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SubjectRe: Flushing the (page?) cache for a block device
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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