Steve Lord wrote:
On Wed, 2002-08-14 at 12:08, Greg Freemyer wrote:
>> The normal way we setup something like this is to use a shared scsi
>> or fiberchannel disk setup, that way you no longer have a singe
>> point of failure. If you are putting the disks into one of the
>> two computers then if that system goes down you are dead in the
>> water. The other aspect of the above setup is using something
>> like failsafe, the two nodes monitor each other, and can (if
>> setup correctly) shoot the other node down and take over the
>> filesystem if it detects problems. failsafe itself is opensource,
>> not sure if the components of it which fail over the filesystem
>> to the other node are. Of course all this assumes using the fs
>> via NFS - the second node takes over the ip address of the failed
>> A quick look at drbd on the net seems to show that it is designed
>> for these sorts of setups, and it has links to all the high
>> availability stuff for linux.
>> That aside, there should be no problem doing this, provided you
>> make sure the unmount from one system is complete before attempting
>> to mount on the other system. Mounting a filesystem which is
>> already mounted elsewhere is not a good thing, the second system
>> will think the fs needs recovery running on it.
I don't think drbd is designed for use in a shared SCSI environment.
I did not intend to imply it was, just presenting a different way
to build a resilient configuration.
This what I'm building isn't shared SCSI.
I'm believe that shared SCSI couldn't survive some kind of crashes.
I prefer two complete, independent machines which are mirroring
some storage via LAN and replicate some data using native protocols
of some services.
It is more of a RAID 1 driver where the 2 halves of the mirror are on the 2 different servers using internal disks.
I believe it has support for ordered writes, but I for one would not simply
assume XFS and drbd are compatible.
Ah, I should have read more.... if the fs is mirrored between the two
hosts then there is a chance it will work OK. However, the interesting
part of XFS is write ordering - there are certain writes which we need
to know have made it down to disk and will survive a crash. In this
sort of setup I really do not know where the data will be once drbd
says it is written. Probably still in cache on the remote box for a
start. There may well be circumstances where loss of both machines
will cause filesystem corruption.
you are right, testing such a setup before going live with it is
important, please do not take my comments as meaning it will definitely
there are few modes of operation of the drbd:
a) - send and forgot
b) - send and OK is when recive acknowledge is returned
c) - send and OK is when write on remote is commited
So, what exactly data should be synced ?
I'm thinking, that during creation of XFS filesystem
on lets say /dev/sdb1 (16GB) i will create logs on
/dev/sdb2 (128MB or 256MB or less - must calculate exact size)
Then: /dev/sdb1 (data) will be mirrored using mode b,
and /dev/sdb2 (logs) will mirror using mode c.
Nothing will use mode a.
Is it wright ?