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Revision 3 (moo, 2007-07-07 15:29) → Revision 4/7 (moo, 2007-07-07 15:30)

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 = Improving the Throughput of File Transfers = 

 Jan has blogged about the development of async io and threaded stat() at 

 http://blog.lighttpd.net/articles/tag/aio 

 == vmstat == 

 <pre> 
 $ vmstat 5 
 </pre> 

 If you are using a threaded io-backend (e.g. gthread-aio) and see that the ``b`` column 
 reaches the value of ``server.max-read-threads`` you know that now all threads are 
 waiting for the disk to respond. 

 == iostat == 

 <pre> 
 $ iostat -x 5 
 </pre> 

 ``r/s`` is the number of requests per second which are handled by the disk. Due to  
 the nature of hard-disks there is a upper limit you can get here: 

 <pre> 
 max-rps = (disk-rpm / 60) * 2 

 ((7200 rot/min) / (60 s/min)) * 2 seek/rot = 120 * 2 seek/rot = 240 seek/s 
 </pre> 

 For each request handle by the disk it has to do a seek. In the best case the next 
 seek is just around the corner, in the worst case it just passed over the block to 
 read and has to do a full rotation to get to the place again. On average this means 
 it has to do half a rotation for a seek or 2 seeks per rotation. 

 If you have more disks the max-rps sums up IF your data is well distributed over all 
 disks. 

 ``util%`` of 100% means that the disk is either at the seek-limit (see r/s) or  
 the throughput limit.