Performance of autofs/amd-managed directory

Performance of autofs/amd-managed directory

[Kenny Simpson]

What is the performance penalty for write access to files in a managed
directory vs. a fstab entry?
I have a program that is very sensitive to write latency, and am being
pressured to use amd or automount to access a NAS instead of my trusty fstab
entry.

My app uses a transactional log file that resides on a NAS.  I'd prefer to keep
a simple fstab entry and use O_DIRECT to access the file, but if there is no
penalty, I would switch.  Does O_DIRECT have meaning for automount or
amd-managed files?  (isn't amd user-space, so it would need to copy data
anyway?)

thanks,
-Kenny



	
		
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Re: Performance of autofs/amd-managed directory

[Ian Kent]

On Tue, 25 May 2004, Kenny Simpson wrote:

> What is the performance penalty for write access to files in a managed
> directory vs. a fstab entry?
> I have a program that is very sensitive to write latency, and am being
> pressured to use amd or automount to access a NAS instead of my trusty fstab
> entry.

autofs only performes the mounts on demand not much more.

Once the mount is done autofs knows is completely out of the way as far as 
mounted filesystem access is concerned.

> 
> My app uses a transactional log file that resides on a NAS.  I'd prefer to keep
> a simple fstab entry and use O_DIRECT to access the file, but if there is no
> penalty, I would switch.  Does O_DIRECT have meaning for automount or
> amd-managed files?  (isn't amd user-space, so it would need to copy data
> anyway?)

I don't know about the amd design and its overhead, sorry.

Ian

Re: Performance of autofs/amd-managed directory

[Mike Waychison]

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Kenny Simpson wrote:
> What is the performance penalty for write access to files in a managed
> directory vs. a fstab entry?
> I have a program that is very sensitive to write latency, and am being
> pressured to use amd or automount to access a NAS instead of my trusty
fstab
> entry.
>
> My app uses a transactional log file that resides on a NAS.  I'd
prefer to keep
> a simple fstab entry and use O_DIRECT to access the file, but if there
is no
> penalty, I would switch.  Does O_DIRECT have meaning for automount or
> amd-managed files?  (isn't amd user-space, so it would need to copy data
> anyway?)
>

As Ian already pointed out, automount simply mounts filesystems in place
when requested and does not affect any file access fast-paths.

Similarly, amd also simply mounts the NFS filesystems, albeit in a
different location and uses symlinks to redirect the user.  This has the
side-effect that a chdir('..') will drop you in a different directory.

As long as you have O_DIRECT support in your NFS client, neither
solution will have any real adverse effects on your transactional log.

- --
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice
mailto: Michael.Waychison@Sun.COM
http://www.sun.com

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