Sounds like you're looking for a combo of overlay mounts on top of tmpfs
- the flush to disk you'd have to manage manually but I've done this
with some success in the past and I do seem to recall it being discussed
to provide pseudo read-write over a read-only medium such as a CDROM
http://netbsd.gw.com/cgi-bin/man-cgi?mount_overlay+8+NetBSD-current
the only caution is that I found it tricky to grok the sense of which
filesystem gets overlaid on what and where writes end up, but some
workshopping should show that up pretty quickly
Regards,
Malcolm
On Wed, Mar 9, 2016, at 10:49, Swift Griggs wrote:
>
> I know NetBSD supports RAM disks. I also know it uses them for
> installation, but I'm not sure if they are overlays on top of the static
> disk images or if they get loaded/populated with the image after the
> system boots. The effect is the same, either way. You end up with a
> writable file system in RAM which is predicated on a disk image. I was
> thinking about this in the context of another problem.
>
> Let's say I've got a machine with a boatload of RAM and I have an
> application which wants extremely fast writes, but doesn't really care
> that much about data integrity, and this case, nor do I. Having the
> system
> come back in the same state after rebooting is "nice to have" in this
> thought experiment. Soo, here is the question:
>
> Since a RAM disk is probably the fastest option, is there a way to
> basically leave uncommitted data until I run "sync" or something calls
> fsync() ? Even better, is there a way to fake a sync until I run
> something
> else ?
>
> In other words, operate out of a RAM disk until my human operator gives
> the go-ahead to flush out the writes to disk or the RAM disk gets full. I
> think Sprite did something like this back in the Jurassic Period. I also
> remember CacheFS for Solaris doing something similar for NFS.
>
> I can think of a few applications:
>
> - Internet cafe style browsing where the machines are using older
> hardware. They might benefit from using a small RAM disk overlay rather
> than having to use something like a PATA disks for it's cache.
>
> - Editing high bandwidth data like video or audio. Most of the time my
> editing sessions don't last more than an hour and once I get a final cut
> I
> just save the project to a server and don't care much about the recording
> or editing workstation.
>
> - Any kind of temporary storage that needs RAM disk speeds, but still
> needs an option to sync/flush the file system back to long term storage.
>
> I'm guessing this would be a little tricky with something like UFS since
> a
> process could alter/write to a file and it'd need to reflect that when
> another process wanted to do the same, fseek(), or dozens of other
> operations. However, I just wonder if there is already a way to do this
> and I'm just not aware of it.
>
> I wonder if the LiveCD's for Linux that let you flush you changes to USB
> do something similar. I doubt it. I'm guessing they just load a RAM disk
> image and you operate on that until you reboot and it's lost.
>
> -Swift
--
Malcolm Herbert
m...@mjch.net
