Eric Roseme
2002-Nov-01 23:43 UTC
[Samba] Oplocks Usage Recommendations Whitepaper (with attachment)
Here is Oplocks Usage Recommendations Whitepaper for Samba on HP-UX
(originally was written for CIFS/9000 Server on HP-UX).
Note that the intended audience is/are HP-UX customers who have
questions
and concerns about when to configure oplocks. This is intended as a
rudimentry guide to help avoid the most obvious oplock pitfalls.
Hopefully the plain text alignments hold up well for most editors. Word
messes things up.
Thanks,
Eric Roseme
Hewlett-Packard
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HP-UX Samba Opportunistic Locking Usage Recommendations
Eric Roseme, Hewlett-Packard
October, 2002
Contents
Legal Notices 2
Chapter 1 Introduction 4
Chapter 2 Opportunistic Locking Overview 5
Chapter 3 Samba Oplock Configuration 7
Chapter 4 Opportunistic Locking Recommendations 9
4.1 Exclusively Accessed Shares 9
4.2 Multiple-Accessed Shares or Files 9
4.3 Unix or NFS Client Accessed Files 10
4.4 Slow and/or Unreliable Networks 10
4.5 Multi-User Databases 10
4.6 PDM Data Shares 10
4.7 Force User 10
4.8 Advanced Samba Opportunistic Locking Parameters 11
4.9 Mission Critical High Availability 11
Chapter 5 Summary 12
Chapter 1 Introduction
Samba on HP-UX manages file access among Windows clients with Windows
style file locking. It applies a very effective set of file locking
features that are managed by the user-space client processes on the
server, and provides excellent data security and integrity in a
multi-user environment. Samba also integrates some Windows locking
protocols with the underlying HP-UX operating system locking protocols,
and therefore provides some interoperability with UNIX and NFS style
file locking.
Opportunistic Locking is a unique Windows file locking feature. It is
not really file locking, but is included in most discussions of Windows
file locking, so is considered a defacto locking feature.
Opportunistic Locking is actually part of the Windows client file
caching mechanism. It is not a particularly robust or reliable feature
when implemented on the variety of customized networks that exist in
enterprise computing, but can be effective in providing modest
perceived performance optimization.
Like Windows, Samba implements Opportunistic Locking as a server-side
component of the client caching mechanism. Because of the lightweight
nature of the Windows feature design, effective configuration of
Opportunistic Locking requires a good understanding of its limitations,
and then applying that understanding when configuring data access for
each particular customized network and client usage state.
Chapter 2 Opportunistic Locking Overview
OPPORTUNISTIC LOCKING (Oplocks) is invoked by the Windows file system
(as opposed to an API) via registry entries (on the server AND client)
for the purpose of enhancing network performance when accessing a file
residing on a server. Performance is enhanced by caching the file
locally on the client which allows:
Read-ahead: The client reads the local copy of the
file, eliminating network latency
Write caching: The client writes to the local copy of the
file, eliminating network latency
Lock caching: The client caches application locks
locally, eliminating network latency
The performance enhancement of oplocks is due to the opportunity of
exclusive access to the file - even if it is opened with deny-none -
because Windows monitors the file's status for concurrent access from
other processes.
Windows defines 4 kinds of Oplocks:
Level1 Oplock - The redirector sees that the file was opened with deny
none (allowing concurrent access), verifies that no
other process is accessing the file, checks that
oplocks are enabled, then grants deny-all/read-write/ex-
clusive access to the file. The client now performs
operations on the cached local file.
If a second process attempts to open the file, the open
is deferred while the redirector "breaks" the original
oplock. The oplock break signals the caching client to
write the local file back to the server, flush the
local locks, and discard read-ahead data. The break is
then complete, the deferred open is granted, and the
multiple processes can enjoy concurrent file access as
dictated by mandatory or byte-range locking options.
However, if the original opening process opened the
file with a share mode other than deny-none, then the
second process is granted limited or no access, despite
the oplock break.
Level2 Oplock - Performs like a level1 oplock, except caching is only
operative for reads. All other operations are performed
on the server disk copy of the file.
Filter Oplock - Does not allow write or delete file access.
Batch Oplock - Manipulates file openings and closings - allows caching
of file attributes.
An important detail is that oplocks are invoked by the file system, not
an application API. Therefore, an application can close an oplocked
file, but the file system does not relinquish the oplock. When the
oplock break is issued, the file system then simply closes the file in
preparation for the subsequent open by the second process.
Chapter 3 CIFS/9000 Oplock Configuration
OPPORTUNISTIC LOCKING (Oplocks) is implemented by Samba on a per-share
basis in the smb.conf file. Oplocks are enabled by default for each
share, which allows the Windows client to cache a local copy of a file
for:
Read-ahead
Write-caching
Lock caching
*Oplocks*are disabled on a per-share basis in the smb.conf file:
[share_name]
oplocks = no
The default is "yes". The default oplock type is Level1.
*Level2 Oplocks* are enabled on a per-share basis in the smb.conf
file:
[share_name]
level2 oplocks = yes
The default is "no". Oplocks must also be set to "yes" for
the Level2
oplock parameter to function.
