NFS Mount Option 'nofsc'

NFS Mount Option 'nofsc'

[Derek McEachern]

I joined the mailing list shortly after Neil sent out a request for 
volunteer to update the nfs man page documenting the 'fsc'/'nofsc' 
options. I suspect this may stem from a ticket we opened with Suse 
inquiring about these options.

Coming from a Solaris background we typically use the 'forcedirectio' 
option for certain mounts and I was looking for the same thing in Linux. 
The typically advice seems to be use 'noac' but the description in the 
man page doesn't seem to match what I would expect from 'forcedirectio', 
namely no buffering on the client.

Poking around the kernel I found the 'fsc'/'nofsc' options and my 
question is does 'nofsc' provide 'forcedirectio' functionality?

Thanks,
Derek

Re: NFS Mount Option 'nofsc'

[Myklebust, Trond]

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Re: NFS Mount Option 'nofsc'

[Harshula]

Hi Trond,

On Wed, 2012-02-08 at 04:55 +0000, Myklebust, Trond wrote:

> Applications that need to use uncached i/o are required to use the
> O_DIRECT open() mode instead, since pretty much all of them need to be
> rewritten to deal with the subtleties involved anyway.

Could you please expand on the subtleties involved that require an
application to be rewritten if forcedirectio mount option was available?

A scenario where forcedirectio would be useful is when an application
reads nearly a TB of data from local disks, processes that data and then
dumps it to an NFS mount. All that happens while other processes are
reading/writing to the local disks. The application does not have an
O_DIRECT option nor is the source code available.

With paged I/O the problem we see is that the NFS client system reaches
dirty_bytes/dirty_ratio threshold and then blocks/forces all the
processes to flush dirty pages. This effectively 'locks' up the NFS
client system while the NFS dirty pages are pushed slowly over the wire
to the NFS server. Some of the processes that have nothing to do with
writing to the NFS mount are badly impacted. A forcedirectio mount
option would be very helpful in this scenario. Do you have any advice on
alleviating such problems on the NFS client by only using existing
tunables?

Thanks,
#

Re: NFS Mount Option 'nofsc'

[Chuck Lever]

On Feb 8, 2012, at 2:43 AM, Harshula wrote:

> Hi Trond,
> 
> On Wed, 2012-02-08 at 04:55 +0000, Myklebust, Trond wrote:
> 
>> Applications that need to use uncached i/o are required to use the
>> O_DIRECT open() mode instead, since pretty much all of them need to be
>> rewritten to deal with the subtleties involved anyway.
> 
> Could you please expand on the subtleties involved that require an
> application to be rewritten if forcedirectio mount option was available?
> 
> A scenario where forcedirectio would be useful is when an application
> reads nearly a TB of data from local disks, processes that data and then
> dumps it to an NFS mount. All that happens while other processes are
> reading/writing to the local disks. The application does not have an
> O_DIRECT option nor is the source code available.
> 
> With paged I/O the problem we see is that the NFS client system reaches
> dirty_bytes/dirty_ratio threshold and then blocks/forces all the
> processes to flush dirty pages. This effectively 'locks' up the NFS
> client system while the NFS dirty pages are pushed slowly over the wire
> to the NFS server. Some of the processes that have nothing to do with
> writing to the NFS mount are badly impacted. A forcedirectio mount
> option would be very helpful in this scenario. Do you have any advice on
> alleviating such problems on the NFS client by only using existing
> tunables?

Using direct I/O would be a work-around.  The fundamental problem is the architecture of the VM system, and over time we have been making improvements there.

Instead of a mount option, you can fix your application to use direct I/O.  Or you can change it to provide the kernel with (better) hints about the disposition of the data it is generating (madvise and fadvise system calls).  (On Linux we assume you have source code and can make such changes.  I realize this is not true for proprietary applications).

You could try using the "sync" mount option to cause the NFS client to push writes to the server immediately rather than delaying them.  This would also slow down applications that aggressively dirties pages on the client.

