Thank you very much for your great explanation.

I little bit doubt that why +/- buffers/cache is not tallying for machine1. I understood after displaying it as mb.Here is output in mb which is not tallied.

[root@machine1 ~]# free -m total used free shared buffers cached Mem: 32183 1309 30873 0 290 485 -/+ buffers/cache: 533 31649 Swap: 51199 0 51199

Thanks again, Ashik

On Tue, Dec 18, 2012 at 9:35 AM, John R Pierce pierce@hogranch.com wrote:

On 12/17/2012 10:22 PM, AshikAli.m wrote:

...

I have two 64 bit cent Os machine. I googled that in linux always show entire memory as used. But below showing only 1 GB detected.

no, that first one shows you have 31gb ram, about 1gb is being used by software, and the other 30GB are in use as buffers/cache

the second shows 11gb ram, 1gb used, and 10gb as buffers/cache

here's a machine fo mine thats been running for a couple months with large databases (both postgres and oracle)

# free -g total used free shared buffers cached Mem: 47 43 3 0 0 38 -/+ buffers/cache: 5 42 Swap: 49 0 49

out of 47gb total, 5gb is actually used, 38gb is in use as disk cache, and 3gb is completely unused currently, so 42gb is available for use by software.

the main line to look at is the -/+ buffers/cache. THAT is the real usage with discardable buffers and cache subtracted and the 'free' value on that line is the buffers + cache + freespace

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* *

yes. yes exactly the calculation is correct. In that machine only DRBD and HA application running.

I tell the story,

I have a telephony server with HA and DRBD mirroring(2 machines) . Unfortunately I installed 32 bit OS on those. I come to know that after upgrading it to 64 system will detect all ram memory which are inserted.

In this HA, important process runs on which is production node. For migration purpose; currently I made machine2 as production node which has 12G ram.

[root@machine2~]# free -m total used free shared buffers cached Mem: 12007 11961 46 0 579 9956 -/+ buffers/cache: 1424 10582 Swap: 51199 0 51199

Here,

Used memory ( 12007 - 46 ) = 11961

In that used memory 11961 - ( 579 + 9956 ) = 1429( Actually in use )

so, 46(free)+579(buffer)+9956(cached) = 10581(fraction) is free.

So, Always look into -/+ buffers/cache. That is the exactly correct.

Thanks, Ashik.

On Tue, Dec 18, 2012 at 10:44 AM, John R Pierce pierce@hogranch.com wrote:

On 12/17/2012 11:19 PM, AshikAli.m wrote:

...

[root@machine1 ~]# free -m total used free shared buffers

cached

...

Mem: 32183 1309 30873 0 290 485 -/+ buffers/cache: 533 31649 Swap: 51199 0 51199

so you have about 32gb total physical memory. 1.3gb of that is 'used' but of that 290MB is buffers and 485MB is cache, so really only 533MB (the 2nd line) is in use by software, and 31649MB is available (which is the 30873 free on the 1st line, plus the 290 buffers plus the 485 cached, as all that is considered 'discardable' [*].

you also have 51199MB of swapfile, entirely unused.

this looks to me like a system that was just booted, and hasn't done much.

[*] under heavy loads, like a buay large scale database server, this sum isn't exactly true, some of the buffer memory is 'dirty' so not considered as free without first flushing it to disk...

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