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On Thu, 2 Apr 2009, Linus Torvalds wrote:On Thu, 2 Apr 2009, Andrew Morton wrote:A suitable design for the streaming might be, every 4MB:Here's an example. I call it "overwrite.c" for obvious reasons.
- run sync_file_range(SYNC_FILE_RANGE_WRITE) to get the 4MB underway
to the disk
- run fadvise(POSIX_FADV_DONTNEED) against the previous 4MB to
discard it from pagecache.
Oh, except my example doesn't do the fadvise. Instead, I make sure to throttle the writes and the old range with
SYNC_FILE_RANGE_WAIT_BEFORE|SYNC_FILE_RANGE_WRITE|SYNC_FILE_RANGE_WAIT_AFTER
which makes sure that the old pages are easily dropped by the VM - and they will be, since they end up always being on the cold list.
I _wanted_ to add a SYNC_FILE_RANGE_DROP but I never bothered because this particular load it didn't matter. The system was perfectly usable while overwriting even huge disks because there was never more than 8MB of dirty data in flight in the IO queues at any time.