Date: Mon, 17 May 1999 11:36:17 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Zhihui Zhang <zzhang@cs.binghamton.edu> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: cylinder group and special device Message-ID: <199905171836.LAA27148@apollo.backplane.com> References: <Pine.GSO.3.96.990517134334.26500A-100000@sol.cs.binghamton.edu>
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:Can anyone answer the following two questions for me:
:
:(1) Does a cylinder group in FFS have to begin at a cylinder boundary?
No. The only thing that happens if it isn't is that the '# (Cyl. .... )'
comment in the disklabel has asterixes in it. Modern hard drives
use variable length tracks so most people do not bother trying to
align the placement of filesystemes on the disk. The filesystem
doesn't care, you shouldn't care, so don't worry about it.
:(2) If we read a block via a special device name (/dev/xxx), will the
:block be buffered as normal file data and used when we need the block
:again?
If you use, for example, /dev/da0a, it will be buffered. If you
use /dev/rda0a, it will not be buffered. 'r' means raw-unbuffered.
Accessing block devices directly is somewhat dangerous. The system
will not allow you to open the buffered device if a mounted filesystem
is using the partition, but it will allow you to open the raw device
even if a filesystem is using the partition. This can be extremely
dangerous so care must be taken when accessing block devices.
-Matt
Matthew Dillon
<dillon@backplane.com>
:Thanks for any help.
:
:--------------------------------------------------
:Zhihui Zhang. Please visit http://www.freebsd.org
:--------------------------------------------------
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