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On Friday 19 November 2010 22:13:50 Mick wrote: |
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> Short of measuring the latency with some system (which I wouldn't |
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> know how) I have experimented with setting the /boot partition on |
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> primary and logical partitions and the difference (on a stopwatch) |
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> was measurable in seconds betweeen having said partition on a |
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> primary and having it on a logical. Furthermore, sda7 was slower |
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> than sda5. |
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> |
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> I haven't measured latencies for first mount and subsequent look ups. |
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> I thought that it would be the same every time a partition fs is |
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> being accessed, no? |
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I shouldn't design it that way. Would you? |
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Consider the layout of the disk. First we have the master boot record, |
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which contains the disk addresses of the four allowable primary |
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partitions*, and not much else besides the primitive boot code to fetch |
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the data from those addresses. Then each primary partition has the |
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address of its first directory containing data. Those five parameters are |
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assumed to be fixed and can be held in a small lookup table in the OS. |
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One primary partition may be declared as an "extended" partition, by the |
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setting of a single bit in its entry in the MBR**. That partition has to |
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have the same header layout as the others, in particular not allowing |
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more than one data address***. In this case it's the address not of the |
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first directory but of the first logical partition - and that partition |
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has to have the same header layout again, because it's just a partition |
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full of data, isn't it? |
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The answer to your question is that only very few values are needed to |
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specify the fixed start points of all the partitions on the disk, and |
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virtually no overhead is involved in storing them for the inevitably |
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frequent use they're going to get. |
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(Sorry if I'm rambling. I've been down with the dreaded lurgy for a day |
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or two, and after a small glass of wine this evening I'm having trouble |
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focusing on the screen, never mind my thoughts.) |
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* One partition is plenty for all normal folk, especially those who run |
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an OS that's convinced it's the only entity in the universe - who could |
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possibly want more than four? And just don't mention 640KB memory unless |
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you're prepared for fisticuffs. |
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** Talk about a single point of failure! |
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*** Well, of course it doesn't, but what would we do without hindsight? |
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-- |
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Rgds |
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Peter. Linux Counter 5290, 1994-04-23. |
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