On Mon, Sep 21, 2009 at 5:39 PM, Scott Marlowe <scott.marlowe@xxxxxxxxx> wrote:
I'm looking at running session servers in ram. All the data is
throw-away data, so my plan is to have a copy of the empty db on the
hard drive ready to go, and have a script that just copies it into ram
and starts the db there. We're currently IO write bound with
fsync=off using a 15k5 seagate SAS drive, so I'm hoping that moving
the db into /dev/shm will help quite a bit here.
Does anybody any real world experience here or any words of sage
advice before I go off and start testing this?
I assume you intend to this or some variation of it.
mount -t tmpfs -o size=1G tmpfs /pgfast
tmpfs file systems, including /dev/shm, ***should not*** be counted on as being RAM only. File systems of this type in Linux, and at least Solaris also, can be swap since the tmpfs type is derived from swap and swap = memory + active swap partitions.
I would think that a ram disk or ramfs may be more what you're after. Add ramdisk_size=X, where X is the maximum size in kilobytes, to the kernel line in your grub.conf file. Unlike tmpfs, the ramdisk_size X parameter cannot be more than the memory of your server. When using a ramdisk the ext2 file system would be best with all the fun noatime and like mount parameters. This is good for a fast, volatile, fixed-size ramdisk.
OTOH, ramfs can be just as fast but with all the fun system-hanging features like growing until all RAM is consumed.
mount -t ramfs ramfs /pgfast
Pro ... it will always be as fast, or possibly a hair faster, than tmpfs.
Con ... it won't show up in a df output unlike tmpfs or ramdisk. Use the mount command with no parameters to look for it and be sure to unmount it when you're done.
Pro/Con ... you can't specify the file system type like with ramdisk.
Pro... it will only take away from memory as space is used, i.e. if you have 500M of memory in use and mount the file system but do nothing else then only 500M of memory is in use. If you then copy a 100M file to it then 600M of memory is in use. Delete that 100M file and you're back to 500M of used memory.
Pro/Con ... unlike other file systems, it will grow with the need... unchecked. It could attempt to consume all available memory pushing all other processes out to swap and this is a bad, bad thing.
I'm sure there are other pro's & con's to ramfs.
HTH.
Greg
