On Sun, Jun 10, 2012 at 01:51:19AM +0300, Serge wrote: > Some people asked for a thread summary. So here it is. [Lots of drivel, including thoroughly debunked statements, snipped. Seriously, can't you even read what's written to you? Sorry for being angry, but there's a limit to how many times you can have folks explain the basics of page cache without starting to suspect malice.] But, for the rest of us, here's a different summary. And note that, even though I'm a strong proponent of tmpfs, I say it might be better to skip it for wheezy -- so there's a chance this summary is not as biased. There are exactly two downsides: * In the "everything on one partition" scheme, there is less space available on swap that on /. • This is the big show-stopping problem. * A test case was shown where tmpfs is somewhat slower (by 15%), with reduced interactivity as well. • Needs some investigation; is not a typical case as you'd need large files on /tmp, nonlinear access patterns and high memory pressure at the same time. Still, it needs to be fixed somehow. And upsides: * Massive speed increase for I/O operations: • if syncs are involved: ~10000x • if the file survives longer than 5-30 seconds: by disk's speed • if the disk is not touched at all¹: ~10x • if there's a writeout: depending on file/metadata ratio (no need for journaling/barriers) Note that these speed-ups touch I/O on /tmp only. Obviously, a process that's CPU-bound won't see noticeable gains, and others typically do quite a lot of things other than I/O. * No spin-ups of laptop drives. • There's always a spin-up even if the file has been deleted before that 5-30 seconds passes. Laptop mode reduces these somehow but you'd have to set dirty_expire_centisecs to some giant value to make them not have a practical effect, risking losing actual data. * Less wear of SSD drives. • Contrary to Serge's claims, SSDs are not an oddity, and it's not unlikely these will be a majority before wheezy becomes oldstable. I intentionally did not include cases not involving typical use, like NFSed or read-only / (we can assume a competent admin there), user errors (like comparing spaces you configured yourself), or merits of LVM with unallocated disk space (ie, a competent or semi-competent admin) vs dynamic swap files. This basically boils down to: efficiency vs failures for a newbie user. Folks in this thread tend to agree that it's the user who can't deal with partitions or the notion of separate space on separate filesystems who needs the most help, as the rest of us can configure the system. Still, it'd not a nice thing to need to do all those repetitive tweaks you may not even know about (like mounting everything noatime, again a good thing even compared to relatime). So, I'd say: * let's play it safe and not default to tmpfs for wheezy * do it properly later Current size defaults for /tmp are naive to the point of brokenness: with modern systems not being expected to ever use swap, we'd want to cap /tmp at 100% of swap rather than mere 20% -- but it's tricky to tell apart that from systems that actually do need swap for memory. And the same RAM/swap combination can need different settings based on what the system is supposed to do. But no matter how we tweak the ratio, it won't let some hapless user plop a 50GB file into /tmp "because I have a lot of free space". This is not an insurmountable problem: /tmp might use some form of overlay that uses tmpfs for regular use and starts shunting to some area other than swap once it sees it is being used for large files. Or alternatively, there could possibly be a dynamic swap file on / earmarked for tmpfs pages only (not implemented yet?). Or... Too bad, any of these solutions would need a lot of testing, something for which there simply isn't time before wheezy. Let's go back here after the release. [¹]. During a short test; there'll be a spin-up later to write the directory even if the file has been deleted already. This is O(1) though. -- I was born an ugly, dumb and work-loving child, then an evil midwife replaced me in the crib.
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