[Samba]DOS app won't run off a Samba drive!

[Rashkae]

I will try to explain my limited understanding. Others can correct me and
fill in the blanks as we go. :)

Oplocks stand for Opportunistic Locking.

Here's how it works.

Client requests to open a file from the server. If no other clients
are using that particular file, the server will grant the client an
"opportunistic lock".  This means that the client will cache a copy of
the file in local memory.  That way, read/write operations to the file
don't have to go over the network.  This is, of course, a
significant performance boost. The server will not allow other clients
to open the file, since the client local cached copy and the server
copy are likely to be out of sync.

The fun begins when a second client wants to open a file that is
oplocked. The server must inform the original client that the lock
will be broken. At this point, Client 1 is supposed to upload any
changes to the file back to the server and continue on the way things
would normally work. All read/write operations are performed over the
network. Databases should lock on a record by record basis for write
operations (Thereby being safe for multiple users.)

If client 1 does not co-operate with the break request, the server has
to force a break. The file to client 1 is closed in whatever state the
cache was last synchronized with the server.

For oplock granting and breaking to work properly requires handshaking
between client user software, client os, server software, and server
os. All possible combinations of the above have their own special
quirks and behavior. (Note: it is only in recent versions of Linux
Kernel that Linux OS participates in the matrix. This is where the
kernel oplocks comes into play. If kernel oplocks are not enabled,
linux will not know that a file in a samba share is 'locked', and will
allow local linux users or NFS users to open the file without
negotiating a break with the SMB client. Chaos ensues. On the other
hand, if Unix processes other than Samba will not be opening the files
for write, disabling kernel oplocks removes yet another possible
source of problems, the server os.)

Long ago when I was young and innocent, and administering NT 4 servers
for a living, I worked out the rule of thumb of always disabling
Oplocks whenever a database was involved. (In Windows, this can be
done with a registry key, but it affects the whole system. Oplocks
cannot be disabled on a file by file or share by share basis.) You can
avoid much database corruption this way. *Especially* Dos Databases,
since older DOS software was not "opportunistic lock aware" (quote
from MS tech support.)


Another neat trick Samba has is the ability to "fake oplocks". This
will make samba always grant oplocks, to multiple simultaneous
clients. This can be a boost to read-only shares, since the files will
never be changed, it doesn't matter if all the clients and server are
in sync.  The problem with this, of course, is that no one can modify
the file while SMB clients are using it. Otherwise, chaos
ensues. :). This is probably only safe on CD-ROM shares.

Oh, BTW, Samba enables oplocks by default.

Personally, I am not a fan of performance over stability 'features.',
but that's just MNSHO. I am not a coder or engineer, and probably
misunderstand many of the concepts involved. I'm just the person who
has to explain to clients why their database is hosed and the previous
2 weeks of tape backups are no good because nobody rotated the tapes
and the one in the drive went bad long time ago.





On Tue, 29 Jan 2002, Greg Conway wrote:

Hi Rashkae,

Well, all these "oplocks" are a little new to me!

I have been running off the same smb.conf for the last six or seven
machines, and this is the first time I have encountered problems since I
spent a long time writing that original smb.conf!

So do I presume...

you can put a "oplocks" section in the [global] section, to enable oplocks -
are they enabled by default? is this why I have never heard of them before?

you can then put a "veto oplocks" section into individual shares ie [data],
to remove oplocks from that particular share.

Sorry to be thick, can somebody please explain to me in windows-users terms
:) exactly what oplocks does? It is similar to SHARE.EXE? Why do my Win2k
machines work be default but not my Win98 machines?

Anyway, just interested in the hows and whys rather than blindly fixing
things and not knowing how I did it!!

Regards,

Greg.

> -----Original Message-----
> From: Rashkae [mailto:rashkae at wealthmap.ca]
> Sent: 29 January 2002 14:33
> To: Dragos
> Cc: Greg Conway; Samba
> Subject: Re: [Samba]DOS app won't run off a Samba drive!
>
>
> Why not just disable openlocks entirely? Makes allot of sense to me for  a
> server that primarily dishes out multi-user files, oplocks at best is a
> performance hindrance, at worst a dangerous database corrupting monster.
>
> (I think your able to disable oplocks on a share by share basis, if you
> have other shares where oplocks would be useful.)
>
> On Tue, 29 Jan 2002, Dragos wrote:
>
> glad to hear that it works...:)
> however, keep the 'veto oplocks files' option handy, because when
> you'll have
> more than one connection to the share (multiple win clients working on the
> database) things will slow down.
>
> dragos

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