On May 13, 2013, at 9:25 AM, Trond Endrestøl
<trond.endres...@fagskolen.gjovik.no> wrote:
>
> I guess it's due to my (mis)understanding that files shorter than 4KB
> stored on 4K drives never will be subject to compression. And as you
> state below, the degree of compression depends largely on the data at
> hand.
Not a misunderstanding at all. With a 4K minimum block size (which is
what a 4K sector size implies), a file less than 4KB will not compress at all.
While ZFS does have a variable block size (512B to 128KB), with a 4K minimum
black size (just like with any fixed block FS with a 4KB block size), small
files take up more pace than they should (a 1KB file takes up an entire 4KB
block). This ends up being an artifact of the block size and not ZFS, any FS on
a 4K sector drive will have similar behavior.
I leave compression off on most of my datasets, only turning it on on
ones where I see a real benefit. /var compresses vert well (I turn off
compression in /etc/newsyslog.conf and let ZFS compress even the current logs
:-), I find that some VM's compress very well, media files do NOT compress very
well (they tend to already be compressed), generic data compresses well, as do
scanned documents (uncompressed PDFs). Your individual results will vary :-)
Also remember, if you start with compression on and after a while you
are not seeing good compression ratios, go ahead and turn it off. The already
written data will remain compressed but new writes will not be.
--
Paul Kraus
Deputy Technical Director, LoneStarCon 3
Sound Coordinator, Schenectady Light Opera Company
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