On Mon, 2011-12-19 at 12:32 -0600, Les Mikesell wrote:
> On Mon, Dec 19, 2011 at 12:04 PM, Jean Spirat <jean.spirat AT squirk DOT org> 
> wrote:
> >
> > I directly mount the nfs share on the backuppc server so no need for
> > rsyncd here this is like local backup with the NFS overhead of course.
> 
> The whole point of rsync is that it can read the files locally with
> block checksums to decide what it really has to copy over the network.
>  Doing it over NFS, you've already had to copy if over the network so
> rsync at the wrong end can read it (and decide that it didn't have
> to...).

I think the real problem is the metadata access, and after a bit of
digging I've dug this up, it's comparing iSCSI with NFS.

But what might help is tweaking the NFS settings to improve the metadata
caching etc.

6.2 Meta-data intensive applications
NFS and iSCSI show their greatest differences in their
handling of meta-data intensive applications. Overall,
we find that iSCSI outperforms NFS for meta-data in-
tensive workloads—workloads where the network traffic
is dominated by meta-data accesses.
The better performance of iSCSI can be attributed to
two factors. First, NFS requires clients to update meta-
data synchronously to the server. In contrast, iSCSI,
when used in conjunction with modern file systems, up-
dates meta-data asynchronously. An additional bene-
fit of asynchronous meta-data updates is that it enables
update aggregation—multiple meta-data updates to the
same cached cached block are aggregated into a single
network write, yielding significant savings. Such opti-
mizations are not possible in NFS v2 or v3 due to their
synchronous meta-data update requirement.
Second, iSCSI also benefits from aggressive meta-
data caching by the file system. Since iSCSI reads are
in granularity of disk blocks, the file system reads and
caches entire blocks containing meta-data; applications
with meta-data locality benefit from such caching. Al-
though the NFS client can also cache meta-data, NFS
clients need to perform periodic consistency checks with
the server to provide weak consistency guarantees across
client machines that share the same NFS namespace.
Since the concept of sharing does not exist in the SCSI
architectural model, the iSCSI protocol also does not pay
the overhead of such a consistency protocol.

Full details are here: http://lass.cs.umass.edu/papers/pdf/FAST04.pdf

-- 
Tim Fletcher <tim AT night-shade.org DOT uk>


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