To flush cache

sync
echo 3 > /proc/sys/vm/drop_caches 

To set readahead (in 512-byte sectors)

blockdev --setra 16384 /dev/sdb

To run hdparm disk throughput benchmark

hdparm -Tt /dev/sdb 

To set kernel request queue size (better to be at least twice higher than queue_depth)

echo 512 > /sys/block/sdb/queue/nr_requests

To set hardware requests queue size (limit 254)

echo 128 > /sys/block/sda/device/queue_depth

To merge files

TChain * chain = new TChain("esdTree");
chain->SetMaxTreeSize(3400000000);
chain->Add("/data.local2/0000/AliESDs.root");
chain->Add("/data.local2/0001/AliESDs.root");
chain->Merge("big.root");

To read a file to memory with C++ code

#include<iostream>
#include<fstream>
int read_cache(const char* a) {
        const size_t BMAX = 1024;
        char buffer[BMAX];
        std::ifstream f(a);
        int i = 0;
        while(f) {
           f.read(buffer,BMAX);
           ++i;
        }
        return i;
}

To cache a file into memory inside ROOT analysis code put the following inside Notify() function in TSelector

int h = read_cache(fChain->GetCurrentFile()->GetName());

or the following inside Notify() in AliAnalysisManager.cxx

int h = read_cache(fTree->GetCurrentFile()->GetName());

-- MishaZ - 05 Mar 2008