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Date: Thu, 28 Aug 1997 05:36:47 +1000
From: Bruce Evans <bde@zeta.org.au>
Message-Id: <199708271936.FAA05649@godzilla.zeta.org.au>
To: dg@root.com, Shimon@i-Connect.Net
Subject: Re: IDE vs SCSI was: flags 80ff work...
Cc: current@freebsd.org
Sender: owner-freebsd-current@freebsd.org
X-Loop: FreeBSD.org
Precedence: bulk

>Copper was left alone, except for running:
>  ``dd if=/dev/sd16s1a of=/dev/null bs=1024k''
>The partition read is a 6GB slice off a 6x4GB RAID-0 array
>...
>While David is right in regards to the filesystem, I did get
>something correctly.  Regardless of block size specified, access
>to a block device is always in 4K blocks.

Nope.  By default, block devices are accessed with the too-small
block size of BLKDEV_IOSIZE = 2K.  The default is used for the
whole-disk device (e.g., /dev/sd16) unless the disk is "dedicated"
and happens to have a BSD file system on its "c" partition.
For block devices that have a BSD file system on them, the block
size is taken from the file system.  You apparently have a BSD
file system with a block size of 4K on /dev/sd16s1a.  4K is a
small block size for a BSD file system (the default is 8K and
the max is 64K), but is good for some applications (ones where
the average write is <= 4K, and ones where disk space is scarce).

Block devices are unsuitable for almost everything except mounting
file systems on.  They are unsuitable for benchmarks because you
can't control the block size.

I just noticed another bug: stat() says that st_blksize is 2K
for all bdevs.  It doesn't know that a different size is used
internally for bdevs with a BSD file system on them.  (st_blksize
is bogus for cdevs too.  It is always MAXBSIZE = 64K, which is
excessive for terminals.  This problem got worse when MAXBSIZE
was increased from 16K to 64K.)

Bruce