Meanwhile, you can dial down the dirty_ratio and especially the dirty_background_ratio settings to trigger earlier writeback.  We've also found increasing min_free_bytes has positive effects.  The exact settings depend on how much memory your client has.  Experimenting yourself is pretty harmless, so I won't give exact settings here.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Re: NFS Mount Option 'nofsc'

[Harshula]

Hi Chuck,

On Wed, 2012-02-08 at 10:40 -0500, Chuck Lever wrote:
> On Feb 8, 2012, at 2:43 AM, Harshula wrote:

> > Could you please expand on the subtleties involved that require an
> > application to be rewritten if forcedirectio mount option was available?
> > 
> > A scenario where forcedirectio would be useful is when an application
> > reads nearly a TB of data from local disks, processes that data and then
> > dumps it to an NFS mount. All that happens while other processes are
> > reading/writing to the local disks. The application does not have an
> > O_DIRECT option nor is the source code available.
> > 
> > With paged I/O the problem we see is that the NFS client system reaches
> > dirty_bytes/dirty_ratio threshold and then blocks/forces all the
> > processes to flush dirty pages. This effectively 'locks' up the NFS
> > client system while the NFS dirty pages are pushed slowly over the wire
> > to the NFS server. Some of the processes that have nothing to do with
> > writing to the NFS mount are badly impacted. A forcedirectio mount
> > option would be very helpful in this scenario. Do you have any advice on
> > alleviating such problems on the NFS client by only using existing
> > tunables?
> 
> Using direct I/O would be a work-around.  The fundamental problem is
> the architecture of the VM system, and over time we have been making
> improvements there.
> 
> Instead of a mount option, you can fix your application to use direct
> I/O.  Or you can change it to provide the kernel with (better) hints
> about the disposition of the data it is generating (madvise and
> fadvise system calls).  (On Linux we assume you have source code and
> can make such changes.  I realize this is not true for proprietary
> applications).
> 
> You could try using the "sync" mount option to cause the NFS client to
> push writes to the server immediately rather than delaying them.  This
> would also slow down applications that aggressively dirties pages on
> the client.
> 
> Meanwhile, you can dial down the dirty_ratio and especially the
> dirty_background_ratio settings to trigger earlier writeback.  We've
> also found increasing min_free_bytes has positive effects.  The exact
> settings depend on how much memory your client has.  Experimenting
> yourself is pretty harmless, so I won't give exact settings here.

Thanks for the reply. Unfortunately, not all vendors provide the source
code, so using O_DIRECT or fsync is not always an option. 

Lowering dirty_bytes/dirty_ratio and
dirty_background_bytes/dirty_background_ratio did help as it smoothed
out the data transfer over the wire by pushing data out to the NFS
server sooner. Otherwise, I was seeing the data transfer over the wire
having idle periods while >10GiB of pages were being dirtied by the
processes, then congestion as soon as the dirty_ratio was reached and
the frantic flushing of dirty pages to the NFS server. However,
modifying dirty_* tunables has a system-wide impact, hence it was not
accepted.

The "sync" option, depending on the NFS server, may impact the NFS
server's performance when serving many NFS clients. But still worth a
try.

The other hack that seems to work is periodically triggering an
nfs_getattr(), via ls -l, to force the dirty pages to be flushed to the
NFS server. Not exactly elegant ...

Thanks,
#

Re: NFS Mount Option 'nofsc'

[Myklebust, Trond]

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Re: NFS Mount Option 'nofsc'

[Harshula]

Hi Trond,

Thanks for the reply. Could you please elaborate on the subtleties
involved that require an application to be rewritten if forcedirectio
mount option was available?

On Thu, 2012-02-09 at 04:12 +0000, Myklebust, Trond wrote:
> On Thu, 2012-02-09 at 14:56 +1100, Harshula wrote:
> >
> > The "sync" option, depending on the NFS server, may impact the NFS
> > server's performance when serving many NFS clients. But still worth a
> > try.
> 
> What on earth makes you think that directio would be any different?

Like I said, sync is still worth a try. I will do O_DIRECT Vs sync mount
option runs and see what the numbers look like. A while back the numbers
for cached Vs direct small random writes showed as the number of threads
increased the cached performance fell well below direct performance. In
this case I'll be looking at large streaming writes, so completely
different scenario, but I'd like to verify the numbers first.

Just to be clear, I am not disagreeing with you. "sync" maybe sufficient
for the scenario I described earlier.

> If
> your performance requirements can't cope with 'sync', then they sure as
> hell won't deal well with 'fsc'.

"fsc"? 

