Matthew Wakeling wrote:
Probably of more use are some of the other settings:
-m reserved-blocks-percentage - this reserves a portion of the filesystem
that only root can write to. If root has no need for it, you can kill
this by setting it to zero. The default is for 5% of the disc to be
wasted.
This is not a good idea. The 5% is NOT reserved for root's use, but rather is
to prevent severe file fragmentation. As the disk gets full, the remaining
empty spaces tend to be small spaces scattered all over the disk, meaning that
even for modest-sized files, the kernel can't allocate contiguous disk blocks.
If you reduce this restriction to 0%, you are virtually guaranteed poor
performance when you fill up your disk, since those files that are allocated
last will be massively fragmented.
Worse, the fragmented files that you create remain fragmented even if you clean
up to get back below the 95% mark. If Postgres happened to insert a lot of
data on a 99% full file system, those blocks could be spread all over the
place, and they'd stay that way forever, even after you cleared some space.
Craig
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