software raid + fs -- performance

[V13]

 Thelo hints & tips pano sto eksis:

 Exo 2 diskoys... Pros to paron toys exo balei na doylevoyn me raid0, alla den 
tha peiraze na einai kai raid1 (ta alla ypotheto oti den ta syzitame stin 
periptosi ayti). To zitoymeno einai oso to dynaton megalyteri taxytita gia 
xrisi apo postgreSQL. H basi ginetai arketa megali (merika GB) kai sxedon ola 
ta queries ginontai me basi indexes. Ayto simenei oti o xronos spatalatai 
kyrios se seek kai oxi se read/write. Oi diskoi einai scsi me 30+ MB/s read 
apo tin epifaneia toy diskoy, eno me to raid exo ftasei ta 60+ MB/s write. 

 Exo dokimasei ext3(me -T largefile kai -T largefile4), raiserfs kai xfs kai 
exo xrisimopoihsei san chunk-size sto raid, times apo 64K mexri 2MB. Par'ola 
ayta den eida kapia aisthiti diafora kai tyxainoyn stigmes opoy to vmstat 
dixnei bi (read apo diskoys) mikroteres apo 1.5MB/s (kai ayto einai kai to 
'bottleneck' tis periptosis') eno oi diskoi doylevoyn san treloi, me ta 
lampakia toys na anaboyn synexeia kai na kanoyn 'arketi' fasaria. Epeksa ligo 
me to stripe toy ext3, alla kai pali den ebgala akri.

 Exo kataliksei sto xfs, giati ap'oti eida ta paei kalytera me megala arxeia 
(to olo 'partition' apoteleitai apo ligotero apo 200 arxeia kai den yparxoyn 
prooptikes na ayksithoyn) kai epidi moy fenetai oti den kathysterei to 
systima otan kanei flush tin cache (antitheta me ta alla 2). 

 Par'ola ayta oti benchmark katafera na bro sto web, exoyn na kanoyn me polla 
arxeia, h me syriaki xrisi megalon arxeion.... 

 Opiadipote protasi/empeiria kalodexoymeni...

 Na shmeiothei oti den me niazei sxedon katholoy h taxytita toy write.. Mono 
sto read/seek thelo taxytita, gia ayto kai dialeksa raid0.

<<V13>>