NFS and iSCSI provide fundamentally different data sharing semantics.
NFS is inherently suitable for data sharing
, since
it enable files to be shared among multiple client machines.
In contrast, a block protocol such as iSCSI supports a single client
for each volume on the block server
. Consequently, iSCSI permits
applications running on a single client machine to share remote data,
but it is not directly suitable for sharing data across
machines. It is possible, however, to employ iSCSI in shared
multi-client environments by designing an appropriate distributed file
system that runs on multiple clients and accesses data from block
server.
The implications of caching are also different in the two scenarios.
In NFS, the file system is located at the server and so is the file
system cache (hits in this cache incur a network hop)
.
NFS clients also employ a cache that can hold both data
and meta-data. To ensure consistency across clients, NFS v2 and v3
require that client perform consistency checks with the server on
cached data and meta-data.
The validity of cached data at the client
is implementation-dependent--in Linux, cached meta-data is treated as
potentially stale after 3 seconds and cached data after 30 seconds.
Thus, meta-data and data reads may trigger a message exchange (i.e., a
consistency check) with the server even in the event of a cache hit.
NFS v4 can avoid this message exchange for data reads if the server
supports file delegation. From the perspective of writes, both data and
meta-data writes in NFS v2 are synchronous. NFS v3 and v4 supports
asynchronous data writes, but meta-data updates continue to be
synchronous. Thus, depending on the version, NFS has different
degrees of write-through caching
.
In iSCSI, the caching policy is governed by the file system. Since
the file system cache is located at the client, both data and
meta-data reads benefit from any cached content. Data updates are
asynchronous in most file systems. In modern file systems, meta-data
updates are also asynchronous, since such systems use log-based
journaling for faster recovery. In the ext3 file system, for
instance, meta-data is written asynchronously at commit points. The
asynchrony and frequency of these commit points is a trade-off between
recovery and performance (ext3 uses a commit interval of 5 seconds).
Thus, when used in conjunction with ext3, iSCSI supports a fully
write-back cache for data and meta-data updates.
Observe that the benefits of asynchronous meta-data update in iSCSI
come at the cost of lower reliability of data and meta-data
persistence than in NFS. Due to synchronous meta-data updates in NFS,
both data and meta-data updates persist across client failure.
However, in iSCSI, meta-data updates as well as related data may be
lost in case client fails prior to flushing the journal and data blocks
to the iSCSI server.
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