Date: Mon, 13 May 2013 09:38:02 -0400 From: Paul Kraus <paul@kraus-haus.org> To: =?iso-8859-1?Q?Trond_Endrest=F8l?= <Trond.Endrestol@fagskolen.gjovik.no> Cc: "freebsd-questions@freebsd.org List" <freebsd-questions@freebsd.org> Subject: Re: ZFS mirror install /mnt is empty Message-ID: <8C7A7E3A-355A-405F-840E-A60B4B6CBB1C@kraus-haus.org> In-Reply-To: <alpine.BSF.2.00.1305131522340.72982@mail.fig.ol.no> References: <5190058D.2030705@micite.net> <alpine.BSF.2.00.1305130743320.72982@mail.fig.ol.no> <472E17AF-B249-4FD3-8F5E-716F8B7867F5@kraus-haus.org> <alpine.BSF.2.00.1305131522340.72982@mail.fig.ol.no>
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On May 13, 2013, at 9:25 AM, Trond Endrest=F8l = <Trond.Endrestol@fagskolen.gjovik.no> wrote: >=20 > I guess it's due to my (mis)understanding that files shorter than 4KB=20= > stored on 4K drives never will be subject to compression. And as you=20= > state below, the degree of compression depends largely on the data at=20= > 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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