[Lustre-devel] WBC HLD outline
Alexander.Zarochentsev at Sun.COM
Mon Mar 23 14:58:30 PDT 2009
here is a wbc hld outline.
Please take a look.
WBC HLD OUTLINE
WBC (MD WBC): (Meta Data) Write Back Cache.
MD operation: whole MD operation over an object:
Reintegration: The process of applying accumulated MD operation to the
MDS/RAW: MDS API extension to do "raw" fs operations: inserting of a
dir entry w/o creating inode and so.
MD update: a part of MD operation to be executed on one server,
contains one or more MDS/RAW operations.
MD batch: a collection of per-server MD updates.
MDTR: MD translator: translates MD operations into MD/Raw ones.
Client application is able to create 64k files/second.
Reintergration moves fs from one consistent state to another
Non-WBC client support w/o visible overhead.
Avoid MDS code rewrite if possible.
* Design outline
** Overall picture
=MD (non-WBC) proto=
[MD CACHE MANAGER] ---> [LDLM]
| | |
| | |
V V V
[MDS1/RAW] [MDS2/RAW] [MDS3/RAW]
WBC client has a MDTR running on client side,
it also can be a proxy server, acting as a server for
non-WBC clients and as a client for MD servers.
*** WBC vs non-WBC
Processing MD operation request (lock enqueue + op intent, by Alex
suggestion), MD server may decide to execute it by itself, or grant a
only a lock (subtree one) and allow client to continue in WBC mode.
needed LDLM locks are taken before operation starts and held until the
corresponded batch is re-integrated.
*** Local cache management
WBC client executes operations locally, modifying local in-memory
objects. WBC client has a (redo-)log of all operations.
The cache manager controls process of MD cache re-integration.
*** MDS/RAW operations
Managing directory entries and inodes, without maintaining
fs consistency automatically.
create/update/delete methods for directory entries and inodes.
MDTR is responsible for converting MD operations into set of
per-server MD/RAW operations.
*** Client re-integration
Periodically, or because of (sub-)lock releasing, dirty memory
flushing or so, WBC client submits batches to all MD servers involved
into the operations.
Process of re-integration is protected by LDLM locks. MD servers are
using WBC protocol.
*** WBC protocol
WBC request contains a set of MD/RAW operations, tagged with one epoch
number. Bulk transfers are used.
*** File data
Flushing file data to the OST servers is delayed until file creation
The redo-log preserved until it is not needed in recovery (i.e. epoch
Client replay the log and re-execute all operations from it, repeating
MDTR processing (dispatching the operation between MD servers).
**** WBC client eviction, uncompleted updates
If client dies until re-integration is completed, there are three
a) Cluster-wide rollback, all servers roll back to the last globally
stable epoch, then clients to replay heir redo-logs.
This scenario should be avoided because a single client failure may
may stop whole cluster for recovery.
b) All servers participating in re-integration coordinate to undo
c) The servers have all information needed to complete re-integration
The recovery strategy is a subject of CMD Recovery Design document,
but a possibility of (c) need a support in the WBC protocol.
*** MD protocol
MD (non-WBC) protocol remains the same as now.
** Use cases
*** WBC / non-WBC decision
1. Check whether server and client can operate in WBC-mode through
2. I they can, a lock enqueue request may contain a request for
WBC-mode, the server may respond with granting WBC-mode and STL or PW
lock on the directory. MD server accepts or rejects WBC-mode request
depending on server rules and per-object access statistics.
*** File creation
client gets a PW lock on directory.
client fetches directory content.
client does file creation locally, in cache, the operation record is
added to the client redo-log.
Another client want to read the directory, lock conflict triggers
MD Cache manager processes the redo-log, prepares batches with MDS/RAW
operations and submits them to the MD servers.
The MD servers integrate the batches.
MD Cache manager frees local cache content and cancels the directory
Q: Can several wbc clients work in one directory simultaneously?
A: If extent locks for directories are implemented, each WBC client
can take a lock on a hash interval.
Q: can wbc clients do massive file creation in one directory
A: the idea that may help: if we can guess that the file names created
by a client are lexicographically ordered, a special hash function
may reduce lock conflicts between clients holding locks on
Alexander "Zam" Zarochentsev
Lustre Group, Sun Microsystems
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