*Kernel oplocks* is a Samba smb.conf parameter that notifies Samba (if
the UNIX kernel has the capability to send a Windows client an oplock
break) when a UNIX process is attempting to open the file that is
cached. This parameter addresses sharing files between UNIX and
Windows with Oplocks enabled on the Samba server: the UNIX process
can open the file that is Oplocked (cached) by the Windows client and
the smbd process will not send an oplock break, which exposes the file
to the risk of data corruption. If the UNIX kernel has the ability to
send an oplock break, then the kernel oplocks parameter enables Samba
to send the oplock break. Kernel oplocks are enabled on a per-server
basis in the smb.conf file.
[global]
kernel oplocks = yes
The default is "no".
*Veto oplocks* is a smb.conf parameter that identifies specific files for
which Oplocks are disabled. When a Windows client opens a file that
has been configured for veto oplocks, the client will not be granted
the oplock, and all operations will be executed on the original file on
disk instead of a client-cached file copy. By explicitly identifying
files that are shared with UNIX processes, and disabling oplocks for
those files, the server-wide Oplock configuration can be enabled to
allow Windows clients to utilize the performance benefit of file
caching without the risk of data corruption. Veto Oplocks can be
enabled on a per-share basis, or globally for the entire server, in the
smb.conf file:
[global]
veto oplock files = /filename.htm/*.txt/
[share_name]
veto oplock files = /*.exe/filename.ext/
*Oplock break wait time" is a smb.conf parameter that adjusts the time
interval for Samba to reply to an oplock break request. Samba
recommends "DO NOT CHANGE THIS PARAMETER UNLESS YOU HAVE READ AND
UNDERSTOOD THE SAMBA OPLOCK CODE." Oplock Break Wait Time can only be
configured globally in the smb.conf file:
[global]
oplock break wait time = 0 (default)
*Oplock break contention limit* is a smb.conf parameter that limits the
response of the Samba server to grant an oplock if the configured
number of contending clients reaches the limit specified by the
parameter. Samba recommends "DO NOT CHANGE THIS PARAMETER UNLESS YOU
HAVE READ AND UNDERSTOOD THE SAMBA OPLOCK CODE." Oplock Break
Contention Limit can be enable on a per-share basis, or globally for
the entire server, in the smb.conf file.
[global]
oplock break contention limit = 2 (default)
[share_name]
oplock break contention limit = 2 (default)
Chapter 4
Opportunistic Locking Recommendations
Opportunistic locking is a desirable feature when it can enhance the
perceived performance of a networked client. However, the
opportunistic locking protocol is not robust, and therefore can
encounter problems when invoked beyond a simplistic configuration, or
on extended, slow, or faulty networks. In these cases, operating
system management of opportunistic locking and/or recovering from
repetitive errors can offset the perceived performance advantage that
it is intended to provide.
"Opportunistic Locking" is actually an improper name for this feature.
The true benefit of this feature is client-side data caching, and
oplocks is merely a notification mechanism for writing data back to the
networked storage disk. The limitation of opportunistic locking is the
reliability of the mechanism to process an oplock break (notification)
between the server and the caching client. If this exchange is faulty
(usually due to timing out for any number of reasons) then the
client-side caching benefit is negated.
The actual decision that a user or administrator should consider is
whether it is sensible to share amongst multiple users data that will
be cached locally on a client. In many cases the answer is no.
Deciding when to cache or not cache data is the real question, and thus
"opportunistic locking" should be treated as a toggle for client-side
caching. Turn it "ON" when client-side caching is desirable and
reliable. Turn it "OFF" when client-side caching is redundant,
unreliable, or counter-productive.
Opportunistic locking is by default set to "on" by Samba on all
configured shares, so careful attention should be given to each case to
determine if the potential benefit is worth the potential for delays.
The following recommendations will help to characterize the environment
where opportunistic locking may be effectively configured.
4.1 Exclusively Accessed Shares
Opportunistic locking is most effective when it is confined to shares
that are exclusively accessed by a single user, or by only one user at
a time. Because the true value of opportunistic locking is the local
client caching of data, any operation that interrupts the caching
mechanism will cause a delay.
Home directories are the most obvious examples of where the performance
benefit of opportunistic locking can be safely realized.
4.2 Multiple-Accessed Shares or Files
As each additional user accesses a file in a share with opportunistic
locking enabled, the potential for delays and resulting perceived poor
performance increases. When multiple users are accessing a file on a
share that has oplocks enabled, the management impact of sending and
receiving oplock breaks, and the resulting latency while other clients
wait for the caching client to flush data, offset the performance gains
of the caching user.
As each additional client attempts to access a file with oplocks set,
the potential performance improvement is negated and eventually results
in a performance bottleneck.
4.3 Unix or NFS Client Accessed Files
Local HP-UX (Unix) and NFS clients access files without a mandatory
file locking mechanism. Thus, these client platforms are incapable of
initiating an oplock break request from the server to a Windows client
that has a file cached. Local HP-UX or NFS file access can therefore
write to a file that has been cached by a Windows client, which
exposes the file to likely data corruption.
