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 ?
Thank you,
Xavi
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