From: David Howells <email@example.com> To: Christoph Hellwig <firstname.lastname@example.org>, Dave Chinner <email@example.com>, "Theodore Ts'o" <firstname.lastname@example.org> Cc: email@example.com, Alexander Viro <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org Subject: How to avoid using bmap in cachefiles -- FS-Cache/CacheFiles rewrite Date: Thu, 14 Nov 2019 13:40:38 +0000 [thread overview] Message-ID: <email@example.com> (raw) In-Reply-To: <firstname.lastname@example.org> Hi Christoph, I've been rewriting cachefiles in the kernel and it now uses kiocbs to do async direct I/O to/from the cache files - which seems to make a 40-48% speed improvement. However, I've replaced the use of bmap internally to detect whether data is present or not - which is dodgy for a number of reasons, not least that extent-based filesystems might insert or remove blocks of zeros to shape the extents better, thereby rendering the metadata information useless for cachefiles. But using a separate map has a couple of problems: (1) The map is metadata kept outside of the filesystem journal, so coherency management is necessary (2) The map gets hard to manage for very large files (I'm using 256KiB granules, so 1 bit per granule means a 512-byte map block can span 1GiB) and xattrs can be of limited capacity. I seem to remember you said something along the lines of it being possible to tell the filesystem not to do discarding and insertion of blocks of zeros. Is there a generic way to do that? Also, is it possible to make it so that I can tell an O_DIRECT read to fail partially or, better, completely if there's no data to be had in part of the range? I can see DIO_SKIP_HOLES, but that only seems to affect writes Thanks, David
prev parent reply other threads:[~2019-11-14 13:40 UTC|newest] Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-11-13 17:55 David Howells 2019-11-13 18:46 ` Jeff Layton 2019-11-14 13:40 ` David Howells [this message]
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