Date: Sat, 28 Dec 1996 18:49:02 +0100 (MET) From: Wilko Bulte <wilko@yedi.iaf.nl> To: joerg_wunsch@uriah.heep.sax.de Cc: freebsd-scsi@FreeBSD.org, ping@red.stepnet.com Subject: Re: Q: ideal block size for exb 8505 Message-ID: <199612281749.SAA10725@yedi.iaf.nl> In-Reply-To: <199612281549.QAA12728@uriah.heep.sax.de> from "J Wunsch" at Dec 28, 96 04:49:51 pm
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As J Wunsch wrote... > As Ping Mai wrote: > > > I am using dump to backup to a exb 8505xl. can anyone > > recommand an ideal block size to get max through > > put? i understand that one can also set the block size > > using mt. how's this diff from blocksize given to > > dump? i am confused. > > You can only set a fixed blocksize with mt(1). Most drives limit the > ability to accept a fixed blocksize to 512 bytes, some also allow for > 1024 bytes. Most (and all modern) drives don't work with a fixed > block size, but with a variable one (expressed as 0). This basically > means they write the amount you pass to the driver in a single > write(2) call into a single tape block. Reading these blocks requires > a read(2) of at least the same (or a larger) size, and will return > exactly the amount of bytes that have been written into that block > (i.e., will yield a `short read' when attempting to read more bytes > than have been there in the tape block -- that's how restore(8) > examines the tape block size). > > Something like 32 KB is a good value. Larger values reduce the > overhead, but the impact is probably getting neglicible above > something like 10 KB. Some programs default to 512 bytes only which Keep in mind that drives that use hardware compression really appreciate a big blocksize. For example DLTs really like 32kB, 10kB is too small (for compressible data) to keep 'm really working at max efficiency. But again: highly data and device dependent. Wilko _ ____________________________________________________________________ | / o / / _ Bulte email: wilko@yedi.iaf.nl - Arnhem, The Netherlands |/|/ / / /( (_) Do, or do not. There is no 'try' - Yoda --------------------------------------------------------------------------
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