> Directio is _synchronous_ just like 'sync'. The big difference is that
> with 'sync' then at least those reads are still cached.

There's another scenario, which we talked about a while back, where the
cached async reads of a slowly growing file (tail) was spitting out
non-exist NULLs to user space. The forcedirectio mount option should
prevent that. Furthermore, the "sync" mount option will not help anymore
because you removed nfs_readpage_sync().

> > The other hack that seems to work is periodically triggering an
> > nfs_getattr(), via ls -l, to force the dirty pages to be flushed to the
> > NFS server. Not exactly elegant ...
> 
> ???????????????????????????????? 

int nfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
{
        struct inode *inode = dentry->d_inode;
        int need_atime = NFS_I(inode)->cache_validity & NFS_INO_INVALID_ATIME;
        int err;

        /* Flush out writes to the server in order to update c/mtime.  */
        if (S_ISREG(inode->i_mode)) {
                err = filemap_write_and_wait(inode->i_mapping);
                if (err)
                        goto out;
        }

Thanks,
#

Re: NFS Mount Option 'nofsc'

[Malahal Naineni]

Harshula [harshula@redhat.com] wrote:
> Hi Trond,
> 
> Thanks for the reply. Could you please elaborate on the subtleties
> involved that require an application to be rewritten if forcedirectio
> mount option was available?
> 
> On Thu, 2012-02-09 at 04:12 +0000, Myklebust, Trond wrote:
> > On Thu, 2012-02-09 at 14:56 +1100, Harshula wrote:
> > >
> > > The "sync" option, depending on the NFS server, may impact the NFS
> > > server's performance when serving many NFS clients. But still worth a
> > > try.
> > 
> > What on earth makes you think that directio would be any different?
> 
> Like I said, sync is still worth a try. I will do O_DIRECT Vs sync mount
> option runs and see what the numbers look like. A while back the numbers
> for cached Vs direct small random writes showed as the number of threads
> increased the cached performance fell well below direct performance. In
> this case I'll be looking at large streaming writes, so completely
> different scenario, but I'd like to verify the numbers first.

directio and sync behavior should be same on server side, but it would
be a different story on the client though. The above behavior you
described is expected on the client.

Thanks, Malahal.

Re: NFS Mount Option 'nofsc'

[Myklebust, Trond]

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Re: NFS Mount Option 'nofsc'

[Harshula]

Hi Trond,

On Thu, 2012-02-09 at 15:31 +0000, Myklebust, Trond wrote:
> On Thu, 2012-02-09 at 16:51 +1100, Harshula wrote:
> > Hi Trond,
> > 
> > Thanks for the reply. Could you please elaborate on the subtleties
> > involved that require an application to be rewritten if forcedirectio
> > mount option was available?
> 
> Firstly, we don't support O_DIRECT+O_APPEND (since the NFS protocol
> itself doesn't support atomic appends), so that would break a bunch of
> applications.
> 
> Secondly, uncached I/O means that read() and write() requests need to be
> serialised by the application itself, since there are no atomicity or
> ordering guarantees at the VFS, NFS or RPC call level. Normally, the
> page cache services read() requests if there are outstanding writes, and
> so provides the atomicity guarantees that POSIX requires.
> IOW: if a write() occurs while you are reading, the application may end
> up retrieving part of the old data, and part of the new data instead of
> either one or the other.
> 
> IOW: your application still needs to be aware of the fact that it is
> using O_DIRECT, and you are better of adding explicit support for it
> rather than hacky cluges such as a forcedirectio option.

Thanks. Would it be accurate to say that if there were only either
streaming writes or (xor) streaming reads to any given file on the NFS
mount, the application would not need to be rewritten? 

Do you see forcedirectio as a sharp object that someone could stab
themselves with?

> > There's another scenario, which we talked about a while back, where the
> > cached async reads of a slowly growing file (tail) was spitting out
> > non-exist NULLs to user space. The forcedirectio mount option should
> > prevent that. Furthermore, the "sync" mount option will not help anymore
> > because you removed nfs_readpage_sync().
> 
> No. See the points about O_APPEND and serialisation of read() and
> write() above. You may still end up seeing NUL characters (and indeed
> worse forms of corruption).

