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From: cgd@eden.CS.Berkeley.EDU (Chris G. Demetriou)
Newsgroups: comp.unix.bsd
Subject: Re: Dumb Question: Why 512 byte block?
Date: 19 Dec 92 00:07:39
Organization: Kernel Hackers 'r' Us
Lines: 62
Message-ID: <CGD.92Dec19000739@eden.CS.Berkeley.EDU>
References: <1992Dec18.005050.20594@decuac.dec.com>
<1992Dec18.030833.7395@fcom.cc.utah.edu> <1gt736INNjje@menudo.uh.edu>
<1992Dec18.235623.27538@fcom.cc.utah.edu>
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In-reply-to: terry@cs.weber.edu's message of 18 Dec 92 23:56:23 GMT
In article <1992Dec18.235623.27538@fcom.cc.utah.edu> terry@cs.weber.edu (A Wizard of Earth C) writes:
>>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?
sorry. you guys *both* sound confused. first of all, i've never
seen a FFS with 1k blocks; the most "standard" configuration
is 4k or 8k blocks, with 8 fragments.
this yields fragments of (obviously) 512bytes and 1k.
you can't split *fragments* between one file.
you *can* split blocks between one file, but this tends not to happen,
because the FFS doesn't do this unless it's necessary.
so: if you have 6 1.5k files, and the following filesystem setups,
you get the following results:
8k blocks, 8frags/block: 6 blocks assigned, but major fragmentation...
(actually, only 12 frags are used. if your
filesystem is near full, < 6 blocks will be
assigned).
total space "actually used": 12k
8k blocks, 4frags/block: 6 blocks assigned, but major fragmentation...
(actually, only 6 frags are used. if your
filesystem is near full, < 6 blocks will be
assigned).
total space "actually used": 12k
4k blocks, 8frags/block: 6 blocks assigned, but major fragmentation...
(actually, only 18 frags are used. if your
filesystem is near full, < 6 blocks will be
assigned).
total space "actually used": 9k
4k blocks, 4frags/block: 6 blocks assigned, fragmentation "not too bad"...
(actually, 12 frags are used. if your
filesystem is near full, < 6 blocks will be
assigned).
total space "actually used": 12k
Chris
--
Chris G. Demetriou cgd@cs.berkeley.edu
"Sometimes it is better to have twenty million instructions by
Friday than twenty million instructions per second." -- Wes Clark