Re: [PATCH 2/2] libext2fs/e2fsck: implement metadata prefetching

[Phillip Susi <psusi@ubuntu.com>]

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On 1/31/2014 8:53 AM, Theodore Ts'o wrote:
> Something that we might need to go to in the future is instead of 
> using mmap(), to maintain our own explicit buffer cache inside 
> unix_io, and use direct I/O to avoid caching the disk blocks
> twice. Then when we use a single-threaded disk prefetcher, managed
> by the unix_io, it will know when a particular I/O request has
> completed, and more importantly, if there is a synchronous read
> request coming in from main body of the program, it can stop
> prefetching and allow the higher priority read to complete.  We can
> also experiment with how many threads might make sense --- even
> with an HDD, using multiple threads so that we can take advantage
> of NCQ might still be a win.

Why build your own cache instead of letting the kernel take care of
it?  I  believe the IO elevator already gives preferential treatment
to blocking reads so just using readahead() to prefetch and sticking
with plain old read() should work nicely.

> Finally, if we are managing our own buffer cache, we should
> consider adding a bforget method to the I/O manager.  That way
> e2fsck can give hints to the caching layer that a block isn't
> needed any more.  If it is in the cache, it can be dropped, to free
> memory, and if it is still on the to-be-prefetched list it should
> also be dropped.  (Of course, if a block is on the to-be-prefetched
> list, and a synchronous read request comes in for that block, we
> should have dropped it from the to-be-prefetched list at that
> point.)  The main use for having a bforget method is for the most
> part, once we are done scanning a non-directory extent tree block,
> we won't be needing it again.

Good idea, but this also could just be translated to posix_fadvise to
have the kernel drop the pages from the cache.

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