If the NFS client only does cached async reads of a slowly growing file
(tail), what's the problem? Is nfs_readpage_sync() gone forever, or
could it be revived?

> > > > The other hack that seems to work is periodically triggering an
> > > > nfs_getattr(), via ls -l, to force the dirty pages to be flushed to the
> > > > NFS server. Not exactly elegant ...
> > > 
> > > ???????????????????????????????? 
> > 
> > int nfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
> > {
> >         struct inode *inode = dentry->d_inode;
> >         int need_atime = NFS_I(inode)->cache_validity & NFS_INO_INVALID_ATIME;
> >         int err;
> > 
> >         /* Flush out writes to the server in order to update c/mtime.  */
> >         if (S_ISREG(inode->i_mode)) {
> >                 err = filemap_write_and_wait(inode->i_mapping);
> >                 if (err)
> >                         goto out;
> >         }
> 
> I'm aware of that code. The point is that '-osync' does that for free.

-osync also impacts the performance of the entire NFS mount. With
aforementioned hack, you can isolate the specific file(s) that need
their dirty pages to be flushed frequently to avoid hitting global dirty
page limit.

cya,
#

Re: NFS Mount Option 'nofsc'

[Myklebust, Trond]

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Re: NFS Mount Option 'nofsc'

[Harshula]

Hi Trond,

On Fri, 2012-02-10 at 16:48 +0000, Myklebust, Trond wrote:
> On Fri, 2012-02-10 at 19:07 +1100, Harshula wrote:

> > Do you see forcedirectio as a sharp object that someone could stab
> > themselves with?
> 
> Yes. It does lead to some very subtle POSIX violations.

I'm trying out the alternatives. Your list of reasons were convincing. Thanks.

> > If the NFS client only does cached async reads of a slowly growing file
> > (tail), what's the problem? Is nfs_readpage_sync() gone forever, or
> > could it be revived?
> 
> It wouldn't help at all. The problem is the VM's handling of pages vs
> the NFS handling of file size.
> 
> The VM basically uses the file size in order to determine how much data
> a page contains. If that file size changed between the instance we
> finished the READ RPC call, and the instance the VM gets round to
> locking the page again, reading the data and then checking the file
> size, then the VM may end up copying data beyond the end of that
> retrieved by the RPC call.

nfs_readpage_sync() keeps doing rsize reads (or PAGE SIZE reads if rsize
> PAGE SIZE) till the entire PAGE has been filled or EOF is hit. Since
these are synchronous reads, the subsequent READ RPC call is not sent
until the previous READ RPC reply arrives. Hence, the READ RPC reply
contains the latest metadata about the file, from the NFS server, before
deciding whether or not to do more READ RPC calls. That is not the case
with the asynchronous READ RPC calls which are queued to be sent before
the replies are received. This results in not READing enough data from
the NFS server even when the READ RPC reply explicitly states that the
file has grown. This mismatch of data and file size is then presented to
the VM.

If you look at nfs_readpage_sync() code, it does not worry about
adjusting the number of bytes to read if it is past the *current* EOF.
Only the async code adjusts the number of bytes to read if it is past
the *current* EOF. Furthermore, testing showed that using -osync (while
nfs_readpage_sync() existed) avoided the NULLs being presented to
userspace.

cya,
#

Re: NFS Mount Option 'nofsc'

[Derek McEachern]

-------- Original Message --------
Subject: Re: NFS Mount Option 'nofsc'
From: Myklebust, Trond <Trond.Myklebust@netapp.com>
To: Derek McEachern <derekm@ti.com>
CC: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Date: Tuesday, February 07, 2012 10:55:04 PM
> On Tue, 2012-02-07 at 20:45 -0600, Derek McEachern wrote:
>> I joined the mailing list shortly after Neil sent out a request for
>> volunteer to update the nfs man page documenting the 'fsc'/'nofsc'
>> options. I suspect this may stem from a ticket we opened with Suse
>> inquiring about these options.
>>
>> Coming from a Solaris background we typically use the 'forcedirectio'
>> option for certain mounts and I was looking for the same thing in Linux.
>> The typically advice seems to be use 'noac' but the description in the
>> man page doesn't seem to match what I would expect from 'forcedirectio',
>> namely no buffering on the client.
>>
>> Poking around the kernel I found the 'fsc'/'nofsc' options and my
>> question is does 'nofsc' provide 'forcedirectio' functionality?
> No. There is no equivalent to the Solaris "forcedirectio" mount option
> in Linux.
> Applications that need to use uncached i/o are required to use the
> O_DIRECT open() mode instead, since pretty much all of them need to be
> rewritten to deal with the subtleties involved anyway.
>
> Trond

So then what exact functionality if provided by the 'nofsc' option? It 
would seem to me from a write perspective that between noac and the sync 
option it is pretty close to forcedirectio.

