On Sat, May 26, 2012 at 04:27:01PM -0400, Joey Hess wrote: > Josselin Mouette wrote: > > Oh stop, there is a difference: in a tmpfs the system doesn’t need to > > commit the data on disk, and therefore can write it to disk whenever it > > likes, especially when the disk is not too used. There is no need to > > keep a journal nor to access the disk several times to update metadata. > > Only unused data is written to disk. Which means a *huge* performance > > improvement. Do the measurements yourself, it works with basically > > anything that makes heavy use of /tmp. > > I'm looking foward to a report from systems instrumented to track all > accesses to /tmp, that analizes the frequency of accesses and gives > numbers about just how huge this performance improvement is. > > Until we have such a report, why are we engaging in premature optimisation? Recently, I had a pile of over a hundred snapshots of a project in tarballs and zips, all with random extra junk, never having the same capitalization of file names, etc. I wanted to put all non-junk data into git. Cue ad-hoc scripts to unzip, fix newlines, check encoding and convert to UTF-8 if needed, etc, etc. Lots of churn but at any given time the working set was a small fraction of memory. Every iteration took 60ish seconds of I/O -- damn frustrating when interspersed with manual work. I then tried it on /tmp/, and suddenly everything went down to well below a second. So don't tell me real filesystems are "almost as good" as tmpfs when memory is not tight so there's no need for writeouts. And on modern systems, memory is hardly ever tight. I think that box had jfs, but other filesystems are no different: for example, ext* will fsync() during a rename() call behind your back even if you don't request it, forcing every file to hit the disk platters even though they'll immediately get changed again. For files outside /tmp/ that makes sense: the system has to guarantee you won't lose data during a crash. But why should I care about not losing temp files? -- I was born an ugly, dumb and work-loving child, then an evil midwife replaced me in the crib.
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