Date: Sat, 1 Jun 1996 01:57:14 +1000 From: Bruce Evans <bde@zeta.org.au> To: freebsd-hackers@freefall.freebsd.org, sextonr.crestvie@squared.com Subject: Re: Disktab and SCSI Performance. Message-ID: <199605311557.BAA11037@godzilla.zeta.org.au>
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> Do the disk parameters in disktab have any meaning? It seems that when For some disks. >we do the mapping from the drive's own private geometry to the scsi block >number, we lose the connection with the drives' own performance >characteristics. I am specifically wondering about the selection of Usually not for SCSI disks. It wouldn't hurt to make the disktab geometry match the actual geometry (if any) for the most active part of the disk. >drive speed, and to what degree the filesystem can use this information. The default newfs parameters are chosen to inhibit the parts of ffs that use this info: -d 0 Rotdelay is only useful for slow disks. All small nonzero rotdelays are equivalent to skipping every second block, i.e., to using an interleave of 2 for blocks. This is in addition to any hardware interleave, and has much the same disaadvantages as hardware interleave: an interleave of N reduces the best-case performance by a factor of N. Disks with their Iown read _and_ write buffering don't need to be interleaved at the block level. For disks with read but not write buffering, you can use -d 0 to get the best possible read performance but abysmal write performance, or -d 4 to get medicocre performance for both read and write. -n 1 This value essentially disables the rotational position table. I think the interleave, rpm and track skew parameters are only used to construct this table, so this option makes them irrelevant. The cylinder skew parameter is never used. -t 1 -u 4096 (one track/cylinder, 4096 sectors/track) This limits the damage from any "optimizations" that weren't stopped by `-d 0 -n 1' to 4096-sector boundaries. Bruce
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