[CentOS] Odd Question... MAX Ram Disk Size
amilivojevic at pbl.ca
Thu Mar 31 18:10:39 UTC 2005
Craig White wrote:
> this is great info and I suppose I might find use for it someday but
> wouldn't a loop mount from an iso file (the disk image) be better for
> this purpose?
It depends on how mounted file system is used.
If you mount read-only (which is the only option with ISO image), than
loop mount would work fine too, or even better. If server has enough
RAM to cache entire image (and keep it cached), than disk would be
accessed only on first read, and subsequent reads would go from cache.
Of course, when system needs memory for something else, cache is the
first thing thrown out. Same goes if you mount read/write with noatime
option and majority of I/O is read with ocasional write.
If you want to write to that file system, and write often (for example,
image contains ext3 or vfat file system), and you don't care information
being lost after reboot (example would be using it for /tmp), than it is
better to use tmpfs. If you have enough free RAM, the information you
don't care about will never be written to the disk. In case of loop
device, eventually it would. So you save some disk I/O.
In both cases, system will use information from RAM whenever possible,
and disk if you get low on free RAM. In both cases, pages with data not
accessed for a long time, will be first to go back to disk (or simply
marked as free if not changed).
This is basically idea why on Solaris /tmp is mounted as tmpfs by
default. On Linux many people argue that files in /tmp "usually" live
for short enough time not to be commited to disk anyhow. Which is true
for some types of files, and usually those are the files you really care
not to have actuall disk I/O. IMO, it never hurts to actually make sure
this happens for any type of temporary file by using tmpfs (provided
there's enough RAM). There's no performance degradation for using tmpfs
(instead of utilizing disk cache), but there is potential benefit.
Aleksandar Milivojevic <amilivojevic at pbl.ca> Pollard Banknote Limited
Systems Administrator 1499 Buffalo Place
Tel: (204) 474-2323 ext 276 Winnipeg, MB R3T 1L7
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