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From: terry@cs.weber.edu (A Wizard of Earth C)
Newsgroups: comp.unix.bsd
Subject: Re: Dumb Question: Why 512 byte block?
Message-ID: <1992Dec18.235623.27538@fcom.cc.utah.edu>
Date: 18 Dec 92 23:56:23 GMT
References: <1992Dec18.005050.20594@decuac.dec.com> <1992Dec18.030833.7395@fcom.cc.utah.edu> <1gt736INNjje@menudo.uh.edu>
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In article <1gt736INNjje@menudo.uh.edu> wjin@cs.uh.edu (W. Woody Jin) writes:
>In article <1992Dec18.030833.7395@fcom.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes:
>>In article <1992Dec18.005050.20594@decuac.dec.com>, darryl@vfofu1.dco.dec.com (Darryl Wagoner) writes:
>>|> Why is everything in 1/2 k block instead the BSD standard of 1024 byte blocks?
[ ... ]
>>
>>If I have a set of 6 512 byte files, I will use up 3K of disk for them;
>>similarly, if I had a blocking factor of 1K, I would use of 6K (since the
>>smallest fragment usable by a file is now 1K.
>>
>>If I have 6 1.5K files, this translates to 9K of disk(512B) or 12K of disk(1K).
>>Obviously, if I have 6 1.6K files, both blocking factors take up 512K.
>
>I did not think that Fast Unix File System works this way.
>If you have 6 1.5K files using 1K blocks,
> 6 1k blocks will take each 1K from each file, and
> 3 1k blocks will take 0.5k from each file.
>This was my understading from the famous FUFS paper.
You can't split blocks between files. A block is, by definition, the
smallest possible allocation unit. Thin about the case where you have
a 1 byte file and a 1 block - 1 byte file; what would you do when
adding one or two characters to the first (1 Byte)? TReallocate? Shift
and reallocate for the last byte of the second file?
But... you're right that it doesn't work the way I've shown... thought
for sure I'd get bombarded on this one:
>>Obviously, if I have 6 1.6K files, both blocking factors take up 512K.
should have read:
>>Obviously, if I have 6 1.6K files, both blocking factors take up 12K.
1.6K rounded to a both a 512b and 1K boundry is 2K, and 2k * 6 = 12k.
>>When you store a lot of little files on the disk, small blocks are wasteful
>>of disk space (for instance, 1024 empty files take 512K vs 1M of disk for
>>the smaller blocking factor).
>
>And choosing a disk block size is a performance issue rather than
>disk space issue. One should tune disk block size and fragment size
>to get the best performance.
>
>I thought that 386BSD uses this Fast Unix File System.
Consider the most ridiculous possible situation: I have a 1G disk with
a 1G blocking factor (impossible without rewriting the buffering code).
How many files can I have on the disk? (Ignore the blocks allocated to inodes
and directoryies 8-P).
The answer is "1"... even if it only has 1 byte in it.
Terry Lambert
terry@icarus.weber.edu
terry_lambert@novell.com
---
Any opinions in this posting are my own and not those of my present
or previous employers.
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