Messages in this thread | | | Date | Tue, 01 Mar 2011 18:27:18 -0800 | From | Linda Walsh <> | Subject | Re: RFE kernel option to do the desirable thing, w/regards to 'O_DIRECT' and mis-aligned data |
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Thanks for the shorthand Dave, but I wasn't really trying to use xfs_mkfs to make a file that was failing -- but was more trying to use it as an example of supporting the idea that both should succeed, and if a write is a partial write to an O_DIRECT file, that it be allowed to succeed and the kernel, knowing the device's minimum write size from the driver, could buffer the last sector.
To deal with back-compat issues, it could be based off of a proc var like /proc/kernel/fs/direct_IO_handling using bitfields (or multiple vars if you don't like bitfields, I s with the bits defined as:
Bit 0 Controlling allowed partial writes that start at an aligned position Bit 1 Controlling allowed non-aligned writes Bit 2 Controlling allowed partial reads that start at aligned position Bit 3 Controlling allowed non-aligned reads Bit 4 Controlling whether to use general FS cache for affected sectors
It's a bit of 'overkill' for what I wanted (just case controlled by Bit 0), but for sake of completeness, I thought all of these combinations should be specified.
Default of 0 = current behavior of mis-aligned data accesses failing, while specifying various combinations would allow for variations with the kernel handling mis-aligned accesses automatically, much like the x86 processor handles mis-aligned integer additions or stacks automatically (perhaps at a performance penalty, but with a tendency toward 'working' rather than failing, if possible).
It seems better to put that logic in the kernel rather than saddle multiple applications using DIRECT I/O with handling the non-aligned cases. This seems especially useful given the long term trend toward increasing use of static-memory devices which will likely support arbitrary direct I/O sizes.
Linda Walsh
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