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Re: [Linux-cluster] gfs2 v. zfs?



On 01/26/2011 02:19 AM, Steven Whitehouse wrote:

I don't know of any reason why the inode number should be related to
back up. The reason why it was suggested that the inode number should be
independent of the physical block number was in order to allow
filesystem shrink without upsetting (for example) NFS which assumed that
its filehandles are valid "forever".

I'm not able to ping-pong too many emails on external mailing lists, at least during week days. However, a quick note on this ...

GFS2 fragments very soon and very badly ! Its blocks are all over the device due to the nature of how the resource group works. That slows down *every* thing, particularly for backup applications. A production deployment will encounter this issue very soon and they'll find the issue more than annoying.

Now, educate me (though I'll probably not read it until weekend) ... how will you defragment the FS with that inode number attaching to physical block number ? You can *not* move these inodes.

-- Wendy
The problem with doing that is that it adds an extra layer of
indirection (and one which had not been written in gfs2 at the point in
time we took that decision). That extra layer of indirection means more
overhead on every lookup of the inode. It would also be a contention
point in a distributed filesystem, since it would be global state.

The dump command directly accesses the filesystem via the block device
which is a problem for GFS2, since there is no guarantee (and in general
it won't be) that the information read via this method will match the
actual content of the filesystem. Unlike ext2/3 etc., GFS2 caches its
metadata in per-inode address spaces which are kept coherent using
glocks. In ext2/3 etc., the metadata is cached in the block device
address space which is why dump can work with them.

With GFS2 the only way to ensure that the block device was consistent
would be to umount the filesystem on all nodes. In that case it is no
problem to simply copy the block device using dd, for example. So dump
is not required.

Ideally we want backup to be online (i.e. with the filesystem mounted),
and we also do not want it to disrupt the workload which the cluster was
designed for, so far as possible. So the best solution is to back up
files from the node which is most likely to be caching them. That also
means that the backup can proceed in parallel across the nodes, reducing
the time taken.

It does mean that a bit more thought has to go into it, since it may not
be immediately obvious what the working set of each node actually is.
Usually though, it is possible to make a reasonable approximation of it,

Steve.


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