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Re: [Linux-cluster] iscsi doubt

On Mon, Apr 20, 2009 at 12:56 PM, ESGLinux <esggrupos gmail com> wrote:

2009/4/20 Gordan Bobic <gordan bobich net>

On Mon, 20 Apr 2009 18:18:22 +0200, ESGLinux <esggrupos gmail com> wrote:
> Hello,
> first, thanks for your answer,
> I suspected it but why can i do it with NFS. ?

Not sure I understand your question. NFS is a network file system like CIFS
specifically designed to be mounted from multiple clients simultaneously.
ext3 is only designed with a single accessor in mind.

I´ll try to explain myself

I have a partition /dev/sda

/dev/sda on /iscsivol type ext3 (rw)

but this partition is a target iscsi on another server. I format the partition with ext3 but its not a local disk, is a target iscsi.

with this configuration I have the filesystem corrupted.

second scenario

I have                       6983168   2839168   3783552  43% /mnt

but the parttion is again ext3 the diference is that I use nfs as network protocol instead of iscsi.

(overly simplified explanation)
This is because with iSCSI you are mounting the filesystem directly as a block device. Through NFS, the filesystem isn't accessed directly. The NFS server handles all the actual I/O, so only one system is actually touching the filesystem.

> the nodes never are going to be active at the same time so I can mount
> shares via NFS. With NFS when I create a file in a share automatically i
> got it in the shared mounted by all the clients.

I still don't understand your question - that is what NFS is designed for.

Yes I agree with you, but I thought with iscsi i can do the same as with NFS.

No, iSCSI and NFS are completely different. One is SCSI encapsulated in Ethernet- the volumes are going to look local to the system. The other is a conventional file sharing protocol.

> In this case I don´t need to write to the share concurrently
> can be this configuration a problem?

No, it's fundamentally impossible. In order to have a FS that can be
mounted simultaneously from multiple nodes, it has to be aware of multiple
nodes accessing it, which means that it needs coherent caching. Local file
systems like ext3 don't have this. When one node writes to the ext3 file
system, the other node will have cached the inodes as they were originally,
and it won't bother hitting the disk to re-read the contents, it'll just
return what it already has cached. And almost certainly corrupt the file
system in the process.

You cannot have a shared ext3 volume with writing enabled. Period.

ok understand it,

but (always there is a but ...)

I only want to share a directory in which one node writes at one and when it fails the other node has the diretory mounted with the data and can write to it.

Before I have known about cluster my decission would been to mount the shares with NFS. Now I want to be more sofisticated and want to use cluster tools, so I thought to mount it with iSCSI instead of NFS, but always with the ext3 as the underlying filesystem.

Perphaps this is my mistake.

any suggestion that makes me see the light ;.)

If you want to have multiple systems access the same volume via iSCSI, you need to use a filesystem that is multiple-node aware such as GFS.

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