zfs discuss - Feb 2007 - Best Practises => Keep Pool Below 80%?

In the ZFS Best Practises Guide here:

   http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide

It says:

   ``Currently, pool performance can degrade when a pool is very full
     and file systems are updated frequently, such as on a busy mail
     server. Under these circumstances, keep pool space under 80%
     utilization to maintain pool performance.''''

What''s the technical reason for this?  Is there a CR describing this?

I tried searching the archive, but didn''t turn up anything obvious...

Thanks,
Jarod

zfs-discuss-bounces at opensolaris.org wrote on 02/13/2007 09:48:54 AM:
> In the ZFS Best Practises Guide here:
>
>
http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide
>
> It says:
>
>    ``Currently, pool performance can degrade when a pool is very full
>      and file systems are updated frequently, such as on a busy mail
>      server. Under these circumstances, keep pool space under 80%
>      utilization to maintain pool performance.''''
>
> What''s the technical reason for this?  Is there a CR describing
this?
>
> I tried searching the archive, but didn''t turn up anything
obvious...
I don''t know of any publicly available tech docs for this
recommendation --
nor do I know that 80% is the "magic" number.  I assume 80% is a safe
percentage to allow for COW/FS/slab overhead on busy filesystems with many
different workloads found in the wild.  This type of recommendation exists
for many filesystems out there, usually 10% -> 20% to allow for best FS
behavior.  For instance on VXFS,  running into low space mode can slow
writes [b]incredibly[b] as free extents and other restructuring become very
high cost.  As with any recommendation,  you may find that on your systems
workload you can get way closer to "full" with no penalties.

-Wade

Jarod Nash - Sun UK wrote:
> In the ZFS Best Practises Guide here:
> 
>   http://www.solarisinternals.com/wiki/index.php/ZFS_Best_Practices_Guide
> 
> It says:
> 
>   ``Currently, pool performance can degrade when a pool is very full
>     and file systems are updated frequently, such as on a busy mail
>     server. Under these circumstances, keep pool space under 80%
>     utilization to maintain pool performance.''''
> 
> What''s the technical reason for this?  Is there a CR describing
this?
One of the main bugs causing this recommendation is 6495013.  Fixing 
this is one of our top priorities.

--matt

Hello Matthew,

Tuesday, February 13, 2007, 9:53:35 PM, you wrote:

MA> One of the main bugs causing this recommendation is 6495013.  Fixing 
MA> this is one of our top priorities.

I would be VERY interested when this is fixed.


-- 
Best regards,
 Robert                            mailto:rmilkowski at task.gda.pl
                                       http://milek.blogspot.com

The space management algorithms in many file systems don''t always
perform well when they can''t find a free block of the desired size.
There''s often a "cliff" where on average, once the file
system is too full, performance drops off exponentially. UFS deals with this by
reserving space explicitly to the root user (in the hope that the system
administrator knows about the problem, and ordinary users can''t trigger
it). ZFS gives all the space to the user, which is good, but it means the user
needs to avoid filling the file system "too much."
 
 
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