On Mar 8, 2006, at 8:01 AM, Helmut Tschemernjak wrote:
Hello Adam, all,
Cross platform locking is defiantly very important. The goal should
be to use the identical sqlite DB via UNIX, AFP, SMB, NFS and
others. My opinion is that it is not needed to have any additional
open parameters, just use the locking features which are in common
between all platforms.
Using an additional TCP/IP server is more complex in term of cross
platform compatibility and may be an overkill in terms of
performance, and will introduce additional problems.
At present the os_unix.c has some problems, I don't recall all its
details so here is just an estimate:
Multiple locks are getting unlocked with one call
- This does not work on AFP and SMB
A Read lock gets turned into a writer lock (and or back?)
- This does not work on AFP, SMB has limited support for it.
The current lock offset will not allow to copy open DB files if the
database gets larger than 0x40000000 bytes. This is because locked
regions cannot be copied under Windows, we changed it to:
#define PENDING_BYTE I64CONST(0x7fffffffffff0000)
#define RESERVED_BYTE I64CONST(0x7fffffffffff0001)
#define SHARED_FIRST I64CONST(0x7fffffffffff0002)
Advisory versus mandatory record locking
AFP and SMB is doing mandatory locking which means other clients
cannot read locked file areas, advisory locking means that we can
ready any data therefore we must call lock first before reading bytes.
The benefit of supporting the OS based locking is that after a
program exits it will automatically cleanup all locks. Windows has
some "oplock" features which will handle remote locks completely on
the Windows SMB clients as long as only one writer uses the file.
It should not be this difficult to use only locking features which
are in common cross major platforms and network file systems.
That's basically the goal, but instead of simply making '.lock' files
the only locking mechanism (the only type of locks that work for some
variants of NFS - thus the only locking features which are in common
across major platforms and network file systems), we want to allow
the developer to make the decision about the 'lowest common
denominator' locking for the environment their database will be used in.
- adam