 From the man page describing sync "any system call that writes data to 
files on that mount point causes that data to be flushed to the server 
before the system call returns control to user  space."

Maybe I've answered one of my questions as flushing the data to the 
server before returning to user space is really what I'm after. The 
userspace app should be blocked until the write has been acknowledged by 
the server and if the server is an NFS appliance then I don't 
necessarily care if it has committed the data to disk as I expect it to 
managed its cache properly.

Though I still want to understand what 'nofsc' is doing.

Derek

Re: NFS Mount Option 'nofsc'

[Chuck Lever]

On Feb 8, 2012, at 1:13 PM, Derek McEachern wrote:

> 
> 
> -------- Original Message --------
> Subject: Re: NFS Mount Option 'nofsc'
> From: Myklebust, Trond <Trond.Myklebust@netapp.com>
> To: Derek McEachern <derekm@ti.com>
> CC: "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
> Date: Tuesday, February 07, 2012 10:55:04 PM
>> On Tue, 2012-02-07 at 20:45 -0600, Derek McEachern wrote:
>>> I joined the mailing list shortly after Neil sent out a request for
>>> volunteer to update the nfs man page documenting the 'fsc'/'nofsc'
>>> options. I suspect this may stem from a ticket we opened with Suse
>>> inquiring about these options.
>>> 
>>> Coming from a Solaris background we typically use the 'forcedirectio'
>>> option for certain mounts and I was looking for the same thing in Linux.
>>> The typically advice seems to be use 'noac' but the description in the
>>> man page doesn't seem to match what I would expect from 'forcedirectio',
>>> namely no buffering on the client.
>>> 
>>> Poking around the kernel I found the 'fsc'/'nofsc' options and my
>>> question is does 'nofsc' provide 'forcedirectio' functionality?
>> No. There is no equivalent to the Solaris "forcedirectio" mount option
>> in Linux.
>> Applications that need to use uncached i/o are required to use the
>> O_DIRECT open() mode instead, since pretty much all of them need to be
>> rewritten to deal with the subtleties involved anyway.
>> 
>> Trond
> 
> So then what exact functionality if provided by the 'nofsc' option? It would seem to me from a write perspective that between noac and the sync option it is pretty close to forcedirectio.
> 
> From the man page describing sync "any system call that writes data to files on that mount point causes that data to be flushed to the server before the system call returns control to user  space."
> 
> Maybe I've answered one of my questions as flushing the data to the server before returning to user space is really what I'm after. The userspace app should be blocked until the write has been acknowledged by the server and if the server is an NFS appliance then I don't necessarily care if it has committed the data to disk as I expect it to managed its cache properly.
> 
> Though I still want to understand what 'nofsc' is doing.

"nofsc" disables file caching on the client's local disk.  It has nothing to do with direct I/O.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Re: NFS Mount Option 'nofsc'

[Derek McEachern]

-------- Original Message --------
Subject: Re: NFS Mount Option 'nofsc'
From: Chuck Lever <chuck.lever@oracle.com>
To: Derek McEachern <derekm@ti.com>
CC: "Myklebust, Trond" <Trond.Myklebust@netapp.com>, 
"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Date: Wednesday, February 08, 2012 12:15:37 PM

>> So then what exact functionality if provided by the 'nofsc' option? It would seem to me from a write perspective that between noac and the sync option it is pretty close to forcedirectio.
>>
>>  From the man page describing sync "any system call that writes data to files on that mount point causes that data to be flushed to the server before the system call returns control to user  space."
>>
>> Maybe I've answered one of my questions as flushing the data to the server before returning to user space is really what I'm after. The userspace app should be blocked until the write has been acknowledged by the server and if the server is an NFS appliance then I don't necessarily care if it has committed the data to disk as I expect it to managed its cache properly.
>>
>> Though I still want to understand what 'nofsc' is doing.
> "nofsc" disables file caching on the client's local disk.  It has nothing to do with direct I/O.
>

If 'nofsc' disables file caching on the client's local disk does that 
mean that write from userspace could go to kernel memory, then 
potentially to client's local disk, before being committed over network 
to the nfs server?

