Messages in this thread | | | Date | Mon, 30 Mar 2009 15:40:52 +0800 | From | Wu Fengguang <> | Subject | Re: Flushing the (page?) cache for a block device |
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Hi Sascha,
On Sun, Mar 29, 2009 at 02:54:39PM +0200, Sascha Silbe wrote: > Hello! > > I'd like to do a verification step after writing to a USB stick (block > device level, no filesystem involved). For this to work as intended, I > need to ensure the reads are not satisfied from blocks still in cache, > i.e.: flush "the cache" (page cache IIUC) for the block device in > question (and _only_ this device). > Some research resulted in the following options: > > 1. Open the target file using O_DIRECT while writing. > Doesn't feel like the right thing to do; only want to flush the > cache, not circumvent it. Might severly limit performance as writes are > synchronous. > Interface isn't very nice (need to ensure alignment to page size in > memory). > > 2. Use posix_fadvise() with POSIX_FADV_DONTNEED. > Probably works with the current kernel (haven't tested yet), but > there's absolutely no guarantee. > Interface much better than O_DIRECT.
FYI, here is a handy fadvise tool: http://www.zip.com.au/~akpm/linux/patches/stuff/ext3-tools.tar.gz
Usage: fadvise /some/file 0 0 dontneed
Thanks, Fengguang
> 3. Flush the entire page cache using procfs. > Brute force, last resort. Will severly impact performance of my > tool (as it's going to write to several devices in parallel). > > > Is there a better method I've not found yet? Nr. 2 sounds like the way > to go, but I don't like that it could (silently!) stop working with any > new kernel version. > > > PS: Not subscribed, so please CC me. > > CU Sascha > > -- > http://sascha.silbe.org/ > http://www.infra-silbe.de/
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