6 Mar
2021
6 Mar
'21
7:16 a.m.
I haven't redirected this reply to COFF, though I was considering
doing so: it
On Friday, 5 March 2021 at 1:50:32 -0800, John Gilmore wrote:
John P. Linderman <jpl.jpl(a)gmail.com> wrote:
This appears to have been inherited from UFS. My recollection is
pretty much the same for UFS, and even the man page entry for EXT4 is
closely related to UFS newfs(8):
Linux:
-m reserved-blocks-percentage
Specify the percentage of the filesystem blocks reserved for
the
super-user. This avoids fragmentation, and allows
root-owned
daemons, such as syslogd(8), to continue to function
correctly
after non-privileged processes are prevented from writing to
the
filesystem. The default percentage is 5%.
FreBSD:
-m free-space
The percentage of space reserved from normal users; the minimum
free space threshold. The default value used is defined by
MINFREE from <ufs/ffs/fs.h>, currently 8%. See tunefs(8) for
more details on how to set this option.
And I can confirm that file systems slow down significantly after the
last 1% of free space has been used.
I have several 12 TB disks scattered about my
house. 5% of 12TB is 600GB.
At one point in hystery, ext2 performance was reported to suffer badly
if there was less than 5% of disk space available in an active
filesystem. My naive belief, probably informed by older and wiser heads
around Sun, was that when the file system was >95% full, ext2 spent a
lot of time seeking around in free lists finding single allocatable
blocks. And there were no built-in "defragmentation" programs that
could easily fix that. Is that still a performance constraint in ext4, which
has had a few
decades to work out those edge performance issues?
Hopefully Ted will comment. But UFS has had even more decades. And
arguably 600 GB on a modern disk is less of a concern than 15 MB on a
300 MB disk 40 years ago.
Greg
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