Messages in this thread | | | Date | Tue, 15 Jan 2002 03:16:45 +0000 | From | Joel Becker <> | Subject | Re: [PATCH] PAGE_SIZE IO for RAW (RAW VARY) |
| |
On Thu, Jan 10, 2002 at 01:15:01PM -0800, Badari Pulavarty wrote: > The issue with my (mostly) PAGE_SIZE RAW IO patch is, it could generate > buffer heads with different b_size in a single IO request. Jens & Ben > have concerns about some of the low level drivers not able to handle > these. (it would be hard to analyse all the drivers to verify this). > > So, Andrea suggested we add a flag in "blk_dev" structure. Only the > drivers which support variable size buffer heads in a single IO will > set it. My RAW VARY patch will use this flag to see if I can do > RAW VARY or not. Makes sense ?
The way I handled it in my O_DIRECT code for the same task (do the largest I/O you can, instead of a hardcoded 512 or sw_blocksize) was to merely choose the minimum alignment. See the patch in <20020109195606.A16884@parcelfarce.linux.theplanet.co.uk>. In your approach, you want leading blocks to be aligned at 512 or so, then all 4k aligned I/Os to be 4k size, then the trailing I/Os are aligned again at their size. My approach was 'if aligned at 512, do all 512'. It is slower than your approach for large I/Os aligned on 512, but it also has the property of submitting identical blocksizes in one request. Andrea has suggested that I work with you to be incremental on top of your code. This would include managing a flag bit to decide if a device can handle variable I/O sizes in one request. The issue I have with that is that in the O_DIRECT case, the fallback is failure, not slow I/O. IOW, in rawio, if the flag is false, you issue all I/Os with a 512 blocksize. That's slow. However, in O_DIRECT, if the flag is false, the sw_blocksize is 4096, and the alignment is 512, you fail with EINVAL. That is bad. In my current patch, the O_DIRECT I/O would succeed at a 512 byte blocksize. However, my patch doesn't touch rawio at all. Please have a look at my patch and maybe we can work on merging our efforts.
Joel
--
"The first thing we do, let's kill all the lawyers." -Henry VI, IV:ii
http://www.jlbec.org/ jlbec@evilplan.org - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |