Date: Mon, 10 Nov 2003 20:40:30 -0800 From: Kirk McKusick <mckusick@beastie.mckusick.com> To: Bruce Evans <bde@zeta.org.au> Cc: arch@freebsd.org Subject: Re: newfs and mount vs. half-baked disks Message-ID: <200311110440.hAB4eUaG002072@beastie.mckusick.com> In-Reply-To: Your message of "Mon, 10 Nov 2003 18:01:59 %2B1100." <20031110174559.Q2074@gamplex.bde.org>
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> Date: Mon, 10 Nov 2003 18:01:59 +1100 (EST) > From: Bruce Evans <bde@zeta.org.au> > To: Kirk McKusick <mckusick@mckusick.com> > cc: arch@freebsd.org > Subject: Re: newfs and mount vs. half-baked disks > X-ASK-Info: Whitelist match > > On Sun, 9 Nov 2003, Kirk McKusick wrote: > > > > From: Bruce Evans <bde@zeta.org.au> > > > > The block count is in units of sector size, so disks much larger than > > > 2TB can be supported by disklabel using (fake if necessary) sector sizes > > > larger than 512. File systems need to use similarly large block (fragment > ^^^^^^^^ > > > for ffs) sizes, and some patches are needed for reading superblocks if > ^^^^^^^ > > > the sector size is larger than 8K. Since ffs uses a block size of 16K > > > by default, a sector size of 16K are not unreasonable and this is > > > sufficent for disks smaller then 64TB. > > > > Actually, FFS requires its fragment size be no smaller than the sector size > > (since it presumes that it cannot do read/write in smaller than sector > > sizes). So, on a 16K filesystem, you get 2K fragments. So your hack only > > gets you to 8TB which is not going to last long at current disk growth > > rates. > > This point was noted in the underlined phrase. The blocks size for ffs is > actually the fragment size in this context. So fragments would be as > large as necessary (16K if that is the sector size), and the block size > (he one given by newfs's -b parameter) would be larger. A fragment size > of 16K may even be the right size for very large disks. My benchmarks > say that 16K/8K block/fragment size is not much slower than 16K/2K on > a 60GB disk, but 16K/16K and 32K/any are significantly slower. > > Bruce Using 16K/16K will not change the speed of operation appreciably, but it will *more than double* the space required to hold the typical FreeBSD filesystem (e.g., what would fit on a 60Gb 16K/2K filesystem will require 120Gb on a 16K/16K filesystem). This happens because most files on a typical filesystem are small. That is a poor tradeoff in my opinion. Kirk McKusick
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