[Linux-cluster] iscsi doubt

Ian Hayes cthulhucalling at gmail.com
Mon Apr 20 17:07:42 UTC 2009


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

> Hello,
>
>
>
> 2009/4/20 Gordan Bobic <gordan at bobich.net>
>
>> On Mon, 20 Apr 2009 18:18:22 +0200, ESGLinux <esggrupos at 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
> 192.168.1.198:/nfsexport/                       6983168   2839168
> 3783552  43% /mnt
>
> but the parttion 192.168.1.198:/nfsexport/ 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
>> the
>> > 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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