Very Computer

I recently went to an interview (hello F if you're subscribing!) and was
asked a question as to why Unix sys admins always leave a partition of 10%
on every disk.  Well, the only thing I could think is that 10% space without
quota would allow creation of log files etc to keep the system up, although
I couldnt think why this would be on a separate partition.  I therefore
enquired whether the question pertained to data disks or the disk with the
root partition.  Apparently it doesnt matter.  Then I turned my answer to
inode tables and reserved space for file table data.  Nope!

So I asked F for the answer. She said that it was so that root could login
to the disk to perform system maintenance.  Now as you can imagine at first
sight this didnt make sense at all (especially if the disk only stores data)
and not wanting to cause confrontation in the interview I let it ride.

Since then I have asked many colleagues and consulted the Solaris Admin
manuals.  FULLY!  This is all I can find (page 10-27 SA-235) and I quote:

Note - Approximately 10 percent of your disk capacity is reserved for file
system proficiency.  This is not reflected in the df -k output.

Anyone have any ideas as to exactly what is this is about?  I failed the
interview apparently because my Unix skills were not strong enough although
I have subsequently had two job offers!

But this has really irritated me and I would like to know: Is she right?

Cheers

Allan

Slightly Disgruntled Unix Sys Admin.

Martin

FYI

mkfs_ufs states :
free=n         The minimum percentage of free  space  to maintain in the file
                   system.  This  space  is  off-limits  to normal users.  Once
                   the file system is  filled  to this   threshold,   only   the

                   superuser can continue writing to   the  file  system.   This

                   parameter can be  subsequently changed  using  the tunefs(1M)

                   command.  The default is 10%.

tunefs states :
DESCRIPTION
     tunefs is designed to change the  dynamic  parameters  of  a
     file system which affect the layout policies.  The file sys-
     tem must be  unmounted  before  using  tunefs.   When  using
     tunefs  with  filesystem, filesystem must be in /etc/vfstab.
     The parameters which are to be changed are indicated by  the
     options given below.

     Generally one should optimize for time unless the file  sys-
     tem is over 90% full.

    -m minfree     Specify the percentage  of  space  held  back
                    from  normal  users;  the  minimum free space
                    threshold.  The default value  used  is  10%.
                    This  value  can be set to 0, however up to a
                    factor of three in throughput  will  be  lost
                    over the performance obtained at a 10% thres-
                    hold.  Note:  If the value  is  raised  above
                    the current usage level, users will be unable
                    to allocate files  until  enough  files  have
                    been  deleted  to get under the higher thres-
                    hold.

fs_ufs states:
fs_minfree
     fs_minfree gives the minimum acceptable percentage  of  file
     system  blocks  which  may  be  free.  If the freelist drops
     below this level only the super-user may continue  to  allo-
     cate  blocks.   fs_minfree  may be set to 0 if no reserve of
     free blocks is deemed necessary, however severe  performance
     degradations  will  be observed if the file system is run at
     greater than 90% full; thus the default value of  fs_minfree
     is 10%.

     Empirically the best trade-off between  block  fragmentation
     and  overall disk utilization at a loading of 90% comes with
     a fragmentation of 8; thus the default fragment size  is  an
     eighth of the block size.

You make your own conclusion sabout how correct F was when she said it was for
system maintenance.

Hope this helps

-AM

Thank you very very much. I feel better now.  That has been nagging at me
for a while and makes a few other things fall into place. You star!

Allan

[deletia]

: Since then I have asked many colleagues and consulted the Solaris Admin
: manuals.  FULLY!  This is all I can find (page 10-27 SA-235) and I quote:

: Note - Approximately 10 percent of your disk capacity is reserved for file
: system proficiency.  This is not reflected in the df -k output.

: Anyone have any ideas as to exactly what is this is about?  I failed the
: interview apparently because my Unix skills were not strong enough although
: I have subsequently had two job offers!

Yep. She's right. However SAs don't do this, it is handled by the OS itself
automagically exactly like it says in the book you quoted. You can see
this for yourself by comparing the partition sizes reported by df and
the actual slice information provided by format.

[deletia]

fpsm
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You're right.  And since the 10% is apparently 1-10% I dont really think she
knew what she was talking about anyway.  She did call it a partition and she
thought it was so that root could play with the fs rather than to maintain
efficiency (despite this no doubt useful side effect).

I guess you win some and you lose some.

Cheers

Allan

: Actually, this explanation does not support the original statement (and I
: don't know any that would, save possibly in the presence of dynamic

Not that I was trying to support it ... 8-)

: concatenation logical volume mechanisms) that a 10% *partition* is left
on
: every disk.  It may simply have been a misstatement of the fact that a

misstatement !!! .... an admin that interchanges partition for reserved
file system space ..... and then proceeds to explain this by saying it's
for roots convenience ... I reckon she's the one that should be looking for
a new job.

: certain percentage of the partition in which the file system resides is
: reserved by the file system.
:

BTW it's not reserved by the file system ...  it's simply not available for
allocation to user data ....it's already part of the filesystem ... but I
guess you know what you mean, right  :-)

-AM

Quote:> I've never seen any formal explaination of this, but I certainly have
seen an
> AIX system which became totally unbootable because the root partition
was
> alowed to fill up completely (a demon running as root went mad and
filled it
> with log files) - even IBM support engineers were unable to get it
restarted
> and they had to wipe it down and rebuild! I have to admit that this
seemed a
> little extream to me, but they assured us that they could not get
round it any
> other way. I have always assumed that it was because there are
processes
> within the kernel that need to be able to write log files and create
tempory
> files and will fail if they cannot hence a machine is likely to become
> unstable if the root and /var partitions fill up. I don't see why
there should
> be similar restrictions on user partitions.

> Martin

But in general it's wise to have this 10% rule for the file systems
were / /var and /tmp are located. For an users filesystem it does not
make much sense, root isn't writing there anyway.

But, vaguely I remenber to have read something as this 10% being used
for defragmentation or preventing huge fragmentation. Can someone
confirm this?

Paul

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