This seems really odd. What would be the use case for this?

Derek

Re: NFS Mount Option 'nofsc'

[Chuck Lever]

On Feb 8, 2012, at 2:52 PM, Derek McEachern wrote:

> 
> 
> -------- Original Message --------
> Subject: Re: NFS Mount Option 'nofsc'
> From: Chuck Lever <chuck.lever@oracle.com>
> To: Derek McEachern <derekm@ti.com>
> CC: "Myklebust, Trond" <Trond.Myklebust@netapp.com>, "linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
> Date: Wednesday, February 08, 2012 12:15:37 PM
> 
>>> So then what exact functionality if provided by the 'nofsc' option? It would seem to me from a write perspective that between noac and the sync option it is pretty close to forcedirectio.
>>> 
>>> From the man page describing sync "any system call that writes data to files on that mount point causes that data to be flushed to the server before the system call returns control to user  space."
>>> 
>>> Maybe I've answered one of my questions as flushing the data to the server before returning to user space is really what I'm after. The userspace app should be blocked until the write has been acknowledged by the server and if the server is an NFS appliance then I don't necessarily care if it has committed the data to disk as I expect it to managed its cache properly.
>>> 
>>> Though I still want to understand what 'nofsc' is doing.
>> "nofsc" disables file caching on the client's local disk.  It has nothing to do with direct I/O.
>> 
> 
> If 'nofsc' disables file caching on the client's local disk does that mean that write from userspace could go to kernel memory, then potentially to client's local disk, before being committed over network to the nfs server?
> 
> This seems really odd. What would be the use case for this?

With "fsc", writes are indeed slower, but reads of a very large file that rarely changes are on average much better.  If a file is significantly larger than a client's page cache, a client can cache that file on its local disk, and get local read speeds instead of going over the wire.

Additionally if multiple clients have to access the same large file, it reduces the load on the storage server if they have their own local copies of that file, since the file is too large for the clients to cache in their page cache.  This also has the benefit of keeping the file data cached across client reboots.

This feature is an optimization for HPC workloads, where a large number of clients access very large read-mostly datasets on a handful of storage servers.  The clients' local fsc absorbs much of the aggregate read workload, allowing storage servers to scale to a larger number of clients.

-- 
Chuck Lever
chuck[dot]lever[at]oracle[dot]com

Re: NFS Mount Option 'nofsc'

[Derek McEachern]

-------- Original Message --------
Subject: Re: NFS Mount Option 'nofsc'
From: Chuck Lever <chuck.lever@oracle.com>
To: Derek McEachern <derekm@ti.com>
CC: "Myklebust, Trond" <Trond.Myklebust@netapp.com>, 
"linux-nfs@vger.kernel.org" <linux-nfs@vger.kernel.org>
Date: Wednesday, February 08, 2012 2:00:24 PM
> On Feb 8, 2012, at 2:52 PM, Derek McEachern wrote:
>
>> If 'nofsc' disables file caching on the client's local disk does that mean that write from userspace could go to kernel memory, then potentially to client's local disk, before being committed over network to the nfs server?
>>
>> This seems really odd. What would be the use case for this?
> With "fsc", writes are indeed slower, but reads of a very large file that rarely changes are on average much better.  If a file is significantly larger than a client's page cache, a client can cache that file on its local disk, and get local read speeds instead of going over the wire.
>
> Additionally if multiple clients have to access the same large file, it reduces the load on the storage server if they have their own local copies of that file, since the file is too large for the clients to cache in their page cache.  This also has the benefit of keeping the file data cached across client reboots.
>
> This feature is an optimization for HPC workloads, where a large number of clients access very large read-mostly datasets on a handful of storage servers.  The clients' local fsc absorbs much of the aggregate read workload, allowing storage servers to scale to a larger number of clients.
>

Thank you, this makes sense for 'fsc'. I'm going to assume then that the 
default is 'nofsc' if nothing is specified.

Derek