Messages in this thread | | | Date | Fri, 14 Mar 2008 08:12:37 -0400 | From | Ric Wheeler <> | Subject | Re: [ANNOUNCE] Ramback: faster than a speeding bullet |
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Benny Amorsen wrote: > Ric Wheeler <ric@emc.com> writes: > >> The only really safe default is to disable the write cache by default >> or possibly dynamically disable the write cache when barriers are not >> supported by a drive. Both have a severe performance impact and I am >> not sure that for most casual users it is a good trade. > > So people ARE running their disks in a mode similar to Ramback. > > > /Benny >
We have been looking at write performance with RAM disk, battery backed Clariion array & slow laptop drives in another thread on fs-devel, but the rough numbers should be interesting.
If you are not doing an fsync() at the end of writing a file, you are writing to the page cache (as long as it fits in DRAM) so you are basically getting thousands of small files/sec.
We did a test which showed the following for synchronous (fsync()) writers of small files with a SLES10/SP1 kernel but the results still hold for upstream kernels (at least for the order of magnitude).
Ramdisk test backed testing showed over 4600 small 4k files/sec with 1 thread.
Midrange array (looks like a ramdisk behind a fibre channel port) hit around 778 files/sec with 1 thread.
With a local disk, write cached enabled and barriers on, you are getting around 47 4k files/sec.
The tests were run on ext3, different file systems perform differently but all fall in the same order of magnitude of performance with the same class of storage behind it ;-)
ric
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