While a fs node may only be reading, does not mean the metadata
on disk is not being updated by some other node. Means, it needs to
take appropriate locks to perform the read... means it needs to have
a lock on the mastered lock resource... means it needs to be part of
the active cluster. The dlm is not only keeping track of the locks but
also the inode (lvb). Bottomline, either you are in a cluster or not.
There is no middle ground. And if not, then all you are doing are
dirty reads.
In ocfs2, we have a readonly and a hard-ro state. As in, we go into the
hard-ro state only if the device is truly ro. In the hard ro, we don't
start a heartbeat nor create a dlm domain. Normal readonly is like
a rw mount, with the exception that the userspace cannot write.
As in, hb is started and the dlm domain is created.
Alexei_Roudnev wrote:
Of course it is cluster operations.
as I said, cluster have a clients like FS. Client can be in 3 modes:
- passive (no reason to fence, just don't allow to switch mode)
- active read only
- active write
Active write requires fencing in all cases, active read status can't transit
into active writes if cluster is not connected, and passive mode never
require fencing (at lest until FS want to switch the mode). FS in passive
mode can run re-initialization without fencing and with 0 risk of corruption
(because server state after the reboot is exactly the same as before
reboot).
Of course, client must make a transitions himself (all writes completed 30
seconds ago, except disk heartbeat - switch to passive mode and inform
cluster manager).
In addition, you can't fully separate cluster manager and FS because FS have
it's own heatbeats and network connections.
I think, that the only way to improve behavior without grand changes (or
risk to have a corruptions) is to monitor FS mode and
switch it to the passive when possible (no activity for some time and all
buffers are flushed out or at least written). Existing implementation can
not be used in many cases (as I descrived in another mail) because it
dramatically decrease cluster reliability.
In addition, if all nodes lost IO access to the disks it don't make sence to
fence as well, until at least one node got access.
PS. I was able to fraud existion OCFSv2 with all it's fencing, by simple
assigning 2 servers the same iSCSI ID. So no one cluster system can protect
from all possible failures anyway. And reboots on each _ap chi_ cause more
problems then bring benefits (except when OCFSv2 is used for critical data
in the 100% time write mode).
----- Original Message -----
From: "Sunil Mushran" <[EMAIL PROTECTED]>
To: "Alexei_Roudnev" <[EMAIL PROTECTED]>
Cc: "David Miller" <[EMAIL PROTECTED]>; <[email protected]>
Sent: Monday, April 09, 2007 4:26 PM
Subject: Re: [Ocfs2-users] Catatonic nodes under SLES10
Fencing is not a fs operation but a cluster operation. The fs is only a
client
of the cluster stack.
Alexei_Roudnev wrote:
It all depends of the usage scenario.
Tipical usage is, for example:
(1) Shared application home. Writes happens once / week during
maintanance,
otehr time files are opened for reading only. Few logfiles
can be redirected if required.
So, when server see a problems, it HAD NOT any pending IO for a 3 days -
so
what the purpose of reboot? It 100% knows that NO ANY IO
is pending, and other nodes have not any IO pending as well.
(2) Backup storage for the RAC. FS is not opened 90% of the time. At
night,
one node opens it and creates a few files. Other node have not any
pending
IO on this FS. Fencing passive node (which dont run any backup) is not
useful because it HAD NOT ANY PENDING IO for a few hours.
(3) WEB server. 10 nodes, 1 only makes updates. The same - most nodes
have
not any pending IO.
Of course there is always a risk of FS corruption in the clusters. Any
layer
can keep pending IO forever (I saw Linux kernel keeping it for 10
minutes).
Problem is that in such cases software fencing can't help as well
because
node is half-dead and can't detect it's own status.
So, the key point here is not in _fence for each ap-chi_ but _keep
system
without pending writes as long as possible and make clean transition
between
active write/active read / passive states. Then you can avoid
self-fencing
in 90% cases (because of server wil be in passive or active reads
state). I
mounT FS but don't cd into it, or just CD but dont read - passive
status. I
read file - active read for 1 minute, tbhnen flush buffers so that it is
in
passive mode again. I began top write - switch system to write mode. I
did
not write blocks for 1 minute - flush everything, wait 1 more minute and
switch to passive mode.
----- Original Message -----
From: "Sunil Mushran" <[EMAIL PROTECTED]>
To: "David Miller" <[EMAIL PROTECTED]>
Cc: <[email protected]>
Sent: Monday, April 09, 2007 3:18 PM
Subject: Re: [Ocfs2-users] Catatonic nodes under SLES10
For io fencing to be graceful, one requires better hardware. Read
expensive.
As in, switches where one can choke off all the ios to the storage from
a specific
node.
Read the following for a discussion on force umounts. In short, not
possible as yet.
http://lwn.net/Articles/192632/
Readonly does not work wrt to io fencing. As in, ro only stops any new
userspace
writes but cannot stop pending writes. And writes could be lodged in
any
io layer.
A reboot is the cheapest way to avoid corruption. (While a reboot is
painful, it is
much less painful than a corrupted fs.)
With 1.2.5 you should be able to increase the network timeouts and
hopefully avoid
the problem.
David Miller wrote:
Alexei_Roudnev wrote:
Did you checked
/proc/sys/kernel/panic /proc/sys/kernel/panic_on_oops
system variables?
No. Maybe I'm missing something here.
Are you saying that a panic/freeze/reboot is the expected/desirable
behavior? That nothing more graceful could be done, like to just
dismount the ocfs2 file systems, or force them to a read-only mount or
something like that? We have to reload the kernel?
Thanks,
--- David
----- Original Message ----- From: "David Miller"
<[EMAIL PROTECTED]>
To: <[email protected]>
Sent: Monday, April 02, 2007 9:01 AM
Subject: [Ocfs2-users] Catatonic nodes under SLES10
[snip]
Both servers will be connected to a dual-host external RAID system.
I've setup ocfs2 on a couple of test systems and everything appears
to work fine.
Until, that is, one of the systems loses network connectivity.
When the systems can't talk to each other anymore, but the disk
heartbeat is still alive, the high numbered node goes catatonic.
Under SLES 9 it fenced itself off with a kernel panic; under 10 it
simply stops responding to network or console. A power cycling is
required to bring it back up.
The desired behavior would be for the higher numbered node to lose
access to the ocfs2 file system(s). I don't really care whether it
would simply timeout ala stale NFS mounts, or immediately error like
access to non-existent files.
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