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Quoting Gordon Henderson <gordon+dcglug@xxxxxxxxxx>:
On Fri, 5 Feb 2010, Rob Beard wrote:One thing I'm not entirely clear about is the buffers and cache. For instance on your embedded system, it says... Mem: 257208k total, 238644k used, 18564k free, 2516k buffers Swap: 0k total, 0k used, 0k free, 35164k cachedSo, about 2.5MB are used for buffers and about 3.5MB used for cache, I'm assuming they are separate? (so combined the buffers and cache are using about 6MB?)35MB cached, not 3.5MB ...
Ahh yeah, I read it wrong.
What I'm wondering is if say, the cache is like the old SmartDrive cache on DOS where rather than writing directly do disc, it holds what it needs to write in a cache for a couple of seconds until the disk is pretty much idle to speed up writing?If that is the case, what is the buffer for?I'm under the impression (someone correct me if I'm wrong!) that 'cached' data belongs to the filesystem and "buffers" belong to the underlying block device(s). So the system has 35MB of files, cached in RAM and not on disk, and 2.5MB worth of buffered disk blocks that it can read from without accessing the disk.
I see.
256MB in total, 140MB for the ramdisk, leaving what's effectively 144MB of system RAM which the kernel, programs and program data has to live in. Off that, some 38MB are being used for cache/buffers by the filing system/OS. I suspect in this instance, it doesn't know that the "disk" is RAM, so it's effectively copied the ramdisk (ext2) into a filesystem cache - although I imagine it's probably quicker to fetch the data from the cache than the (ram)disk though as it probably has less code to execute. (getting from a very simplified cache filesystem vs. traversing an ext2 filesystem)
Yep, I presume not much difference in speed, but I guess every little helps. I suppose it's also simplier this way too not having to change things to disable the buffers.
Some of the files in the cache will be regularly executed executables - cron jobs, etc. and some will be the aterisk config files, and php source code files, images, etc. the web server reads, and so on. Also the music on hold files and other audio files it has read out of the flash drive...
Ahh, so the actual space occupied by Asterisk etc isn't much then I guess?
They may actually be every single file the system has read since boot time, if it's never had to flush the cache since boot to make space for something new... Wonder if there's a way to find out what files are in the cache...
Not sure, I'm sure someone has written a tool to do that though. Rob -- The Mailing List for the Devon & Cornwall LUG http://mailman.dclug.org.uk/listinfo/list FAQ: http://www.dcglug.org.uk/linux_adm/list-faq.html