think it would be very useful. It seems to me that it works correctly in all situations, however I don't have all the experience that other developers have with the healing functions of AFR, so I will be happy to answer any question or suggestion to solve problems it may have or to improve it.
What do you think about it ?
The goals you state above are all valid. What would really help (adoption) is if you can implement this as a modification of AFR by utilizing all the work already done, and you get brownie points if it is backward compatible with existing AFR. If you already have any code in a publishable state, please share it with us (github link?).
Avati
