[TriLUG] SAN file locking
jackhill at jackhill.us
Fri Dec 16 20:36:41 EST 2011
If you're interested in mounting a filesystem simultaniously on multiple
machines you could look at something like OCFS2 or GFS. Or depending on
what your trying to do something like AFS.
On Fri, 16 Dec 2011, bak wrote:
> SAN storage is not meant to be shared in the way that you describe, at
> the file level. Individual files are not shared in SAN setups, so file
> locking is not a relevant concept.
> Think of SAN as presenting some blocks on a disk somewhere to a server
> such that it believes those blocks are on a directly connected disk of
> an arbitrary size.
> Just like you wouldn't have two PCs connected to the same USB hard drive
> trying to access its blocks at the same time, as a rule, two servers
> won't access the same set of blocks on a SAN pretending to be a disk at
> the same time. Filesystem layout is just too complex to coordinate in
> this way and keep things running at low latencies. (A side effect of
> this is that usually a SAN system can't see 'into' a disk used by a
> server, and wouldn't for example know how many files are in there, how
> full it is, etc. All it sees are blocks.)
> As a rule, a SAN setup will use various kinds of masking to keep hosts
> from seeing blocks that don't "belong" to them in the closet of disk
> drives that makes up a SAN array.
> Now, there are exceptions to the "only one at a time rule", usually made
> for clustered systems. They often have a disk that they share in order
> to coordinate cluster activities. In the case of a Microsoft Exchange
> cluster, or a Solaris cluster, data is not served from there.
> VMWare clusters also share disks, but they have a very specialized
> cluster-aware filesystem that lets them pass the "ownership" of
> individual block ranges back and forth to cluster members as you do
> things like VMotion.
> The use cases you describe are not reasons why you would deploy a SAN.
> Sometimes-access to one or more files at a time is exactly why you use a
> NAS -- let some other system handle the filesystem and serve things out
> as needed, one file at a time.
> SANs are for low-latency, high-throughput situations. In an Oracle or
> OLTP situation, getting to the data fast and crunching through it to
> return a result is the goal, so fast access to a relatively small
> working set is necessary. Contrast this with a NAS situation, where you
> might have a music file that gets read slowly every day or two.
> Another traditional place for SAN are things like Exchange where
> everyone has to talk to one or two relatively beefy servers to begin
> with, so you don't need a whole farm of them sharing files.
> Although you can run VMWare over NFS these days, often SANs are deployed
> to run ESX clusters, because you want your VM guests to have fast,
> low-latency access to their virtual disks so that they don't bog down
> waiting for I/O, and because a shared storage backend enables all kinds
> of nifty VMWare magic.
> SAN is all about 'limited space, one server at a time, but very fast' --
> databases, virtual disks... whereas NAS is about monster buckets of data
> that are used more rarely -- home directories, pictures.
> On 12/16/11 7:42 PM, Joseph Mack NA3T wrote:
>> I haven't setup a SAN so don't know how people handle file locking. The
>> problem is that any host on the SAN network can in principle access a
>> file. Here's the only thing I've found about SAN file locking, a
>> metadata broker
>> I assume the SAN file locking problem is the same as for other shared
>> file systems eg. for multiple hosts mounting disks via NFS.
>> This wiki page
>> shows that some implementations of NFS have file locking and
>> some don't.
>> Is everyone just mounting /usr (ro) and sharing that, while mounting
>> home directories (rw) expecting the user to handle the exclusive writing
>> manually? In this case there is no real sharing. If there's no real
>> sharing, what are SANs being used for? - stupendously large amounts of
>> home directories (eg cloud storage of pictures, music, movies)? What
>> about all this data storage we're always hearing about that is storing
>> more data than all the National Geographics in dusty boxes in people's
>> creeking attics - is this all (ro) to make webpages etc?
>> How are large companies managing their inventories, production reports,
>> financial statements, stocks, puts/calls? How does Apple, GM,
>> Goldman-Sachs store their data and handle filelocking. I assume they're
>> all on SANs. Is the data all handled by the database (Oracle?) which
>> handles the locking for the user and the user never sees the shared blocks?
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