Watching performance on a DHCP Server

Simon Hobson dhcp1 at
Tue Feb 12 22:17:19 UTC 2008

Olaf van der Spek wrote:

>  >  fsyncs are the most I/O intensive operation you can perform on magnetic
>>  media. If I remember correctly it takes at least 2 full rotations of the
>>  disk for an fsync to occur, although it may take longer to commit more data.
>>  This is were solid state rules over magnetic storage.
>Why are two required?
>Even then, you could commit lots of transactions in a single go.

I'm wondering if that's a hangover from RAID5 thinking - where 
(unless the data is already in cache), every write is preceded by a 
read. In fact, to write a single block, the minimum is to read the 
pre-update version of that block and the pre-update parity block that 
goes with it, re-compute the parity, and write the new data and 
parity. In that situation, you must read and then write the same 
block, so a minimum of just over one revolution, with an average of a 
little over 1 1/2 revs - all assuming zero overhead in computing the 
new parity and synced spindles.

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