If files are shared between Windows clients, and either local HP-UX
(Unix) or NFS users, then turn opportunistic locking off.
4.4 Slow and/or Unreliable Networks
The biggest potential performance improvement for opportunistic locking
occurs when the client-side caching of reads and writes delivers the
most differential over sending those reads and writes over the wire.
This is most likely to occur when the network is extremely slow,
congested, or distributed (as in a WAN). However, network latency also
has a very high impact on the reliability of the oplock break
mechanism, and thus increases the likelihood of encountering oplock
problems that more than offset the potential perceived performance
gain. Of course, if an oplock break never has to be sent, then this is
the most advantageous scenario to utilize opportunistic locking.
If the network is slow, unreliable, or a WAN, then do not configure
opportunistic locking if there is any chance of multiple users
regularly opening the same file.
4.5 Multi-User Databases
Multi-user databases clearly pose a risk due to their very nature -
they are typically heavily accessed by numerous users at random
intervals. Placing a multi-user database on a share with opportunistic
locking enabled will likely result in a locking management bottleneck
on the Samba server. Whether the database application is developed
in-house or a commercially available product, ensure that the share
has opportunistic locking disabled.
4.6 PDM Data Shares
Process Data Management (PDM) applications such as IMAN, Enovia, and
Clearcase, are increasing in usage with Windows client platforms, and
therefore SMB data stores. PDM applications manage multi-user
environments for critical data security and access. The typical PDM
environment is usually associated with sophisticated client design
applications that will load data locally as demanded. In addition, the
PDM application will usually monitor the data-state of each client.
In this case, client-side data caching is best left to the local
application and PDM server to negotiate and maintain. It is
appropriate to eliminate the client OS from any caching tasks, and the
server from any oplock management, by disabling opportunistic locking on
the share.
4.7 Force User
Samba includes an smb.conf parameter called "force user" that changes
the user accessing a share from the incoming user to whatever user is
defined by the smb.conf variable. If opportunistic locking is enabled
on a share, the change in user access causes an oplock break to be sent
to the client, even if the user has not explicitly loaded a file. In
cases where the network is slow or unreliable, an oplock break can
become lost without the user even accessing a file. This can cause
apparent performance degradation as the client continually reconnects
to overcome the lost oplock break.
So avoid the following combination:
* "force user" in smb.conf share configuration, and
* Slow or unreliable networks, and
* Opportunistic locking
4.8 Advanced Samba Opportunistic Locking Parameters
Samba provides opportunistic locking parameters that allow the
administrator to adjust various properties of the oplock mechanism to
account for timing and usage levels. These parameters provide good
versatility for implementing oplocks in environments where they would
likely cause problems. The parameters are:
* oplock break wait time
* oplock contention limit
For most users, administrators, and environments, if these parameters
are required, then the better option is to simply turn oplocks off.
The samba SWAT help text for both parameters reads "DO NOT CHANGE THIS
PARAMETER UNLESS YOU HAVE READ AND UNDERSTOOD THE SAMBA OPLOCK CODE."
This is good advice.
4.9 Mission Critical High Availability
In mission critical high availability environments, data integrity is
often a priority. Complex and expensive configurations are implemented
to ensure that if a client loses connectivity with a file server, a
failover replacement will be available immediately to provide
continuous data availability.
Windows client failover behavior is more at risk of application
interruption than other platforms because it is dependant upon an
established TCP transport connection. If the connection is interrupted
- as in a file server failover - a new session must be established.
It is rare for Windows client applications to be coded to recover
correctly from a transport connection loss, therefore most applications
will experience some sort of interruption - at worst, abort and
require restarting.
If a client session has been caching writes and reads locally due to
opportunistic locking, it is likely that the data will be lost when the
application restarts, or recovers from the TCP interrupt. When the TCP
connection drops, the client state is lost. When the file server
recovers, an oplock break is not sent to the client. In this case, the
work from the prior session is lost. Observing this scenario with
oplocks disabled, and the client was writing data to the file server
real-time, then the failover will provide the data on disk as it
existed at the time of the disconnect.
In mission critical high availability environments, careful attention
should be given to opportunistic locking. Ideally, comprehensive
testing should be done with all affected applications with oplocks
enabled and disabled.
Chapter 5
Summary
Windows Opportunistic Locking is a lightweight performance-enhancing
feature. It is not a robust and reliable protocol. Every
implementation of Opportunistic Locking should be evaluated as a
tradeoff between perceived performance and reliability. Reliability
decreases as each successive rule above is not enforced. Consider a
share with oplocks enabled, over a wide area network, to a client on a
South Pacific atoll, on a high-availability server, serving a
mission-critical multi-user corporate database, during a tropical
storm. This configuration will likely encounter problems with oplocks.
Oplocks can be beneficial to perceived client performance when treated
as a configuration toggle for client-side data caching. If the data
caching is likely to be interrupted, then oplock usage should be
reviewed. Samba enables opportunistic locking by default on all
shares. Careful attention should be given to the client usage of
shared data on the server, the server network reliability, and the
opportunistic locking configuration of each share.
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